Java Strings String s new String silly

Question

I m a C   guy learning Java  I m reading Effective Java and something confused me  It says never to write code like this   String s   new String  silly      Because it creates unnecessary String objects  But instead it should be written like this   String s    No longer silly     Ok fine so far   However  given this class   public final class CaseInsensitiveString       private String s      public CaseInsensitiveString String s            if  s    null                throw new NullPointerException                      this s   s                       CaseInsensitiveString cis   new CaseInsensitiveString  Polish    String s    polish      Why is the first statement ok  Shouldn t it be   CaseInsensitiveString cis    Polish   How do I make CaseInsensitiveString behave like String so the above statement is OK  with and without extending String   What is it about String that makes it OK to just be able to pass it a literal like that  From my understanding there is no  copy constructor  concept in Java

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In your first example  you are creating a String   silly  and then passing it as a parameter to another String s copy constructor  which makes a second String which is identical to the first   Since Java Strings are immutable  something that frequently stings people who are used to C strings   this is a needless waste of resources   You should instead use the second example because it skips several needless steps   However  the String literal is not a CaseInsensitiveString so there you cannot do what you want in your last example   Furthermore  there is no way to overload a casting operator like you can in C   so there is literally no way to do what you want   You must instead pass it in as a parameter to your class s constructor   Of course  I d probably just use String toLowerCase   and be done with it   Also  your CaseInsensitiveString should implement the CharSequence interface as well as probably the Serializable and Comparable interfaces   Of course  if you implement Comparable  you should override equals   and hashCode   as well

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when they say to write  String s    Silly     instead of   String s   new String  Silly      they mean it when creating a String object because both of the above statements create a String object but the new String   version creates two String objects  one in heap and the other in string constant pool  Hence using more memory   But when you write  CaseInsensitiveString cis   new CaseInsensitiveString  Polish      you are not creating a String instead you are creating an object of class CaseInsensitiveString  Hence you need to use the new operator

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In most versions of the JDK the two versions will be the same   String s   new String  silly     String s    No longer silly    Because strings are immutable the compiler maintains a list of string constants and if you try to make a new one will first check to see if the string is already defined  If it is then a reference to the existing immutable string is returned   To clarify - when you say  String s     you are defining a new variable which takes up space on the stack - then whether you say  No longer silly  or new String  silly   exactly the same thing happens - a new constant string is compiled into your application and the reference points to that   I dont see the distinction here  However for your own class  which is not immutable  this behaviour is irrelevant and you must call your constructor   UPDATE  I was wrong  Based on a down vote and comment attached I tested this and realise that my understanding is wrong - new String  Silly   does indeed create a new string rather than reuse the existing one  I am unclear why this would be  what is the benefit   but code speaks louder than words

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Strings are special in Java - they re immutable  and string constants are automatically turned into String objects   There s no way for your SomeStringClass cis    value  example to apply to any other class   Nor can you extend String  because it s declared as final  meaning no sub-classing is allowed

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- How do i make CaseInsensitiveString behave like String so the above statement is ok  with and w out extending String   What is it about String that makes it ok to just be able to pass it a literal like that  From my understanding there is no  copy constructor  concept in Java right    Enough has been said from the first point   Polish  is an string literal and cannot be assigned to the CaseInsentiviveString class   Now about the second point  Although you can t create new literals  you can follow the first item of that book for a  similar  approach so the following statements are true          Lets test the insensitiveness     CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg        CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG         assert cis5    cis6      assert cis5 equals cis6     Here s the code   C  oreyes samples java insensitive gt type CaseInsensitiveString java import java util Map  import java util HashMap   public final class CaseInsensitiveString          private static final Map lt String CaseInsensitiveString gt  innerPool                                    new HashMap lt String CaseInsensitiveString gt          private final String s           Effective Java Item 1  Consider providing static factory methods instead of constructors     public static CaseInsensitiveString valueOf  String s              if   s    null                 return null                    String value   s toLowerCase             if    CaseInsensitiveString innerPool containsKey  value                    CaseInsensitiveString innerPool put  value   new CaseInsensitiveString  value                           return CaseInsensitiveString innerPool get  value                     Class constructor  This creates a new instance each time it is invoked      public CaseInsensitiveString String s           if  s    null                throw new NullPointerException                                 this s   s toLowerCase               public boolean equals  Object other              if   other instanceof CaseInsensitiveString                   CaseInsensitiveString otherInstance     CaseInsensitiveString   other               return this s equals  otherInstance s                         return false              public int hashCode             return this s hashCode               Test the class using the  assert  keyword      public static void main  String    args                 Creating two different objects as in new String  Polish      new String  Polish   is false         CaseInsensitiveString cis1   new CaseInsensitiveString  Polish            CaseInsensitiveString cis2   new CaseInsensitiveString  Polish                references cis1 and cis2 points to differents objects             so the following is true         assert cis1     cis2          Yes they re different         assert cis1 equals cis2       Yes they re equals thanks to the equals method             Now let s try the valueOf idiom         CaseInsensitiveString cis3   CaseInsensitiveString valueOf  Polish            CaseInsensitiveString cis4   CaseInsensitiveString valueOf  Polish                References cis3 and cis4 points to same  object             so the following is true         assert cis3    cis4          Yes they point to the same object         assert cis3 equals cis4      and still equals              Lets test the insensitiveness         CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg            CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG             assert cis5    cis6          assert cis5 equals cis6               Futhermore         CaseInsensitiveString cis7   CaseInsensitiveString valueOf  SomethinG            CaseInsensitiveString cis8   CaseInsensitiveString valueOf  someThing             assert cis8    cis5  amp  amp  cis7    cis6          assert cis7 equals cis5   amp  amp  cis6 equals cis8             C  oreyes samples java insensitive gt javac CaseInsensitiveString java   C  oreyes samples java insensitive gt java -ea CaseInsensitiveString  C  oreyes samples java insensitive gt    That is  create an internal pool of CaseInsensitiveString  objects  and return the corrensponding instance from there   This way the      operator returns true for two objects references representing the same value    This is useful when similar objects are used very frequently and creating cost is expensive   The string class documentation states that the class uses an internal pool  The class is not complete  some interesting issues arises when we try to walk the contents of the object at implementing the CharSequence interface  but this code is good enough to show how that item in the Book could be applied    It is important to notice that by using the internalPool object  the references are not released and thus not garbage collectible  and that may become an issue if a lot of objects are created    It works for the String class because it is used intensively and the pool is constituted of  interned  object only   It works well for the Boolean class too   because there are only two possible values   And finally that s also the reason why valueOf int  in class Integer is limited to -128 to 127 int values

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Strings are treated a bit specially in java  they re immutable so it s safe for them to be handled by reference counting    If you write  String s    Polish   String t    Polish     then s and t actually refer to the same object  and s  t will return true  for      for objects read  is the same object   or can  anyway  I m not sure if this is part of the actual language spec or simply a detail of the compiler implementation-so maybe it s not safe to rely on this      If you write   String s   new String  Polish    String t   new String  Polish      then s  t  because you ve explicitly created a new string  although s equals t  will return true  because string adds this behavior to equals     The thing you want to write    CaseInsensitiveString cis    Polish     can t work because you re thinking that the quotations are some sort of short-circuit constructor for your object  when in fact this only works for plain old java lang Strings

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Java creates a String object for each string literal you use in your code  Any time    is used  it is the same as calling new String     Strings are complex data that just  act  like primitive data  String literals are actually objects even though we pretend they re primitive literals  like 6  6 0   c   etc  So the String  literal   text  returns a new String object with value char   value     t   e   x   t   Therefore  calling   new String  text       is actually akin to calling   new String new String new char    t   e   x   t        Hopefully from here  you can see why your textbook considers this redundant     For reference  here is the implementation of String  http   www docjar com html api java lang String java html   It s a fun read and might inspire some insight  It s also great for beginners to read and try to understand  as the code demonstrates very professional and convention-compliant code   Another good reference is the Java tutorial on Strings   http   docs oracle com javase tutorial java data strings html

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In Java the syntax  text  creates an instance of class java lang String   The assignment   String foo    text     is a simple assignment  with no copy constructor necessary   MyString bar    text     Is illegal whatever you do because the MyString class isn t either java lang String or a superclass of java lang String

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Strings are special in Java - they re immutable  and string constants are automatically turned into String objects   There s no way for your SomeStringClass cis    value  example to apply to any other class   Nor can you extend String  because it s declared as final  meaning no sub-classing is allowed

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I would just add that Java has Copy constructors     Well  that s an ordinary constructor with an object of same type as argument

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Java strings are interesting   It looks like the responses have covered some of the interesting points   Here are my two cents   strings are immutable  you can never change them   String x    x   x    Y       The first line will create a variable x which will contain the string value  x    The JVM will look in its pool of string values and see if  x  exists  if it does  it will point the variable x to it  if it does not exist  it will create it and then do the assignment The second line will remove the reference to  x  and see if  Y  exists in the pool of string values   If it does exist  it will assign it  if it does not  it will create it first then assignment   As the string values are used or not  the memory space in the pool of string values will be reclaimed    string comparisons are contingent on what you are comparing  String a1   new String  A     String a2   new String  A       a1 does not equal a2 a1 and a2 are object references When string is explicitly declared  new instances are created and their references will not be the same    I think you re on the wrong path with trying to use the caseinsensitive class   Leave the strings alone   What you really care about is how you display or compare the values   Use another class to format the string or to make comparisons   i e    TextUtility compare string 1  string 2   TextUtility compareIgnoreCase string 1  string 2  TextUtility camelHump string 1    Since you are making up the class  you can make the compares do what you want - compare the text values

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The best way to answer your question would be to make you familiar with the  String constant pool   In java string objects are immutable  i e their values cannot be changed once they are initialized   so when editing a string object you end up creating a new edited string object wherease the old object just floats around in a special memory ares called the  string constant pool   creating a new string object by  String s    Hello     will only create a string object in the pool and the reference s will refer to it  but by using  String s   new String  Hello      you create two string objects  one in the pool and the other in the heap  the reference will refer to the object in the heap

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CaseInsensitiveString is not a String although it contains a String  A String literal e g  example  can be only assigned to a String

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- How do i make CaseInsensitiveString behave like String so the above statement is ok  with and w out extending String   What is it about String that makes it ok to just be able to pass it a literal like that  From my understanding there is no  copy constructor  concept in Java right    Enough has been said from the first point   Polish  is an string literal and cannot be assigned to the CaseInsentiviveString class   Now about the second point  Although you can t create new literals  you can follow the first item of that book for a  similar  approach so the following statements are true          Lets test the insensitiveness     CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg        CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG         assert cis5    cis6      assert cis5 equals cis6     Here s the code   C  oreyes samples java insensitive gt type CaseInsensitiveString java import java util Map  import java util HashMap   public final class CaseInsensitiveString          private static final Map lt String CaseInsensitiveString gt  innerPool                                    new HashMap lt String CaseInsensitiveString gt          private final String s           Effective Java Item 1  Consider providing static factory methods instead of constructors     public static CaseInsensitiveString valueOf  String s              if   s    null                 return null                    String value   s toLowerCase             if    CaseInsensitiveString innerPool containsKey  value                    CaseInsensitiveString innerPool put  value   new CaseInsensitiveString  value                           return CaseInsensitiveString innerPool get  value                     Class constructor  This creates a new instance each time it is invoked      public CaseInsensitiveString String s           if  s    null                throw new NullPointerException                                 this s   s toLowerCase               public boolean equals  Object other              if   other instanceof CaseInsensitiveString                   CaseInsensitiveString otherInstance     CaseInsensitiveString   other               return this s equals  otherInstance s                         return false              public int hashCode             return this s hashCode               Test the class using the  assert  keyword      public static void main  String    args                 Creating two different objects as in new String  Polish      new String  Polish   is false         CaseInsensitiveString cis1   new CaseInsensitiveString  Polish            CaseInsensitiveString cis2   new CaseInsensitiveString  Polish                references cis1 and cis2 points to differents objects             so the following is true         assert cis1     cis2          Yes they re different         assert cis1 equals cis2       Yes they re equals thanks to the equals method             Now let s try the valueOf idiom         CaseInsensitiveString cis3   CaseInsensitiveString valueOf  Polish            CaseInsensitiveString cis4   CaseInsensitiveString valueOf  Polish                References cis3 and cis4 points to same  object             so the following is true         assert cis3    cis4          Yes they point to the same object         assert cis3 equals cis4      and still equals              Lets test the insensitiveness         CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg            CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG             assert cis5    cis6          assert cis5 equals cis6               Futhermore         CaseInsensitiveString cis7   CaseInsensitiveString valueOf  SomethinG            CaseInsensitiveString cis8   CaseInsensitiveString valueOf  someThing             assert cis8    cis5  amp  amp  cis7    cis6          assert cis7 equals cis5   amp  amp  cis6 equals cis8             C  oreyes samples java insensitive gt javac CaseInsensitiveString java   C  oreyes samples java insensitive gt java -ea CaseInsensitiveString  C  oreyes samples java insensitive gt    That is  create an internal pool of CaseInsensitiveString  objects  and return the corrensponding instance from there   This way the      operator returns true for two objects references representing the same value    This is useful when similar objects are used very frequently and creating cost is expensive   The string class documentation states that the class uses an internal pool  The class is not complete  some interesting issues arises when we try to walk the contents of the object at implementing the CharSequence interface  but this code is good enough to show how that item in the Book could be applied    It is important to notice that by using the internalPool object  the references are not released and thus not garbage collectible  and that may become an issue if a lot of objects are created    It works for the String class because it is used intensively and the pool is constituted of  interned  object only   It works well for the Boolean class too   because there are only two possible values   And finally that s also the reason why valueOf int  in class Integer is limited to -128 to 127 int values

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String is a special built-in class of the language   It is for the String class only in which you should avoid saying  String s   new String  Polish      Because the literal  Polish  is already of type String  and you re creating an extra unnecessary object   For any other class  saying  CaseInsensitiveString cis   new CaseInsensitiveString  Polish      is the correct  and only  in this case  thing to do

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Java strings are interesting   It looks like the responses have covered some of the interesting points   Here are my two cents   strings are immutable  you can never change them   String x    x   x    Y       The first line will create a variable x which will contain the string value  x    The JVM will look in its pool of string values and see if  x  exists  if it does  it will point the variable x to it  if it does not exist  it will create it and then do the assignment The second line will remove the reference to  x  and see if  Y  exists in the pool of string values   If it does exist  it will assign it  if it does not  it will create it first then assignment   As the string values are used or not  the memory space in the pool of string values will be reclaimed    string comparisons are contingent on what you are comparing  String a1   new String  A     String a2   new String  A       a1 does not equal a2 a1 and a2 are object references When string is explicitly declared  new instances are created and their references will not be the same    I think you re on the wrong path with trying to use the caseinsensitive class   Leave the strings alone   What you really care about is how you display or compare the values   Use another class to format the string or to make comparisons   i e    TextUtility compare string 1  string 2   TextUtility compareIgnoreCase string 1  string 2  TextUtility camelHump string 1    Since you are making up the class  you can make the compares do what you want - compare the text values

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I believe the main benefit of using the literal form  ie   foo  rather than new String  foo    is that all String literals are  interned  by the VM  In other words it is added to a pool such that any other code that creates the same string will use the pooled String rather than creating a new instance   To illustrate  the following code will print true for the first line  but false for the second   System out println  foo      foo    System out println new String  bar      new String  bar

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- How do i make CaseInsensitiveString behave like String so the above statement is ok  with and w out extending String   What is it about String that makes it ok to just be able to pass it a literal like that  From my understanding there is no  copy constructor  concept in Java right    Enough has been said from the first point   Polish  is an string literal and cannot be assigned to the CaseInsentiviveString class   Now about the second point  Although you can t create new literals  you can follow the first item of that book for a  similar  approach so the following statements are true          Lets test the insensitiveness     CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg        CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG         assert cis5    cis6      assert cis5 equals cis6     Here s the code   C  oreyes samples java insensitive gt type CaseInsensitiveString java import java util Map  import java util HashMap   public final class CaseInsensitiveString          private static final Map lt String CaseInsensitiveString gt  innerPool                                    new HashMap lt String CaseInsensitiveString gt          private final String s           Effective Java Item 1  Consider providing static factory methods instead of constructors     public static CaseInsensitiveString valueOf  String s              if   s    null                 return null                    String value   s toLowerCase             if    CaseInsensitiveString innerPool containsKey  value                    CaseInsensitiveString innerPool put  value   new CaseInsensitiveString  value                           return CaseInsensitiveString innerPool get  value                     Class constructor  This creates a new instance each time it is invoked      public CaseInsensitiveString String s           if  s    null                throw new NullPointerException                                 this s   s toLowerCase               public boolean equals  Object other              if   other instanceof CaseInsensitiveString                   CaseInsensitiveString otherInstance     CaseInsensitiveString   other               return this s equals  otherInstance s                         return false              public int hashCode             return this s hashCode               Test the class using the  assert  keyword      public static void main  String    args                 Creating two different objects as in new String  Polish      new String  Polish   is false         CaseInsensitiveString cis1   new CaseInsensitiveString  Polish            CaseInsensitiveString cis2   new CaseInsensitiveString  Polish                references cis1 and cis2 points to differents objects             so the following is true         assert cis1     cis2          Yes they re different         assert cis1 equals cis2       Yes they re equals thanks to the equals method             Now let s try the valueOf idiom         CaseInsensitiveString cis3   CaseInsensitiveString valueOf  Polish            CaseInsensitiveString cis4   CaseInsensitiveString valueOf  Polish                References cis3 and cis4 points to same  object             so the following is true         assert cis3    cis4          Yes they point to the same object         assert cis3 equals cis4      and still equals              Lets test the insensitiveness         CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg            CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG             assert cis5    cis6          assert cis5 equals cis6               Futhermore         CaseInsensitiveString cis7   CaseInsensitiveString valueOf  SomethinG            CaseInsensitiveString cis8   CaseInsensitiveString valueOf  someThing             assert cis8    cis5  amp  amp  cis7    cis6          assert cis7 equals cis5   amp  amp  cis6 equals cis8             C  oreyes samples java insensitive gt javac CaseInsensitiveString java   C  oreyes samples java insensitive gt java -ea CaseInsensitiveString  C  oreyes samples java insensitive gt    That is  create an internal pool of CaseInsensitiveString  objects  and return the corrensponding instance from there   This way the      operator returns true for two objects references representing the same value    This is useful when similar objects are used very frequently and creating cost is expensive   The string class documentation states that the class uses an internal pool  The class is not complete  some interesting issues arises when we try to walk the contents of the object at implementing the CharSequence interface  but this code is good enough to show how that item in the Book could be applied    It is important to notice that by using the internalPool object  the references are not released and thus not garbage collectible  and that may become an issue if a lot of objects are created    It works for the String class because it is used intensively and the pool is constituted of  interned  object only   It works well for the Boolean class too   because there are only two possible values   And finally that s also the reason why valueOf int  in class Integer is limited to -128 to 127 int values

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String is a special built-in class of the language   It is for the String class only in which you should avoid saying  String s   new String  Polish      Because the literal  Polish  is already of type String  and you re creating an extra unnecessary object   For any other class  saying  CaseInsensitiveString cis   new CaseInsensitiveString  Polish      is the correct  and only  in this case  thing to do

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First  you can t make a class that extends from String  because String is a final class  And java manage Strings differently from other classes so only with String you can do  String s    Polish     But whit your class you have to invoke the constructor  So  that code is fine

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String is one of the special classes in which you can create them without the new Sring part  it s the same as  int x   y   or   char c

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CaseInsensitiveString and String are different objects   You can t do   CaseInsensitiveString cis    Polish     because  Polish  is a String  not a CaseInsensitiveString   If String extended CaseInsensitiveString String then you d be OK  but obviously it doesn t   And don t worry about the construction here  you won t be making unecessary objects   If you look at the code of the constructor  all it s doing is storing a reference to the string you passed in   Nothing extra is being created   In the String s   new String  foobar   case it s doing something different   You are first creating the literal string  foobar   then creating a copy of it by constructing a new string out of it   There s no need to create that copy

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CaseInsensitiveString and String are different objects   You can t do   CaseInsensitiveString cis    Polish     because  Polish  is a String  not a CaseInsensitiveString   If String extended CaseInsensitiveString String then you d be OK  but obviously it doesn t   And don t worry about the construction here  you won t be making unecessary objects   If you look at the code of the constructor  all it s doing is storing a reference to the string you passed in   Nothing extra is being created   In the String s   new String  foobar   case it s doing something different   You are first creating the literal string  foobar   then creating a copy of it by constructing a new string out of it   There s no need to create that copy

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The best way to answer your question would be to make you familiar with the  String constant pool   In java string objects are immutable  i e their values cannot be changed once they are initialized   so when editing a string object you end up creating a new edited string object wherease the old object just floats around in a special memory ares called the  string constant pool   creating a new string object by  String s    Hello     will only create a string object in the pool and the reference s will refer to it  but by using  String s   new String  Hello      you create two string objects  one in the pool and the other in the heap  the reference will refer to the object in the heap

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In your first example  you are creating a String   silly  and then passing it as a parameter to another String s copy constructor  which makes a second String which is identical to the first   Since Java Strings are immutable  something that frequently stings people who are used to C strings   this is a needless waste of resources   You should instead use the second example because it skips several needless steps   However  the String literal is not a CaseInsensitiveString so there you cannot do what you want in your last example   Furthermore  there is no way to overload a casting operator like you can in C   so there is literally no way to do what you want   You must instead pass it in as a parameter to your class s constructor   Of course  I d probably just use String toLowerCase   and be done with it   Also  your CaseInsensitiveString should implement the CharSequence interface as well as probably the Serializable and Comparable interfaces   Of course  if you implement Comparable  you should override equals   and hashCode   as well

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First  you can t make a class that extends from String  because String is a final class  And java manage Strings differently from other classes so only with String you can do  String s    Polish     But whit your class you have to invoke the constructor  So  that code is fine

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Strings are special in Java - they re immutable  and string constants are automatically turned into String objects   There s no way for your SomeStringClass cis    value  example to apply to any other class   Nor can you extend String  because it s declared as final  meaning no sub-classing is allowed

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In Java the syntax  text  creates an instance of class java lang String   The assignment   String foo    text     is a simple assignment  with no copy constructor necessary   MyString bar    text     Is illegal whatever you do because the MyString class isn t either java lang String or a superclass of java lang String

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- How do i make CaseInsensitiveString behave like String so the above statement is ok  with and w out extending String   What is it about String that makes it ok to just be able to pass it a literal like that  From my understanding there is no  copy constructor  concept in Java right    Enough has been said from the first point   Polish  is an string literal and cannot be assigned to the CaseInsentiviveString class   Now about the second point  Although you can t create new literals  you can follow the first item of that book for a  similar  approach so the following statements are true          Lets test the insensitiveness     CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg        CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG         assert cis5    cis6      assert cis5 equals cis6     Here s the code   C  oreyes samples java insensitive gt type CaseInsensitiveString java import java util Map  import java util HashMap   public final class CaseInsensitiveString          private static final Map lt String CaseInsensitiveString gt  innerPool                                    new HashMap lt String CaseInsensitiveString gt          private final String s           Effective Java Item 1  Consider providing static factory methods instead of constructors     public static CaseInsensitiveString valueOf  String s              if   s    null                 return null                    String value   s toLowerCase             if    CaseInsensitiveString innerPool containsKey  value                    CaseInsensitiveString innerPool put  value   new CaseInsensitiveString  value                           return CaseInsensitiveString innerPool get  value                     Class constructor  This creates a new instance each time it is invoked      public CaseInsensitiveString String s           if  s    null                throw new NullPointerException                                 this s   s toLowerCase               public boolean equals  Object other              if   other instanceof CaseInsensitiveString                   CaseInsensitiveString otherInstance     CaseInsensitiveString   other               return this s equals  otherInstance s                         return false              public int hashCode             return this s hashCode               Test the class using the  assert  keyword      public static void main  String    args                 Creating two different objects as in new String  Polish      new String  Polish   is false         CaseInsensitiveString cis1   new CaseInsensitiveString  Polish            CaseInsensitiveString cis2   new CaseInsensitiveString  Polish                references cis1 and cis2 points to differents objects             so the following is true         assert cis1     cis2          Yes they re different         assert cis1 equals cis2       Yes they re equals thanks to the equals method             Now let s try the valueOf idiom         CaseInsensitiveString cis3   CaseInsensitiveString valueOf  Polish            CaseInsensitiveString cis4   CaseInsensitiveString valueOf  Polish                References cis3 and cis4 points to same  object             so the following is true         assert cis3    cis4          Yes they point to the same object         assert cis3 equals cis4      and still equals              Lets test the insensitiveness         CaseInsensitiveString cis5   CaseInsensitiveString valueOf  sOmEtHiNg            CaseInsensitiveString cis6   CaseInsensitiveString valueOf  SoMeThInG             assert cis5    cis6          assert cis5 equals cis6               Futhermore         CaseInsensitiveString cis7   CaseInsensitiveString valueOf  SomethinG            CaseInsensitiveString cis8   CaseInsensitiveString valueOf  someThing             assert cis8    cis5  amp  amp  cis7    cis6          assert cis7 equals cis5   amp  amp  cis6 equals cis8             C  oreyes samples java insensitive gt javac CaseInsensitiveString java   C  oreyes samples java insensitive gt java -ea CaseInsensitiveString  C  oreyes samples java insensitive gt    That is  create an internal pool of CaseInsensitiveString  objects  and return the corrensponding instance from there   This way the      operator returns true for two objects references representing the same value    This is useful when similar objects are used very frequently and creating cost is expensive   The string class documentation states that the class uses an internal pool  The class is not complete  some interesting issues arises when we try to walk the contents of the object at implementing the CharSequence interface  but this code is good enough to show how that item in the Book could be applied    It is important to notice that by using the internalPool object  the references are not released and thus not garbage collectible  and that may become an issue if a lot of objects are created    It works for the String class because it is used intensively and the pool is constituted of  interned  object only   It works well for the Boolean class too   because there are only two possible values   And finally that s also the reason why valueOf int  in class Integer is limited to -128 to 127 int values

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Just because you have the word String in your class  does not mean you get all the special features of the built-in String class

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Just because you have the word String in your class  does not mean you get all the special features of the built-in String class

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Strings are treated a bit specially in java  they re immutable so it s safe for them to be handled by reference counting    If you write  String s    Polish   String t    Polish     then s and t actually refer to the same object  and s  t will return true  for      for objects read  is the same object   or can  anyway  I m not sure if this is part of the actual language spec or simply a detail of the compiler implementation-so maybe it s not safe to rely on this      If you write   String s   new String  Polish    String t   new String  Polish      then s  t  because you ve explicitly created a new string  although s equals t  will return true  because string adds this behavior to equals     The thing you want to write    CaseInsensitiveString cis    Polish     can t work because you re thinking that the quotations are some sort of short-circuit constructor for your object  when in fact this only works for plain old java lang Strings

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In most versions of the JDK the two versions will be the same   String s   new String  silly     String s    No longer silly    Because strings are immutable the compiler maintains a list of string constants and if you try to make a new one will first check to see if the string is already defined  If it is then a reference to the existing immutable string is returned   To clarify - when you say  String s     you are defining a new variable which takes up space on the stack - then whether you say  No longer silly  or new String  silly   exactly the same thing happens - a new constant string is compiled into your application and the reference points to that   I dont see the distinction here  However for your own class  which is not immutable  this behaviour is irrelevant and you must call your constructor   UPDATE  I was wrong  Based on a down vote and comment attached I tested this and realise that my understanding is wrong - new String  Silly   does indeed create a new string rather than reuse the existing one  I am unclear why this would be  what is the benefit   but code speaks louder than words

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String is a special built-in class of the language   It is for the String class only in which you should avoid saying  String s   new String  Polish      Because the literal  Polish  is already of type String  and you re creating an extra unnecessary object   For any other class  saying  CaseInsensitiveString cis   new CaseInsensitiveString  Polish      is the correct  and only  in this case  thing to do

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I believe the main benefit of using the literal form  ie   foo  rather than new String  foo    is that all String literals are  interned  by the VM  In other words it is added to a pool such that any other code that creates the same string will use the pooled String rather than creating a new instance   To illustrate  the following code will print true for the first line  but false for the second   System out println  foo      foo    System out println new String  bar      new String  bar

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when they say to write  String s    Silly     instead of   String s   new String  Silly      they mean it when creating a String object because both of the above statements create a String object but the new String   version creates two String objects  one in heap and the other in string constant pool  Hence using more memory   But when you write  CaseInsensitiveString cis   new CaseInsensitiveString  Polish      you are not creating a String instead you are creating an object of class CaseInsensitiveString  Hence you need to use the new operator

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I believe the main benefit of using the literal form  ie   foo  rather than new String  foo    is that all String literals are  interned  by the VM  In other words it is added to a pool such that any other code that creates the same string will use the pooled String rather than creating a new instance   To illustrate  the following code will print true for the first line  but false for the second   System out println  foo      foo    System out println new String  bar      new String  bar

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Java strings are interesting   It looks like the responses have covered some of the interesting points   Here are my two cents   strings are immutable  you can never change them   String x    x   x    Y       The first line will create a variable x which will contain the string value  x    The JVM will look in its pool of string values and see if  x  exists  if it does  it will point the variable x to it  if it does not exist  it will create it and then do the assignment The second line will remove the reference to  x  and see if  Y  exists in the pool of string values   If it does exist  it will assign it  if it does not  it will create it first then assignment   As the string values are used or not  the memory space in the pool of string values will be reclaimed    string comparisons are contingent on what you are comparing  String a1   new String  A     String a2   new String  A       a1 does not equal a2 a1 and a2 are object references When string is explicitly declared  new instances are created and their references will not be the same    I think you re on the wrong path with trying to use the caseinsensitive class   Leave the strings alone   What you really care about is how you display or compare the values   Use another class to format the string or to make comparisons   i e    TextUtility compare string 1  string 2   TextUtility compareIgnoreCase string 1  string 2  TextUtility camelHump string 1    Since you are making up the class  you can make the compares do what you want - compare the text values

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String is a special built-in class of the language   It is for the String class only in which you should avoid saying  String s   new String  Polish      Because the literal  Polish  is already of type String  and you re creating an extra unnecessary object   For any other class  saying  CaseInsensitiveString cis   new CaseInsensitiveString  Polish      is the correct  and only  in this case  thing to do

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String s1  foo     literal will go in pool and s1 will refer   String s2  foo     this time it will check  foo  literal is already available in StringPool or not as now it exist so s2 will refer the same literal   String s3 new String  foo       foo  literal will be created in StringPool first then through string arg constructor String Object will be created i e  foo  in the heap due to object creation through new operator then s3 will refer it   String s4 new String  foo      same as s3  so System out println s1  s2       true   due to literal comparison   and System out println s3  s4       false   due to object  comparison s3 and s4 is created at different places in heap

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Strings are special in Java - they re immutable  and string constants are automatically turned into String objects   There s no way for your SomeStringClass cis    value  example to apply to any other class   Nor can you extend String  because it s declared as final  meaning no sub-classing is allowed

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I would just add that Java has Copy constructors     Well  that s an ordinary constructor with an object of same type as argument

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Java strings are interesting   It looks like the responses have covered some of the interesting points   Here are my two cents   strings are immutable  you can never change them   String x    x   x    Y       The first line will create a variable x which will contain the string value  x    The JVM will look in its pool of string values and see if  x  exists  if it does  it will point the variable x to it  if it does not exist  it will create it and then do the assignment The second line will remove the reference to  x  and see if  Y  exists in the pool of string values   If it does exist  it will assign it  if it does not  it will create it first then assignment   As the string values are used or not  the memory space in the pool of string values will be reclaimed    string comparisons are contingent on what you are comparing  String a1   new String  A     String a2   new String  A       a1 does not equal a2 a1 and a2 are object references When string is explicitly declared  new instances are created and their references will not be the same    I think you re on the wrong path with trying to use the caseinsensitive class   Leave the strings alone   What you really care about is how you display or compare the values   Use another class to format the string or to make comparisons   i e    TextUtility compare string 1  string 2   TextUtility compareIgnoreCase string 1  string 2  TextUtility camelHump string 1    Since you are making up the class  you can make the compares do what you want - compare the text values

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CaseInsensitiveString is not a String although it contains a String  A String literal e g  example  can be only assigned to a String

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If I understood it correctly  your question means why we cannot create an object by directly assigning it a value  lets not restrict it to a Wrapper of String class in java   To answer that I would just say  purely Object Oriented Programming languages have some constructs and it says  that all the literals when written alone can be directly transformed into an object of the given type   That precisely means  if the interpreter sees 3 it will be converted into an Integer object because integer is the type defined for such literals   If the interpreter sees any thing in single quotes like  a  it will directly create an object of type character  you do not need to specify it as the language defines the default object of type character for it   Similarly if the interpreter sees something in    it will be considered as an object of its default type i e  string  This is some native code working in the background   Thanks to MIT video lecture course 6 00 where I got the hint for this answer

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String str1    foo     String str2    foo      Both str1 and str2 belongs to tha same String object   foo   b coz Java manages Strings in StringPool  so if a new variable refers to the same String  it doesn t create another one rather assign the same alerady present in StringPool      String str1   new String  foo      String str2   new String  foo      Here both str1 and str2 belongs to different Objects  b coz new String   forcefully create a new String Object

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In your first example  you are creating a String   silly  and then passing it as a parameter to another String s copy constructor  which makes a second String which is identical to the first   Since Java Strings are immutable  something that frequently stings people who are used to C strings   this is a needless waste of resources   You should instead use the second example because it skips several needless steps   However  the String literal is not a CaseInsensitiveString so there you cannot do what you want in your last example   Furthermore  there is no way to overload a casting operator like you can in C   so there is literally no way to do what you want   You must instead pass it in as a parameter to your class s constructor   Of course  I d probably just use String toLowerCase   and be done with it   Also  your CaseInsensitiveString should implement the CharSequence interface as well as probably the Serializable and Comparable interfaces   Of course  if you implement Comparable  you should override equals   and hashCode   as well

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If I understood it correctly  your question means why we cannot create an object by directly assigning it a value  lets not restrict it to a Wrapper of String class in java   To answer that I would just say  purely Object Oriented Programming languages have some constructs and it says  that all the literals when written alone can be directly transformed into an object of the given type   That precisely means  if the interpreter sees 3 it will be converted into an Integer object because integer is the type defined for such literals   If the interpreter sees any thing in single quotes like  a  it will directly create an object of type character  you do not need to specify it as the language defines the default object of type character for it   Similarly if the interpreter sees something in    it will be considered as an object of its default type i e  string  This is some native code working in the background   Thanks to MIT video lecture course 6 00 where I got the hint for this answer

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You can t  Things in double-quotes in Java are specially recognised by the compiler as Strings  and unfortunately you can t override this  or extend java lang String - it s declared final

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In most versions of the JDK the two versions will be the same   String s   new String  silly     String s    No longer silly    Because strings are immutable the compiler maintains a list of string constants and if you try to make a new one will first check to see if the string is already defined  If it is then a reference to the existing immutable string is returned   To clarify - when you say  String s     you are defining a new variable which takes up space on the stack - then whether you say  No longer silly  or new String  silly   exactly the same thing happens - a new constant string is compiled into your application and the reference points to that   I dont see the distinction here  However for your own class  which is not immutable  this behaviour is irrelevant and you must call your constructor   UPDATE  I was wrong  Based on a down vote and comment attached I tested this and realise that my understanding is wrong - new String  Silly   does indeed create a new string rather than reuse the existing one  I am unclear why this would be  what is the benefit   but code speaks louder than words

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I believe the main benefit of using the literal form  ie   foo  rather than new String  foo    is that all String literals are  interned  by the VM  In other words it is added to a pool such that any other code that creates the same string will use the pooled String rather than creating a new instance   To illustrate  the following code will print true for the first line  but false for the second   System out println  foo      foo    System out println new String  bar      new String  bar

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You can t  Things in double-quotes in Java are specially recognised by the compiler as Strings  and unfortunately you can t override this  or extend java lang String - it s declared final

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In Java the syntax  text  creates an instance of class java lang String   The assignment   String foo    text     is a simple assignment  with no copy constructor necessary   MyString bar    text     Is illegal whatever you do because the MyString class isn t either java lang String or a superclass of java lang String

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It is a basic law that Strings in java are immutable and case sensitive

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CaseInsensitiveString and String are different objects   You can t do   CaseInsensitiveString cis    Polish     because  Polish  is a String  not a CaseInsensitiveString   If String extended CaseInsensitiveString String then you d be OK  but obviously it doesn t   And don t worry about the construction here  you won t be making unecessary objects   If you look at the code of the constructor  all it s doing is storing a reference to the string you passed in   Nothing extra is being created   In the String s   new String  foobar   case it s doing something different   You are first creating the literal string  foobar   then creating a copy of it by constructing a new string out of it   There s no need to create that copy

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In your first example  you are creating a String   silly  and then passing it as a parameter to another String s copy constructor  which makes a second String which is identical to the first   Since Java Strings are immutable  something that frequently stings people who are used to C strings   this is a needless waste of resources   You should instead use the second example because it skips several needless steps   However  the String literal is not a CaseInsensitiveString so there you cannot do what you want in your last example   Furthermore  there is no way to overload a casting operator like you can in C   so there is literally no way to do what you want   You must instead pass it in as a parameter to your class s constructor   Of course  I d probably just use String toLowerCase   and be done with it   Also  your CaseInsensitiveString should implement the CharSequence interface as well as probably the Serializable and Comparable interfaces   Of course  if you implement Comparable  you should override equals   and hashCode   as well

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First  you can t make a class that extends from String  because String is a final class  And java manage Strings differently from other classes so only with String you can do  String s    Polish     But whit your class you have to invoke the constructor  So  that code is fine

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I would just add that Java has Copy constructors     Well  that s an ordinary constructor with an object of same type as argument

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In Java the syntax  text  creates an instance of class java lang String   The assignment   String foo    text     is a simple assignment  with no copy constructor necessary   MyString bar    text     Is illegal whatever you do because the MyString class isn t either java lang String or a superclass of java lang String

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String is one of the special classes in which you can create them without the new Sring part  it s the same as  int x   y   or   char c

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String s1  foo     literal will go in pool and s1 will refer   String s2  foo     this time it will check  foo  literal is already available in StringPool or not as now it exist so s2 will refer the same literal   String s3 new String  foo       foo  literal will be created in StringPool first then through string arg constructor String Object will be created i e  foo  in the heap due to object creation through new operator then s3 will refer it   String s4 new String  foo      same as s3  so System out println s1  s2       true   due to literal comparison   and System out println s3  s4       false   due to object  comparison s3 and s4 is created at different places in heap

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You can t  Things in double-quotes in Java are specially recognised by the compiler as Strings  and unfortunately you can t override this  or extend java lang String - it s declared final

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String str1    foo     String str2    foo      Both str1 and str2 belongs to tha same String object   foo   b coz Java manages Strings in StringPool  so if a new variable refers to the same String  it doesn t create another one rather assign the same alerady present in StringPool      String str1   new String  foo      String str2   new String  foo      Here both str1 and str2 belongs to different Objects  b coz new String   forcefully create a new String Object

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You can t  Things in double-quotes in Java are specially recognised by the compiler as Strings  and unfortunately you can t override this  or extend java lang String - it s declared final

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In most versions of the JDK the two versions will be the same   String s   new String  silly     String s    No longer silly    Because strings are immutable the compiler maintains a list of string constants and if you try to make a new one will first check to see if the string is already defined  If it is then a reference to the existing immutable string is returned   To clarify - when you say  String s     you are defining a new variable which takes up space on the stack - then whether you say  No longer silly  or new String  silly   exactly the same thing happens - a new constant string is compiled into your application and the reference points to that   I dont see the distinction here  However for your own class  which is not immutable  this behaviour is irrelevant and you must call your constructor   UPDATE  I was wrong  Based on a down vote and comment attached I tested this and realise that my understanding is wrong - new String  Silly   does indeed create a new string rather than reuse the existing one  I am unclear why this would be  what is the benefit   but code speaks louder than words

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It is a basic law that Strings in java are immutable and case sensitive

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Java creates a String object for each string literal you use in your code  Any time    is used  it is the same as calling new String     Strings are complex data that just  act  like primitive data  String literals are actually objects even though we pretend they re primitive literals  like 6  6 0   c   etc  So the String  literal   text  returns a new String object with value char   value     t   e   x   t   Therefore  calling   new String  text       is actually akin to calling   new String new String new char    t   e   x   t        Hopefully from here  you can see why your textbook considers this redundant     For reference  here is the implementation of String  http   www docjar com html api java lang String java html   It s a fun read and might inspire some insight  It s also great for beginners to read and try to understand  as the code demonstrates very professional and convention-compliant code   Another good reference is the Java tutorial on Strings   http   docs oracle com javase tutorial java data strings html

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First  you can t make a class that extends from String  because String is a final class  And java manage Strings differently from other classes so only with String you can do  String s    Polish     But whit your class you have to invoke the constructor  So  that code is fine

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Strings are treated a bit specially in java  they re immutable so it s safe for them to be handled by reference counting    If you write  String s    Polish   String t    Polish     then s and t actually refer to the same object  and s  t will return true  for      for objects read  is the same object   or can  anyway  I m not sure if this is part of the actual language spec or simply a detail of the compiler implementation-so maybe it s not safe to rely on this      If you write   String s   new String  Polish    String t   new String  Polish      then s  t  because you ve explicitly created a new string  although s equals t  will return true  because string adds this behavior to equals     The thing you want to write    CaseInsensitiveString cis    Polish     can t work because you re thinking that the quotations are some sort of short-circuit constructor for your object  when in fact this only works for plain old java lang Strings

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CaseInsensitiveString and String are different objects   You can t do   CaseInsensitiveString cis    Polish     because  Polish  is a String  not a CaseInsensitiveString   If String extended CaseInsensitiveString String then you d be OK  but obviously it doesn t   And don t worry about the construction here  you won t be making unecessary objects   If you look at the code of the constructor  all it s doing is storing a reference to the string you passed in   Nothing extra is being created   In the String s   new String  foobar   case it s doing something different   You are first creating the literal string  foobar   then creating a copy of it by constructing a new string out of it   There s no need to create that copy

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Strings are treated a bit specially in java  they re immutable so it s safe for them to be handled by reference counting    If you write  String s    Polish   String t    Polish     then s and t actually refer to the same object  and s  t will return true  for      for objects read  is the same object   or can  anyway  I m not sure if this is part of the actual language spec or simply a detail of the compiler implementation-so maybe it s not safe to rely on this      If you write   String s   new String  Polish    String t   new String  Polish      then s  t  because you ve explicitly created a new string  although s equals t  will return true  because string adds this behavior to equals     The thing you want to write    CaseInsensitiveString cis    Polish     can t work because you re thinking that the quotations are some sort of short-circuit constructor for your object  when in fact this only works for plain old java lang Strings

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I would just add that Java has Copy constructors     Well  that s an ordinary constructor with an object of same type as argument

[java] Java Strings: "String s = new String("silly");"

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