What is meant by immutable

Question

This could be the dumbest question ever asked but I think it is quite confusing for a Java newbie     Can somebody clarify what is meant by immutable   Why is a String immutable  What are the advantages disadvantages of the immutable objects  Why should a mutable object such as StringBuilder be preferred over String and vice-verse    A nice example  in Java  will be really appreciated

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Actually String is not immutable if you use the wikipedia definition suggested above.

String's state does change post construction. Take a look at the hashcode() method. String caches the hashcode value in a local field but does not calculate it until the first call of hashcode(). This lazy evaluation of hashcode places String in an interesting position as an immutable object whose state changes, but it cannot be observed to have changed without using reflection.

So maybe the definition of immutable should be an object that cannot be observed to have changed.

If the state changes in an immutable object after it has been created but no-one can see it (without reflection) is the object still immutable?

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Immutable objects are objects that can t be changed programmatically  They re especially good for multi-threaded environments or other environments where more than one process is able to alter  mutate  the values in an object   Just to clarify  however  StringBuilder is actually a mutable object  not an immutable one  A regular java String is immutable  meaning that once it s been created you cannot change the underlying string without changing the object    For example  let s say that I have a class called ColoredString that has a String value and a String color   public class ColoredString        private String color      private String string       public ColoredString String color  String string            this color    color          this string   string             public String getColor      return this color         public String getString     return this string         public void setColor String newColor            this color   newColor             In this example  the ColoredString is said to be mutable because you can change  mutate  one of its key properties without creating a new ColoredString class  The reason why this may be bad is  for example  let s say you have a GUI application which has multiple threads and you are using ColoredStrings to print data to the window  If you have an instance of ColoredString which was created as  new ColoredString  Blue    This is a blue string       Then you would expect the string to always be  Blue   If another thread  however  got ahold of this instance and called  blueString setColor  Red      You would suddenly  and probably unexpectedly  now have a  Red  string when you wanted a  Blue  one  Because of this  immutable objects are almost always preferred when passing instances of objects around  When you have a case where mutable objects are really necessary  then you would typically guard the objet by only passing copies out from your specific field of control   To recap  in Java  java lang String is an immutable object  it cannot be changed once it s created  and java lang StringBuilder is a mutable object because it can be changed without creating a new instance

User · Answer

Actually String is not immutable if you use the wikipedia definition suggested above.

String's state does change post construction. Take a look at the hashcode() method. String caches the hashcode value in a local field but does not calculate it until the first call of hashcode(). This lazy evaluation of hashcode places String in an interesting position as an immutable object whose state changes, but it cannot be observed to have changed without using reflection.

So maybe the definition of immutable should be an object that cannot be observed to have changed.

If the state changes in an immutable object after it has been created but no-one can see it (without reflection) is the object still immutable?

User · Answer

I really like the explaination from SCJP Sun Certified Programmer for Java 5 Study Guide      To make Java more memory efficient  the JVM sets aside a special area of  memory called the  String constant pool   When the compiler encounters a String literal  it checks the pool to see if  an identical String already exists  If  a match is found  the reference to the new literal is directed to the existing String  and no new String literal object is created

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Immutable means that once the object is created  non of its members will change  String is immutable since you can not change its content  For example   String s1      abc     String s2   s1 trim      In the code above  the string s1 did not change  another object  s2  was created using s1

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As the accepted answer doesn t answer all the questions  I m forced to give an answer after 11 years and 6 months   Can somebody clarify what is meant by immutable   Hope you meant immutable object  because we could think about immutable reference   An object is immutable  iff once created  they always represent the same value  doesn t have any method that change the value    Why is a String immutable   Respect the above definition which could be checked by looking into the Sting java source code   What are the advantages disadvantages of the immutable objects  immutable types are     safer from bugs   easier to understand   and more ready for change     Why should a mutable object such as StringBuilder be preferred over String and vice-verse   Narrowing the question Why do we need the mutable StringBuilder in programming  A common use for it is to concatenate a large number of strings together  like this  String s    quot  quot   for  int i   0  i  lt  n    i        s   s   n     Using immutable strings  this makes a lot of temporary copies     the first number of the string   quot 0 quot   is actually copied n times in the course of building up the final string  the second number is copied n-1 times  and so on  It actually costs O n2  time just to do all that copying  even though we only concatenated n elements  StringBuilder is designed to minimize this copying  It uses a simple but clever internal data structure to avoid doing any copying at all until the very end  when you ask for the final String with a toString   call  StringBuilder sb   new StringBuilder    for  int i   0  i  lt  n    i      sb append String valueOf n      String s   sb toString     Getting good performance is one reason why we use mutable objects  Another is convenient sharing  two parts of your program can communicate more conveniently by sharing a common mutable data structure  More could be found here   https   web mit edu 6 005 www fa15 classes 09-immutability  useful immutable types

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An immutable object is the one you cannot modify after you create it  A typical example are string literals   A D programming language  which becomes increasingly popular  has a notion of  immutability  through  invariant  keyword  Check this Dr Dobb s article about it - http   dobbscodetalk com index php option com myblog amp show Invariant-Strings html amp Itemid 29   It explains the problem perfectly

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Immutable means that once the constructor for an object has completed execution that instance can t be altered   This is useful as it means you can pass references to the object around  without worrying that someone else is going to change its contents  Especially when dealing with concurrency  there are no locking issues with objects that never change  e g   class Foo        private final String myvar        public Foo final String initialValue                  this myvar   initialValue               public String getValue                   return this myvar             Foo doesn t have to worry that the caller to getValue   might change the text in the string   If you imagine a similar class to Foo  but with a StringBuilder rather than a String as a member  you can see that a caller to getValue   would be able to alter the StringBuilder attribute of a Foo instance   Also beware of the different kinds of immutability you might find  Eric Lippert wrote a blog article about this  Basically you can have objects whose interface is immutable but behind the scenes actual mutables private state  and therefore can t be shared safely between threads

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Once instanciated  cannot be altered  Consider a class that an instance of might be used as the key for a hashtable or similar  Check out Java best practices

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Once instanciated  cannot be altered  Consider a class that an instance of might be used as the key for a hashtable or similar  Check out Java best practices

User · Answer

One meaning has to do with how the value is stored in the computer   For a  Net string for example  it means that the string in memory cannot be changed    When you think you re changing it  you are in fact creating a new string in memory and pointing the existing variable  which is just a pointer to the actual collection of characters somewhere else  to the new string

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Immutable objects are objects that can t be changed programmatically  They re especially good for multi-threaded environments or other environments where more than one process is able to alter  mutate  the values in an object   Just to clarify  however  StringBuilder is actually a mutable object  not an immutable one  A regular java String is immutable  meaning that once it s been created you cannot change the underlying string without changing the object    For example  let s say that I have a class called ColoredString that has a String value and a String color   public class ColoredString        private String color      private String string       public ColoredString String color  String string            this color    color          this string   string             public String getColor      return this color         public String getString     return this string         public void setColor String newColor            this color   newColor             In this example  the ColoredString is said to be mutable because you can change  mutate  one of its key properties without creating a new ColoredString class  The reason why this may be bad is  for example  let s say you have a GUI application which has multiple threads and you are using ColoredStrings to print data to the window  If you have an instance of ColoredString which was created as  new ColoredString  Blue    This is a blue string       Then you would expect the string to always be  Blue   If another thread  however  got ahold of this instance and called  blueString setColor  Red      You would suddenly  and probably unexpectedly  now have a  Red  string when you wanted a  Blue  one  Because of this  immutable objects are almost always preferred when passing instances of objects around  When you have a case where mutable objects are really necessary  then you would typically guard the objet by only passing copies out from your specific field of control   To recap  in Java  java lang String is an immutable object  it cannot be changed once it s created  and java lang StringBuilder is a mutable object because it can be changed without creating a new instance

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I really like the explaination from SCJP Sun Certified Programmer for Java 5 Study Guide      To make Java more memory efficient  the JVM sets aside a special area of  memory called the  String constant pool   When the compiler encounters a String literal  it checks the pool to see if  an identical String already exists  If  a match is found  the reference to the new literal is directed to the existing String  and no new String literal object is created

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String s1  Hi   String s2 s1  s1  Bye    System out println s2     Hi   if String was mutable output would be  Bye  System out println s1     Bye   s1  Hi    an object s1 was created with  Hi  value in it   s2 s1     an object s2 is created with reference to s1 object   s1  Bye    the previous s1 object s value doesn t change because s1 has String type and String type is an immutable type  instead compiler create a new String object with  Bye  value and s1 referenced to it  here when we print s2 value  the result will be  Hi  not  Bye  because s2 referenced to previous s1 object which had  Hi  value

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java time  It might be a bit late but in order to understand what an immutable object is  consider the following example from the new Java 8 Date and Time API  java time   As you probably know all date objects from Java 8 are immutable so in the following example  LocalDate date   LocalDate of 2014  3  18    date plusYears 2   System out println date     Output      2014-03-18   This prints the same year as the initial date because the plusYears 2  returns a new object so the old date is still unchanged because it s an immutable object  Once created you cannot further modify it and the date variable still points to it   So  that code example should capture and use the new object instantiated and returned by that call to plusYears   LocalDate date   LocalDate of 2014  3  18    LocalDate dateAfterTwoYears   date plusYears 2        date toString      2014-03-18      dateAfterTwoYears toString      2016-03-18

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Immutable means that once the constructor for an object has completed execution that instance can t be altered   This is useful as it means you can pass references to the object around  without worrying that someone else is going to change its contents  Especially when dealing with concurrency  there are no locking issues with objects that never change  e g   class Foo        private final String myvar        public Foo final String initialValue                  this myvar   initialValue               public String getValue                   return this myvar             Foo doesn t have to worry that the caller to getValue   might change the text in the string   If you imagine a similar class to Foo  but with a StringBuilder rather than a String as a member  you can see that a caller to getValue   would be able to alter the StringBuilder attribute of a Foo instance   Also beware of the different kinds of immutability you might find  Eric Lippert wrote a blog article about this  Basically you can have objects whose interface is immutable but behind the scenes actual mutables private state  and therefore can t be shared safely between threads

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An immutable object is the one you cannot modify after you create it  A typical example are string literals   A D programming language  which becomes increasingly popular  has a notion of  immutability  through  invariant  keyword  Check this Dr Dobb s article about it - http   dobbscodetalk com index php option com myblog amp show Invariant-Strings html amp Itemid 29   It explains the problem perfectly

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java time  It might be a bit late but in order to understand what an immutable object is  consider the following example from the new Java 8 Date and Time API  java time   As you probably know all date objects from Java 8 are immutable so in the following example  LocalDate date   LocalDate of 2014  3  18    date plusYears 2   System out println date     Output      2014-03-18   This prints the same year as the initial date because the plusYears 2  returns a new object so the old date is still unchanged because it s an immutable object  Once created you cannot further modify it and the date variable still points to it   So  that code example should capture and use the new object instantiated and returned by that call to plusYears   LocalDate date   LocalDate of 2014  3  18    LocalDate dateAfterTwoYears   date plusYears 2        date toString      2014-03-18      dateAfterTwoYears toString      2016-03-18

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Immutable Objects  An object is considered immutable if its state cannot change after it is constructed  Maximum reliance on immutable objects is widely accepted as a sound strategy for creating simple  reliable code   Immutable objects are particularly useful in concurrent applications  Since they cannot change state  they cannot be corrupted by thread interference or observed in an inconsistent state   Programmers are often reluctant to employ immutable objects  because they worry about the cost of creating a new object as opposed to updating an object in place  The impact of object creation is often overestimated  and can be offset by some of the efficiencies associated with immutable objects  These include decreased overhead due to garbage collection  and the elimination of code needed to protect mutable objects from corruption   The following subsections take a class whose instances are mutable and derives a class with immutable instances from it  In so doing  they give general rules for this kind of conversion and demonstrate some of the advantages of immutable objects   Source

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Objects which are immutable can not have their state changed after they have been created   There are three main reasons to use immutable objects whenever you can  all of which will help to reduce the number of bugs you introduce in your code    It is much easier to reason about how your program works when you know that an object s state cannot be changed by another method Immutable objects are automatically thread safe  assuming they are published safely  so will never be the cause of those hard-to-pin-down multithreading bugs Immutable objects will always have the same Hash code  so they can be used as the keys in a HashMap  or similar   If the hash code of an element in a hash table was to change  the table entry would then effectively be lost  since attempts to find it in the table would end up looking in the wrong place  This is the main reason that String objects are immutable - they are frequently used as HashMap keys    There are also some other optimisations you might be able to make in code when you know that the state of an object is immutable - caching the calculated hash  for example - but these are optimisations and therefore not nearly so interesting

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Immutable simply mean unchangeable or unmodifiable  Once string object is created its data or state can t be changed  Consider bellow example   class Testimmutablestring      public static void main String args           String s  Future         s concat   World     concat   method appends the string at the end       System out println s    will print Future because strings are immutable objects                Let s get idea considering bellow diagram     In this diagram  you can see new object created as  Future World   But not change  Future  Because String is immutable  s  still refer to  Future   If you need to call  Future World    String s  Future     s s concat   World      System out println s    print Future World   Why are string objects immutable in java      Because Java uses the concept of string literal  Suppose there are 5 reference variables  all refers to one object  Future  If one reference variable changes the value of the object  it will be affected to all the reference variables  That is why string objects are immutable in java

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An immutable object is an object where the internal fields  or at least  all the internal fields that affect its external behavior  cannot be changed   There are a lot of advantages to immutable strings   Performance  Take the following operation   String substring   fullstring substring x y     The underlying C for the substring   method is probably something like this      Assume string is stored like this  struct String   char  characters  unsigned int length         Passing pointers because Java is pass-by-reference struct String  substring struct String  in  unsigned int begin  unsigned int end        struct String  out   malloc sizeof struct String        out- gt characters   in- gt characters   begin      out- gt length   end - begin      return out      Note that none of the characters have to be copied   If the String object were mutable  the characters could change later  then you would have to copy all the characters  otherwise changes to characters in the substring would be reflected in the other string later   Concurrency  If the internal structure of an immutable object is valid  it will always be valid   There s no chance that different threads can create an invalid state within that object  Hence  immutable objects are Thread Safe   Garbage collection  It s much easier for the garbage collector to make logical decisions about immutable objects   However  there are also downsides to immutability   Performance  Wait  I thought you said performance was an upside of immutability   Well  it is sometimes  but not always   Take the following code   foo   foo substring 0 4     a    foo substring 5       foo is a String bar replace 4 5  a       bar is a StringBuilder   The two lines both replace the fourth character with the letter  a    Not only is the second piece of code more readable  it s faster   Look at how you would have to do the underlying code for foo   The substrings are easy  but now because there s already a character at space five and something else might be referencing foo  you can t just change it  you have to copy the whole string  of course some of this functionality is abstracted into functions in the real underlying C  but the point here is to show the code that gets executed all in one place    struct String  concatenate struct String  first  struct String  second        struct String  new   malloc sizeof struct String        new- gt length   first- gt length   second- gt length       new- gt characters   malloc new- gt length        int i       for i   0  i  lt  first- gt length  i            new- gt characters i    first- gt characters i        for   i - first- gt length  lt  second- gt length  i            new- gt characters i    second- gt characters i - first- gt length        return new        The code that executes struct String  astring  char a    a   astring- gt characters    amp a  astring- gt length   1  foo   concatenate concatenate slice foo 0 4  astring  slice foo 5 foo- gt length      Note that concatenate gets called twice meaning that the entire string has to be looped through   Compare this to the C code for the bar operation   bar- gt characters 4     a     The mutable string operation is obviously much faster   In Conclusion  In most cases  you want an immutable string   But if you need to do a lot of appending and inserting into a string  you need the mutability for speed   If you want the concurrency safety and garbage collection benefits with it the key is to keep your mutable objects local to a method      This will have awful performance if you don t use mutable strings String join String   strings  String separator        StringBuilder mutable      boolean first   true       for int i   0  i  lt  strings length  i                  if  first  first   false          else mutable append separator            mutable append strings i               return mutable toString        Since the mutable object is a local reference  you don t have to worry about concurrency safety  only one thread ever touches it    And since it isn t referenced anywhere else  it is only allocated on the stack  so it is deallocated as soon as the function call is finished  you don t have to worry about garbage collection    And you get all the performance benefits of both mutability and immutability

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Objects which are immutable can not have their state changed after they have been created   There are three main reasons to use immutable objects whenever you can  all of which will help to reduce the number of bugs you introduce in your code    It is much easier to reason about how your program works when you know that an object s state cannot be changed by another method Immutable objects are automatically thread safe  assuming they are published safely  so will never be the cause of those hard-to-pin-down multithreading bugs Immutable objects will always have the same Hash code  so they can be used as the keys in a HashMap  or similar   If the hash code of an element in a hash table was to change  the table entry would then effectively be lost  since attempts to find it in the table would end up looking in the wrong place  This is the main reason that String objects are immutable - they are frequently used as HashMap keys    There are also some other optimisations you might be able to make in code when you know that the state of an object is immutable - caching the calculated hash  for example - but these are optimisations and therefore not nearly so interesting

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immutable  means you cannot change value  If you have an instance of String class  any method you call which seems to modify the value  will actually create another String   String foo    Hello   foo substring 3    lt -- foo here still has the same value  Hello    To preserve changes you should do something like this     foo   foo sustring 3    Immutable vs mutable can be funny when you work with collections  Think about what will happen if you use mutable object as a key for map and then change the value  tip  think about equals and hashCode

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Immutable objects are objects that can t be changed programmatically  They re especially good for multi-threaded environments or other environments where more than one process is able to alter  mutate  the values in an object   Just to clarify  however  StringBuilder is actually a mutable object  not an immutable one  A regular java String is immutable  meaning that once it s been created you cannot change the underlying string without changing the object    For example  let s say that I have a class called ColoredString that has a String value and a String color   public class ColoredString        private String color      private String string       public ColoredString String color  String string            this color    color          this string   string             public String getColor      return this color         public String getString     return this string         public void setColor String newColor            this color   newColor             In this example  the ColoredString is said to be mutable because you can change  mutate  one of its key properties without creating a new ColoredString class  The reason why this may be bad is  for example  let s say you have a GUI application which has multiple threads and you are using ColoredStrings to print data to the window  If you have an instance of ColoredString which was created as  new ColoredString  Blue    This is a blue string       Then you would expect the string to always be  Blue   If another thread  however  got ahold of this instance and called  blueString setColor  Red      You would suddenly  and probably unexpectedly  now have a  Red  string when you wanted a  Blue  one  Because of this  immutable objects are almost always preferred when passing instances of objects around  When you have a case where mutable objects are really necessary  then you would typically guard the objet by only passing copies out from your specific field of control   To recap  in Java  java lang String is an immutable object  it cannot be changed once it s created  and java lang StringBuilder is a mutable object because it can be changed without creating a new instance

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Objects which are immutable can not have their state changed after they have been created   There are three main reasons to use immutable objects whenever you can  all of which will help to reduce the number of bugs you introduce in your code    It is much easier to reason about how your program works when you know that an object s state cannot be changed by another method Immutable objects are automatically thread safe  assuming they are published safely  so will never be the cause of those hard-to-pin-down multithreading bugs Immutable objects will always have the same Hash code  so they can be used as the keys in a HashMap  or similar   If the hash code of an element in a hash table was to change  the table entry would then effectively be lost  since attempts to find it in the table would end up looking in the wrong place  This is the main reason that String objects are immutable - they are frequently used as HashMap keys    There are also some other optimisations you might be able to make in code when you know that the state of an object is immutable - caching the calculated hash  for example - but these are optimisations and therefore not nearly so interesting

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immutable  means you cannot change value  If you have an instance of String class  any method you call which seems to modify the value  will actually create another String   String foo    Hello   foo substring 3    lt -- foo here still has the same value  Hello    To preserve changes you should do something like this     foo   foo sustring 3    Immutable vs mutable can be funny when you work with collections  Think about what will happen if you use mutable object as a key for map and then change the value  tip  think about equals and hashCode

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Immutable means that once the constructor for an object has completed execution that instance can t be altered   This is useful as it means you can pass references to the object around  without worrying that someone else is going to change its contents  Especially when dealing with concurrency  there are no locking issues with objects that never change  e g   class Foo        private final String myvar        public Foo final String initialValue                  this myvar   initialValue               public String getValue                   return this myvar             Foo doesn t have to worry that the caller to getValue   might change the text in the string   If you imagine a similar class to Foo  but with a StringBuilder rather than a String as a member  you can see that a caller to getValue   would be able to alter the StringBuilder attribute of a Foo instance   Also beware of the different kinds of immutability you might find  Eric Lippert wrote a blog article about this  Basically you can have objects whose interface is immutable but behind the scenes actual mutables private state  and therefore can t be shared safely between threads

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One meaning has to do with how the value is stored in the computer   For a  Net string for example  it means that the string in memory cannot be changed    When you think you re changing it  you are in fact creating a new string in memory and pointing the existing variable  which is just a pointer to the actual collection of characters somewhere else  to the new string

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Once instanciated  cannot be altered  Consider a class that an instance of might be used as the key for a hashtable or similar  Check out Java best practices

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Once instanciated  cannot be altered  Consider a class that an instance of might be used as the key for a hashtable or similar  Check out Java best practices

User · Answer

One meaning has to do with how the value is stored in the computer   For a  Net string for example  it means that the string in memory cannot be changed    When you think you re changing it  you are in fact creating a new string in memory and pointing the existing variable  which is just a pointer to the actual collection of characters somewhere else  to the new string

User · Answer

Objects which are immutable can not have their state changed after they have been created   There are three main reasons to use immutable objects whenever you can  all of which will help to reduce the number of bugs you introduce in your code    It is much easier to reason about how your program works when you know that an object s state cannot be changed by another method Immutable objects are automatically thread safe  assuming they are published safely  so will never be the cause of those hard-to-pin-down multithreading bugs Immutable objects will always have the same Hash code  so they can be used as the keys in a HashMap  or similar   If the hash code of an element in a hash table was to change  the table entry would then effectively be lost  since attempts to find it in the table would end up looking in the wrong place  This is the main reason that String objects are immutable - they are frequently used as HashMap keys    There are also some other optimisations you might be able to make in code when you know that the state of an object is immutable - caching the calculated hash  for example - but these are optimisations and therefore not nearly so interesting

User · Answer

An immutable object is the one you cannot modify after you create it  A typical example are string literals   A D programming language  which becomes increasingly popular  has a notion of  immutability  through  invariant  keyword  Check this Dr Dobb s article about it - http   dobbscodetalk com index php option com myblog amp show Invariant-Strings html amp Itemid 29   It explains the problem perfectly

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An immutable object is the one you cannot modify after you create it  A typical example are string literals   A D programming language  which becomes increasingly popular  has a notion of  immutability  through  invariant  keyword  Check this Dr Dobb s article about it - http   dobbscodetalk com index php option com myblog amp show Invariant-Strings html amp Itemid 29   It explains the problem perfectly

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One meaning has to do with how the value is stored in the computer   For a  Net string for example  it means that the string in memory cannot be changed    When you think you re changing it  you are in fact creating a new string in memory and pointing the existing variable  which is just a pointer to the actual collection of characters somewhere else  to the new string

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An immutable object is an object where the internal fields  or at least  all the internal fields that affect its external behavior  cannot be changed   There are a lot of advantages to immutable strings   Performance  Take the following operation   String substring   fullstring substring x y     The underlying C for the substring   method is probably something like this      Assume string is stored like this  struct String   char  characters  unsigned int length         Passing pointers because Java is pass-by-reference struct String  substring struct String  in  unsigned int begin  unsigned int end        struct String  out   malloc sizeof struct String        out- gt characters   in- gt characters   begin      out- gt length   end - begin      return out      Note that none of the characters have to be copied   If the String object were mutable  the characters could change later  then you would have to copy all the characters  otherwise changes to characters in the substring would be reflected in the other string later   Concurrency  If the internal structure of an immutable object is valid  it will always be valid   There s no chance that different threads can create an invalid state within that object  Hence  immutable objects are Thread Safe   Garbage collection  It s much easier for the garbage collector to make logical decisions about immutable objects   However  there are also downsides to immutability   Performance  Wait  I thought you said performance was an upside of immutability   Well  it is sometimes  but not always   Take the following code   foo   foo substring 0 4     a    foo substring 5       foo is a String bar replace 4 5  a       bar is a StringBuilder   The two lines both replace the fourth character with the letter  a    Not only is the second piece of code more readable  it s faster   Look at how you would have to do the underlying code for foo   The substrings are easy  but now because there s already a character at space five and something else might be referencing foo  you can t just change it  you have to copy the whole string  of course some of this functionality is abstracted into functions in the real underlying C  but the point here is to show the code that gets executed all in one place    struct String  concatenate struct String  first  struct String  second        struct String  new   malloc sizeof struct String        new- gt length   first- gt length   second- gt length       new- gt characters   malloc new- gt length        int i       for i   0  i  lt  first- gt length  i            new- gt characters i    first- gt characters i        for   i - first- gt length  lt  second- gt length  i            new- gt characters i    second- gt characters i - first- gt length        return new        The code that executes struct String  astring  char a    a   astring- gt characters    amp a  astring- gt length   1  foo   concatenate concatenate slice foo 0 4  astring  slice foo 5 foo- gt length      Note that concatenate gets called twice meaning that the entire string has to be looped through   Compare this to the C code for the bar operation   bar- gt characters 4     a     The mutable string operation is obviously much faster   In Conclusion  In most cases  you want an immutable string   But if you need to do a lot of appending and inserting into a string  you need the mutability for speed   If you want the concurrency safety and garbage collection benefits with it the key is to keep your mutable objects local to a method      This will have awful performance if you don t use mutable strings String join String   strings  String separator        StringBuilder mutable      boolean first   true       for int i   0  i  lt  strings length  i                  if  first  first   false          else mutable append separator            mutable append strings i               return mutable toString        Since the mutable object is a local reference  you don t have to worry about concurrency safety  only one thread ever touches it    And since it isn t referenced anywhere else  it is only allocated on the stack  so it is deallocated as soon as the function call is finished  you don t have to worry about garbage collection    And you get all the performance benefits of both mutability and immutability

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String s1  Hi   String s2 s1  s1  Bye    System out println s2     Hi   if String was mutable output would be  Bye  System out println s1     Bye   s1  Hi    an object s1 was created with  Hi  value in it   s2 s1     an object s2 is created with reference to s1 object   s1  Bye    the previous s1 object s value doesn t change because s1 has String type and String type is an immutable type  instead compiler create a new String object with  Bye  value and s1 referenced to it  here when we print s2 value  the result will be  Hi  not  Bye  because s2 referenced to previous s1 object which had  Hi  value

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Immutable Objects  An object is considered immutable if its state cannot change after it is constructed  Maximum reliance on immutable objects is widely accepted as a sound strategy for creating simple  reliable code   Immutable objects are particularly useful in concurrent applications  Since they cannot change state  they cannot be corrupted by thread interference or observed in an inconsistent state   Programmers are often reluctant to employ immutable objects  because they worry about the cost of creating a new object as opposed to updating an object in place  The impact of object creation is often overestimated  and can be offset by some of the efficiencies associated with immutable objects  These include decreased overhead due to garbage collection  and the elimination of code needed to protect mutable objects from corruption   The following subsections take a class whose instances are mutable and derives a class with immutable instances from it  In so doing  they give general rules for this kind of conversion and demonstrate some of the advantages of immutable objects   Source

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In large applications its common for string literals to occupy large bits of memory  So to efficiently handle the memory  the JVM allocates an area called  String constant pool   Note that in memory even an unreferenced String carries around a char    an int for its length  and another for its hashCode  For a number  by contrast  a maximum of eight immediate bytes is required  When complier comes across a String literal it checks the pool to see if there is an identical literal already present  And if one is found  the reference to the new literal is directed to the existing String  and no new  String literal object  is created the existing String simply gets an additional reference    Hence   String mutability saves memory    But when any of the variables change value  Actually - it s only their reference that s changed  not the value in memory hence it will not affect the other variables referencing it  as seen below         String s1    Old string       s1 variable  refers to string in memory         reference                       MEMORY                 variables                                                   s1    --------------- gt      Old String        String s2   s1      s2 refers to same string as s1                                                                    s1    --------------- gt      Old String                  s2    ------------------------    s1    New String      s1 deletes reference to old string and points to the newly created one             s1    ----- --------- gt      New String                                                                                              X     Old String                  s2    ------------------------       The original string  in memory  didn t change  but the   reference variable was changed so that it refers to the new string    And if we didn t have s2   Old String  would still be in the memory but   we ll not be able to access it

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Immutable means that once the object is created  non of its members will change  String is immutable since you can not change its content  For example   String s1      abc     String s2   s1 trim      In the code above  the string s1 did not change  another object  s2  was created using s1

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Actually String is not immutable if you use the wikipedia definition suggested above.

String's state does change post construction. Take a look at the hashcode() method. String caches the hashcode value in a local field but does not calculate it until the first call of hashcode(). This lazy evaluation of hashcode places String in an interesting position as an immutable object whose state changes, but it cannot be observed to have changed without using reflection.

So maybe the definition of immutable should be an object that cannot be observed to have changed.

If the state changes in an immutable object after it has been created but no-one can see it (without reflection) is the object still immutable?

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As the accepted answer doesn t answer all the questions  I m forced to give an answer after 11 years and 6 months   Can somebody clarify what is meant by immutable   Hope you meant immutable object  because we could think about immutable reference   An object is immutable  iff once created  they always represent the same value  doesn t have any method that change the value    Why is a String immutable   Respect the above definition which could be checked by looking into the Sting java source code   What are the advantages disadvantages of the immutable objects  immutable types are     safer from bugs   easier to understand   and more ready for change     Why should a mutable object such as StringBuilder be preferred over String and vice-verse   Narrowing the question Why do we need the mutable StringBuilder in programming  A common use for it is to concatenate a large number of strings together  like this  String s    quot  quot   for  int i   0  i  lt  n    i        s   s   n     Using immutable strings  this makes a lot of temporary copies     the first number of the string   quot 0 quot   is actually copied n times in the course of building up the final string  the second number is copied n-1 times  and so on  It actually costs O n2  time just to do all that copying  even though we only concatenated n elements  StringBuilder is designed to minimize this copying  It uses a simple but clever internal data structure to avoid doing any copying at all until the very end  when you ask for the final String with a toString   call  StringBuilder sb   new StringBuilder    for  int i   0  i  lt  n    i      sb append String valueOf n      String s   sb toString     Getting good performance is one reason why we use mutable objects  Another is convenient sharing  two parts of your program can communicate more conveniently by sharing a common mutable data structure  More could be found here   https   web mit edu 6 005 www fa15 classes 09-immutability  useful immutable types

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Immutable means that once the object is created  non of its members will change  String is immutable since you can not change its content  For example   String s1      abc     String s2   s1 trim      In the code above  the string s1 did not change  another object  s2  was created using s1

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Actually String is not immutable if you use the wikipedia definition suggested above.

String's state does change post construction. Take a look at the hashcode() method. String caches the hashcode value in a local field but does not calculate it until the first call of hashcode(). This lazy evaluation of hashcode places String in an interesting position as an immutable object whose state changes, but it cannot be observed to have changed without using reflection.

So maybe the definition of immutable should be an object that cannot be observed to have changed.

If the state changes in an immutable object after it has been created but no-one can see it (without reflection) is the object still immutable?

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I really like the explaination from SCJP Sun Certified Programmer for Java 5 Study Guide      To make Java more memory efficient  the JVM sets aside a special area of  memory called the  String constant pool   When the compiler encounters a String literal  it checks the pool to see if  an identical String already exists  If  a match is found  the reference to the new literal is directed to the existing String  and no new String literal object is created

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Immutable means that once the constructor for an object has completed execution that instance can t be altered   This is useful as it means you can pass references to the object around  without worrying that someone else is going to change its contents  Especially when dealing with concurrency  there are no locking issues with objects that never change  e g   class Foo        private final String myvar        public Foo final String initialValue                  this myvar   initialValue               public String getValue                   return this myvar             Foo doesn t have to worry that the caller to getValue   might change the text in the string   If you imagine a similar class to Foo  but with a StringBuilder rather than a String as a member  you can see that a caller to getValue   would be able to alter the StringBuilder attribute of a Foo instance   Also beware of the different kinds of immutability you might find  Eric Lippert wrote a blog article about this  Basically you can have objects whose interface is immutable but behind the scenes actual mutables private state  and therefore can t be shared safely between threads

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Immutable objects are objects that can t be changed programmatically  They re especially good for multi-threaded environments or other environments where more than one process is able to alter  mutate  the values in an object   Just to clarify  however  StringBuilder is actually a mutable object  not an immutable one  A regular java String is immutable  meaning that once it s been created you cannot change the underlying string without changing the object    For example  let s say that I have a class called ColoredString that has a String value and a String color   public class ColoredString        private String color      private String string       public ColoredString String color  String string            this color    color          this string   string             public String getColor      return this color         public String getString     return this string         public void setColor String newColor            this color   newColor             In this example  the ColoredString is said to be mutable because you can change  mutate  one of its key properties without creating a new ColoredString class  The reason why this may be bad is  for example  let s say you have a GUI application which has multiple threads and you are using ColoredStrings to print data to the window  If you have an instance of ColoredString which was created as  new ColoredString  Blue    This is a blue string       Then you would expect the string to always be  Blue   If another thread  however  got ahold of this instance and called  blueString setColor  Red      You would suddenly  and probably unexpectedly  now have a  Red  string when you wanted a  Blue  one  Because of this  immutable objects are almost always preferred when passing instances of objects around  When you have a case where mutable objects are really necessary  then you would typically guard the objet by only passing copies out from your specific field of control   To recap  in Java  java lang String is an immutable object  it cannot be changed once it s created  and java lang StringBuilder is a mutable object because it can be changed without creating a new instance

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I really like the explaination from SCJP Sun Certified Programmer for Java 5 Study Guide      To make Java more memory efficient  the JVM sets aside a special area of  memory called the  String constant pool   When the compiler encounters a String literal  it checks the pool to see if  an identical String already exists  If  a match is found  the reference to the new literal is directed to the existing String  and no new String literal object is created

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immutable  means you cannot change value  If you have an instance of String class  any method you call which seems to modify the value  will actually create another String   String foo    Hello   foo substring 3    lt -- foo here still has the same value  Hello    To preserve changes you should do something like this     foo   foo sustring 3    Immutable vs mutable can be funny when you work with collections  Think about what will happen if you use mutable object as a key for map and then change the value  tip  think about equals and hashCode

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In large applications its common for string literals to occupy large bits of memory  So to efficiently handle the memory  the JVM allocates an area called  String constant pool   Note that in memory even an unreferenced String carries around a char    an int for its length  and another for its hashCode  For a number  by contrast  a maximum of eight immediate bytes is required  When complier comes across a String literal it checks the pool to see if there is an identical literal already present  And if one is found  the reference to the new literal is directed to the existing String  and no new  String literal object  is created the existing String simply gets an additional reference    Hence   String mutability saves memory    But when any of the variables change value  Actually - it s only their reference that s changed  not the value in memory hence it will not affect the other variables referencing it  as seen below         String s1    Old string       s1 variable  refers to string in memory         reference                       MEMORY                 variables                                                   s1    --------------- gt      Old String        String s2   s1      s2 refers to same string as s1                                                                    s1    --------------- gt      Old String                  s2    ------------------------    s1    New String      s1 deletes reference to old string and points to the newly created one             s1    ----- --------- gt      New String                                                                                              X     Old String                  s2    ------------------------       The original string  in memory  didn t change  but the   reference variable was changed so that it refers to the new string    And if we didn t have s2   Old String  would still be in the memory but   we ll not be able to access it

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immutable  means you cannot change value  If you have an instance of String class  any method you call which seems to modify the value  will actually create another String   String foo    Hello   foo substring 3    lt -- foo here still has the same value  Hello    To preserve changes you should do something like this     foo   foo sustring 3    Immutable vs mutable can be funny when you work with collections  Think about what will happen if you use mutable object as a key for map and then change the value  tip  think about equals and hashCode

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Immutable means that once the object is created  non of its members will change  String is immutable since you can not change its content  For example   String s1      abc     String s2   s1 trim      In the code above  the string s1 did not change  another object  s2  was created using s1

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Immutable simply mean unchangeable or unmodifiable  Once string object is created its data or state can t be changed  Consider bellow example   class Testimmutablestring      public static void main String args           String s  Future         s concat   World     concat   method appends the string at the end       System out println s    will print Future because strings are immutable objects                Let s get idea considering bellow diagram     In this diagram  you can see new object created as  Future World   But not change  Future  Because String is immutable  s  still refer to  Future   If you need to call  Future World    String s  Future     s s concat   World      System out println s    print Future World   Why are string objects immutable in java      Because Java uses the concept of string literal  Suppose there are 5 reference variables  all refers to one object  Future  If one reference variable changes the value of the object  it will be affected to all the reference variables  That is why string objects are immutable in java

[java] What is meant by immutable?

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