Difference between string object and string literal

Question

What is the difference between   String str   new String  abc      and   String str    abc

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In addition to the answers already posted  also see this excellent article on javaranch

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According to String class documentation they are equivalent   Documentation for String String original  also says that  Unless an explicit copy of original is needed  use of this constructor is unnecessary since Strings are immutable   Look for other responses  because it seems that Java documentation is misleading

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In the first case  there are two objects created   In the second case  it s just one   Although both ways str is referring to  abc

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The following are some comparisons   String s1    Hello   String s2    Hello   String s3   new String  Hello     System out println s1    s2     true System out println s1 equals s2      true  System out println s1    s3       false System out println s1 equals s3      true  s3   s3 intern    System out println s1    s3     true System out println s1 equals s3      true   When intern   is called the reference is changed

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String s   new String  FFFF   creates 2 objects   FFFF  string and String object  which point to  FFFF  string  so it is like pointer to pointer  reference to reference  I am not keen with terminology    It is said you should never use new String  FFFF

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When you use a string literal the string can be interned  but when you use new String        you get a new string object   In this example both string literals refer the same object   String a    abc    String b    abc   System out println a    b       true   Here  2 different objects are created and they have different references   String c   new String  abc    String d   new String  abc    System out println c    d       false   In general  you should use the string literal notation when possible  It is easier to read and it gives the compiler a chance to optimize your code

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A String literal is a Java language concept  This is a String literal    a String literal    A String object is an individual instance of the java lang String class    String s1    abcde   String s2   new String  abcde    String s3    abcde     All are valid  but have a slight difference  s1 will refer to an interned String object  This means  that the character sequence  abcde  will be stored at a central place  and whenever the same literal  abcde  is used again  the JVM will not create a new String object but use the reference of the cached String   s2 is guranteed to be a new String object  so in this case we have   s1    s2    is false s1    s3    is true s1 equals s2     is true

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String is a class in Java different from other programming languages  So as for every class the object declaration and initialization is   String st1   new String      or  String st2   new String  Hello     String st3   new String  Hello      Here  st1  st2 and st3 are different objects   That is   st1    st2    false st1    st3    false st2    st3    false   Because st1  st2  st3 are referencing 3 different objects  and    checks for the equality in memory location  hence the result    But   st1 equals st2     false st2 equals st3     true   Here  equals   method checks for the content  and the content of st1       st2    hello  and st3    hello   Hence the result   And in the case of the String declaration   String st    hello     Here  intern   method of String class is called  and checks if  hello  is in intern pool  and if not  it is added to intern pool  and if  hello  exist in intern pool  then st will point to the memory of the existing  hello     So in case of   String st3    hello   String st4    hello      Here   st3    st4    true   Because st3 and st4 pointing to same memory address    Also   st3 equals st4       true as usual

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abc  is a literal String    In Java  these literal strings are pooled internally and the same String instance of  abc  is used where ever you have that string literal declared in your code   So  abc      abc  will always be true as they are both the same String instance   Using the String intern   method you can add any string you like to the internally pooled strings  these will be kept in memory until java exits   On the other hand  using new String  abc   will create a new string object in memory  which is logically the same as the  abc  literal    abc     new String  abc   will always be false  as although they are logically equal they refer to different instances   Wrapping a String constructor around a string literal is of no value  it just needlessly uses more memory than it needs to

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Some disassembly is always interesting       cat Test java  public class Test       public static void main String    args            String abc    abc           String def   new String  def               javap -c -v Test Compiled from  Test java  public class Test extends java lang Object   SourceFile   Test java    minor version  0   major version  50   Constant pool  const  1   Method   7  16       java lang Object   lt init gt     V const  2   String   17          abc const  3   class    18          java lang String const  4   String   19          def const  5   Method   3  20       java lang String   lt init gt    Ljava lang String  V const  6   class    21          Test const  7   class    22          java lang Object const  8   Asciz    lt init gt          public Test             public static void main java lang String       Code     Stack 3  Locals 3  Args size 1     0     ldc  2               Load string constant  abc      2     astore 1             Store top of stack onto local variable 1     3     new  3               class java lang String     6     dup                  duplicate top of stack     7     ldc  4               Load string constant  def      9     invokespecial  5     Invoke constructor    12     astore 2             Store top of stack onto local variable 2    13     return

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There is a subtle differences between String object and string literal   String s    abc      creates one String object and one reference variable   In this simple case   abc  will go in the pool and s will refer to it   String s   new String  abc       creates two objects and one reference variable   In this case  because we used the new keyword  Java will create a new String object in normal  non-pool  memory  and s will refer to it  In addition  the literal  abc  will be placed in the pool

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The long answer is available here  so I ll give you the short one    When you do this   String str    abc     You are calling the intern   method on String  This method references an internal pool of String objects  If the String you called intern   on already resides in the pool  then a reference to that String is assigned to str  If not  then the new String is placed in the pool  and a reference to it is then assigned to str   Given the following code   String str    abc   String str2    abc   boolean identity   str    str2    When you check for object identity by doing     you are literally asking  do these two references point to the same object    you get true    However  you don t need to intern   Strings  You can force the creation on a new Object on the Heap by doing this   String str   new String  abc    String str2   new String  abc    boolean identity   str    str2    In this instance  str and str2 are references to different Objects  neither of which have been interned  so that when you test for Object identity using     you will get false   In terms of good coding practice  do not use    to check for String equality  use  equals   instead

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As Strings are immutable  when you do   String a    xyz    while creating the string  the JVM searches in the pool of strings if there already exists a string value  xyz   if so  a  will simply be a reference of that string and no new String object is created   But if you say   String a   new String  xyz     you force JVM to create a new String reference  even if  xyz  is in its pool   For more information read this

[java] Difference between string object and string literal

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