String is immutable What exactly is the meaning

Question

I wrote the following code on immutable Strings   public class ImmutableStrings        public static void main String   args            testmethod               private static void testmethod             String a    a           System out println  a 1-- gt     a           a    ty           System out println  a 2-- gt     a             Output   a 1-- gt a   a 2-- gt ty   Here the value of variable a has been changed  while many say that contents of the immutable objects cannot be changed   But what exactly does one mean by saying String is immutable  Could you please clarify this topic for me   source   https   docs oracle com javase tutorial java nutsandbolts datatypes html

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Only the reference is changing  First a was referencing to the string  a   and later you changed it to  ty   The string  a  remains the same

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String is immutable means that you cannot change the object itself  but you can change the reference to the object    When you execute a    ty   you are actually changing the reference of a to a new object created by the String literal  ty    Changing an object means to use its methods to change one of its fields  or the fields are public and not final  so that they can be updated from outside without accessing them via methods   for example   Foo x   new Foo  the field    x setField  a new field    System out println x getField        prints  a new field    While in an immutable class  declared as final  to prevent modification via inheritance  its methods cannot modify its fields  and also the fields are always private and recommended to be final   for example String  you cannot change the current String but you can return a new String  i e   String s    some text   s substring 0 4   System out println s      still printing  some text  String a   s substring 0 4   System out println a      prints  some

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Java String is immutable  String will Store the value in the form of object  so if u assign the value String a  a   it will create an object and the value is stored in that and again if you are assigning value a  ty  means it will create an another object store the value in that  if you want to understand clearly  check the has code for the String

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In your example  a refers first to  a   and then to  ty   You re not mutating any String instance  you re just changing which String instance a refers to  For example  this   String a    a   String b   a     b refers to the same String as a a    b      a now refers to a different instance System out println b     prints  a   because we never mutate the String instance that b points to

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Before proceeding further with the fuss of immutability  let s just take a look into the String class and its functionality a little before coming to any conclusion   This is how String works   String str    knowledge     This  as usual  creates a string containing  knowledge  and assigns it a reference str  Simple enough  Lets perform some more functions    String s   str         assigns a new reference to the same string  knowledge    Lets see how the below statement works     str   str concat   base      This appends a string   base  to str  But wait  how is this possible  since String objects are immutable  Well to your surprise  it is   When the above statement is executed  the VM takes the value of String str  i e   knowledge  and appends   base   giving us the value  knowledge base   Now  since Strings are immutable  the VM can t assign this value to str  so it creates a new String object  gives it a value  knowledge base   and gives it a reference str   An important point to note here is that  while the String object is immutable  its reference variable is not  So that s why  in the above example  the reference was made to refer to a newly formed String object   At this point in the example above  we have two String objects  the first one we created with value  knowledge   pointed to by s  and the second one  knowledge base   pointed to by str  But  technically  we have three String objects  the third one being the literal  base  in the concat statement   Important Facts about String and Memory usage  What if we didn t have another reference s to  knowledge   We would have lost that String  However  it still would have existed  but would be considered lost due to having no references  Look at one more example below  String s1    java   s1 concat   rules    System out println  s1 refers to   s1       Yes  s1 still refers to  java    What s happening     The first line is pretty straightforward  create a new String  java  and refer s1 to it  Next  the VM creates another new String  java rules   but nothing refers to it  So  the second String is instantly lost  We can t reach it    The reference variable s1 still refers to the original String  java    Almost every method  applied to a String object in order to modify it  creates new String object  So  where do these String objects go  Well  these exist in memory  and one of the key goals of any programming language is to make efficient use of memory   As applications grow  it s very common for String literals to occupy large area of memory  which can even cause redundancy  So  in order to make Java more efficient  the JVM sets aside a special area of memory called the  String constant pool    When the compiler sees a String literal  it looks for the String in the pool  If a match is found  the reference to the new literal is directed to the existing String and no new String object is created  The existing String simply has one more reference  Here comes the point of making String objects immutable   In the String constant pool  a String object is likely to have one or many references  If several references point to same String without even knowing it  it would be bad if one of the references modified that String value  That s why String objects are immutable   Well  now you could say  what if someone overrides the functionality of String class  That s the reason that the String class is marked final so that nobody can override the behavior of its methods

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String S1  abc   S1 concat  xyz    System out println  S1 is     S1   String S2 S1 concat  def    System out println  S2 is     S2     This shows that once a string object is create it cannot be changed  EveryTime you need to create new and put in another String  S

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If some object bar holds a reference to a mutable object foo and encapsulates some of its state in mutable aspects of foo s state  that will allow code which can change those aspects of foo to change the corresponding aspects of bar s state without actually touching bar or even knowing of its existence   Generally  this means that objects which encapsulate their own state using mutable objects must ensure that no references to those objects are exposed to any code which might unexpectedly mutate them   By contrast  if bar holds a reference to an object moo and only uses immutable aspects of moo other than identity to encapsulate its state  then bar can freely expose moo to outside code without worrying about anything the outside code might do to it

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Hope the below code would clarify your doubts    public static void testString         String str    Hello       System out println  Before String Concat    str       str concat  World        System out println  After String Concat    str       StringBuffer sb   new StringBuffer  Hello        System out println  Before StringBuffer Append    sb       sb append  World        System out println  After StringBuffer Append    sb       Before String Concat  Hello After  String Concat  Hello Before StringBuffer Append  Hello After StringBuffer Append  HelloWorld

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String is immutable it means that the content of the String Object can t be change  once it is created  If you want to modify the content then you can go for StringBuffer StringBuilder instead of String  StringBuffer and StringBuilder are mutable classes

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You are actually getting a reference to a new string  the string itself is not being changed as it is immutable   This is relevant   See  Immutable objects on Wikipedia

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You re changing what a refers to  Try this   String a  a   System out println  a 1-- gt   a   String b a  a  ty   System out println  a 2-- gt   a   System out println  b  -- gt   b     You will see that the object to which a and then b refers has not changed   If you want to prevent your code from changing which object a refers to  try   final String a  a

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see here   class ImmutableStrings        public static void main String   args            testmethod               private static void testmethod         String a  a       System out println  a 1-- gt   a       System out println  a 1 address-- gt   a hashCode          a    ty       System out println  a 2-- gt   a           System out println  a 2 address-- gt   a hashCode               output   a 1-- gt a a 1 address-- gt 97 a 2-- gt ty a 2 address-- gt 3717   This indicates that whenever you are modifying the content of immutable string object a a new object will be created  i e you are not allowed to change the content of immutable object  that s why the address are different for both the object

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I think the following code clears the difference   String A   new String  Venugopal    String B   A   A   A   mitul    System out println  A is     A   System out println  B is     B    StringBuffer SA   new StringBuffer  Venugopal    StringBuffer SB   SA   SA   SA append  mitul     System out println  SA is     SA   System out println  SB is     SB

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A string is a char   containing a series of UTF-16 code units  an int offset into that array  and an int length   For example   String s   It creates space for a string reference   Assigning copies references around but does not modify the objects to which those references refer   You should also be aware that  new String s    doesn t really do anything useful   It merely creates another instance backed by the same array  offset  and length as s   There is very rarely a reason to do this so it is considered bad practice by most Java programmers   Java double quoted strings like  my string  are really references to interned String instances so  bar  is a reference to the same String instance regardless of how many times it appears in your code     The  hello  creates one instance that is pooled  and the new String      creates a non-pooled instance  Try System out println   hello      hello            new String  hello       hello            new String  hello      new String  hello      and you should see true false false

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immutable means you can t not change the value of the same referance every time you required to create new referance means new memory location  ex   String str  abc   str  bcd     here  in the above code  in the memory there are 2 blocks for storing the value the first for value  abc  and second for  bcd  the second value is not replace to first value   this is call the immutable

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An immutable object is an object whose state cannot be modified after it is created    So a    ABC   lt -- immutable object   a  holds reference to the object  And  a    DEF   lt -- another immutable object   a  holds reference to it now   Once you assign a string object  that object can not be changed in memory   In summary  what you did is to change the reference of  a  to a new string object

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You are not changing the object in the assignment statement  you replace one immutable object with another one  Object String  a   does not change to String  ty    it gets discarded  and a reference to ty gets written into a in its stead   In contrast  StringBuffer represents a mutable object  You can do this   StringBuffer b   new StringBuffer  Hello    System out writeln b   b append    world     System out writeln b     Here  you did not re-assign b  it still points to the same object  but the content of that object has changed

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In your example  the variable a is just a reference to an instance of a string object   When you say a    ty   you are not actually changing the string object  but rather pointing the reference at an entirely different instance of the string class

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Probably every answer provided above is right  but my answer is specific to use of hashCode   method  to prove the points like  String    once created can t be modified and modifications will results in new value at different memory location   public class ImmutabilityTest        private String changingRef    TEST STRING        public static void main String a               ImmutabilityTest dn   new ImmutabilityTest             System out println  ChangingRef for TEST STRING OLD                       dn changingRef hashCode              dn changingRef    NEW TEST STRING           System out println  ChangingRef for NEW TEST STRING                       dn changingRef hashCode              dn changingRef    TEST STRING           System out println  ChangingRef for TEST STRING BACK                       dn changingRef hashCode              dn changingRef    NEW TEST STRING           System out println  ChangingRef for NEW TEST STRING BACK                       dn changingRef hashCode              String str   new String  STRING1            System out println  String Class STRING1       str hashCode              str   new String  STRING2            System out println  String Class STRING2       str hashCode              str   new String  STRING1            System out println  String Class STRING1 BACK       str hashCode              str   new String  STRING2            System out println  String Class STRING2 BACK       str hashCode                OUTPUT  ChangingRef for TEST STRING OLD   247540830 ChangingRef for NEW TEST STRING   970356767 ChangingRef for TEST STRING BACK   247540830 ChangingRef for NEW TEST STRING BACK   970356767 String Class STRING1   -1163776448 String Class STRING2   -1163776447 String Class STRING1 BACK   -1163776448 String Class STRING2 BACK   -1163776447

[java] String is immutable. What exactly is the meaning?

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