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Thread: mutable and immutable concept of java

  1. #1
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    Default mutable and immutable concept of java

    String str=new String("JAVA");
     System.out.println("value of str before "+str); //JAVA
     String str2=null;
     str=str2;
     System.out.println("value of str"+str);  //null
     System.out.println("value of str2"+str2);//null
     
     str="Prog";
     System.out.println("value of str"+str); //prog 
     System.out.println("value of str2"+str2);//null
    Ques 1 If string is immutable why is value of str changing??

    Student stu= new Student(123);
    System.out.println("value of stu before "+stu); //some address is printed
    Student stu2=null;
    stu=stu2;
    System.out.println("value of stu"+stu); //null
    System.out.println("value of stu2"+stu2);//null
    Student stu3=new Student(456);
    stu=stu3;
    System.out.println("value of stu"+stu); //some new address
    System.out.println("value of stu2"+stu2);//null
    Ques 2.String and Object are behaving in same way .Then why String is immutable and Object mutable. Where's the difference


  2. #2
    Super Moderator jps's Avatar
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    Default Re: mutable and immutable concept of java

    The old string goes bye-bye, a new string is created and assigned to the variable

  3. #3
    Super Moderator helloworld922's Avatar
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    Default Re: mutable and immutable concept of java

    Variables "refer" to an immutable String object, the variable themselves are mutable, and are free to point to whatever string object you want.

    Similarly, you can have immutable variables which point to mutable (or immutable) objects by declaring the variable final.

    final String str = "hello world";
    str = "blah"; // compiler error: trying to change the actual variable
    final ArrayList<Integer> list = new ArrayList<Integer>();
    list.add(5); // perfectly fine, the variable list can't be modified, but the object refered to by the variable list can be modified.
    list = some_other_list; // compiler error again

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    Default Re: mutable and immutable concept of java

    Hello.
    Every object you create in Java is always of type java.lang.Object.
    So a String object is ultimately an object of java.lang.Object.
    There is no question of String different than Object.

    Mutable means the properties of an object can be modified anytime after its creation (Ex: collections such as ArrayList, HashMap, etc).
    Immutable means once an object is created its properties will never ever be modified again (Ex: String).

    Syed.

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