Thread: 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

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

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.