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.

1. Can somebody clarify what is meant by immutable? <

Answer 1

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.