What is difference between mutable and immutable String in java

Question

As per my knowledge,

a mutable string can be changed, and an immutable string cannot be changed.

Here I want to change the value of String like this,

String str="Good";
str=str+" Morning";

and other way is,

StringBuffer str= new StringBuffer("Good");
str.append(" Morning");

In both the cases I am trying to alter the value of str. Can anyone tell me, what is difference in both case and give me clear picture of mutable and immutable objects.

Answer 1

Case 1:

String str = "Good";
str = str + " Morning";

In the above code you create 3 String Objects.

1. "Good" it goes into the String Pool.
2. " Morning" it goes into the String Pool as well.
3. "Good Morning" created by concatenating "Good" and " Morning". This guy goes on the Heap.

Note: Strings are always immutable. There is no, such thing as a mutable String. str is just a reference which eventually points to "Good Morning". You are actually, not working on 1 object. you have 3 distinct String Objects.

---

Case 2:

StringBuffer str = new StringBuffer("Good"); 
str.append(" Morning");

StringBuffer contains an array of characters. It is not same as a String. The above code adds characters to the existing array. Effectively, StringBuffer is mutable, its String representation isn't.