What is difference between mutable and immutable String in java

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.

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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.