What is difference between mutable and immutable String in java

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.

Solution 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.

Solution 2:

What is difference between mutable and immutable String in java

immutable exist, mutable don't.

Solution 3:

In Java, all strings are immutable. When you are trying to modify a String, what you are really doing is creating a new one. However, when you use a StringBuilder, you are actually modifying the contents, instead of creating a new one.

Solution 4:

Java Strings are immutable.

In your first example, you are changing the reference to the String, thus assigning it the value of two other Strings combined: str + " Morning".

On the contrary, a StringBuilder or StringBuffer can be modified through its methods.