why stringbuffer is immutable in java code example

Example 1: string vs stringbuffer

The main similarity between String and StringBuffer class is that both are
thread safe. 
The main difference is that String is immutable; StringBuffer is mutable.

Example 2: difference between string vs stringbuffer

The main similarity between String and StringBuffer class is that both are 
thread safe.
The main difference is that String is immutable; StringBuffer is mutable.