Similarities: Both StringBuilder and StringBuffer are mutable. That means you can change the content of them, with in the same location.

Differences: StringBuffer is mutable and synchronized as well. Where as StringBuilder is mutable but not synchronized by default.

Meaning of synchronized (synchronization): When some thing is synchronized, then multiple threads can access, and modify it with out any problem or side effect. StringBuffer is synchronized, so you can use it with multiple threads with out any problem.

Which one to use when? StringBuilder : When you need a string, which can be modifiable, and only one thread is accessing and modifying it. StringBuffer : When you need a string, which can be modifiable, and multiple threads are accessing and modifying it.

Note : Don’t use StringBuffer unnecessarily, i.e., don’t use it if only one thread is modifying and accessing it because it has lot of locking and unlocking code for synchronization which will unnecessarily take up CPU time. Don’t use locks unless it is required.

Simply use StringBuilder unless you really are trying to share a buffer between threads. StringBuilder is the unsynchronized (less overhead = more efficient)

When a String is there and you add use concat like one below

  
 String x = "Java";
 x.concat("Rules !");
 sysout(x);

Now in the above there would be 2 string object created with output Java.When you used append in stringBuilder or stringBuffer the changes were applied over the object itself

  
 StringBuiler x = new StringBuiler("Java");
 x.append("Rules !");
 sysout(x);

the output would be Java Rules !