StringBuilder

Just to add my experience working with StringBuilder:

StringBuilder  hasn't got an overridden equals() method, so if you want to see whether two StringBuilders have the same contents try

builder1.toString().equals(builder2.toString()).

For example:

StringBuilder a = new StringBuilder("rupali");
StringBuilder b = new StringBuilder("rupali");

If we want to check if content of a and b are same (as we do in StringBuffer or String class with equals() method)

if (a.equals(b))
{ System.out.println("Equal"); }
else {System.our.println("Not equal"); }

Unlike String or StringBuffer, this will give "Not equal" as answer.

The correct implementation to check equals() in case of StringBuilder is:

if (a.toString().equals(b.toString()))
{ System.out.println("Equal"); }
else {System.our.println("Not equal"); }

The answer is "Equal".

Criteria to choose among String, StringBuffer and StringBuilder

   1. If your text is not going to change use a string Class because a String object is immutable.
   2. If your text can change and will only be accessed from a single thread, use a StringBuilder because StringBuilder is unsynchronized.
   3. If your text can change, and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous.

Thanks
Rupali