Difference between string and stringbuffer class

String

• It is used to manipulate character strings that cannot be changed (read-only and immutable). 

StringBuffer:

• A StringBuffer (or its non-synchronized cousin StringBuilder) is used when you need to construct a string piece by piece without the performance overhead of constructing lots of little Strings along the way.
• StringBuffer is used to represent characters that can be modified.
• Performance wise, StringBuffer is faster when performing concatenations. This is because when you concatenate a String, you are creating a new object (internally) every time since String is immutable.

Example:

public class Concat
{
    public static String concatWithString()
    {
        String t = "Cat";
        for (int i=0; i<10000; i++)
        {
            t = t + "Dog";
        }
        return t;
    }
    public static String concatWithStringBuffer()
    {
        StringBuffer sb = new StringBuffer("Cat");
        for (int i=0; i<10000; i++)
        {
            sb.append("Dog");
        }
        return sb.toString();
    }
    public static void main(String[] args)
    {
        long start = System.currentTimeMillis();
        concatWithString();
        System.out.println("Concat with String took: " + (System.currentTimeMillis() - start) + "ms");
        start = System.currentTimeMillis();
        concatWithStringBuffer();
        System.out.println("Concat with StringBuffer took: " + (System.currentTimeMillis() - start) + "ms");
    }
}