What is the difference between String and StringBuffer in Java?

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

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.

You can also use StringBuilder which is similar to StringBuffer except it is not synchronized. The maximum size for either of these is Integer.MAX_VALUE (2³¹ - 1 = 2,147,483,647) or maximum heap size divided by 2 (see How many characters can a Java String have?). More information here.

A String is immutable, i.e. when it's created, it can never change.

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.

The maximum length for both is Integer.MAX_VALUE, because they are stored internally as arrays, and Java arrays only have an int for their length pseudo-field.

The performance improvement between Strings and StringBuffers for multiple concatenation is quite significant. If you run the following test code, you will see the difference. On my ancient laptop with Java 6, I get these results:

Concat with String took: 1781ms
Concat with StringBuffer took: 0ms

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");
    }
}