What are String and StringBuffer classes of Java?

How are Strings different from StringBuffers ?

Write the return type of the following library functions:

1. isLetterOrDigit(char)
2. replace(char, char)

State the data type and values of a and b after the following segment is executed:

String s1 = "Computer", s2 = "Applications"
a = (s1.compareTo(s2));
b = (s1.equals(s2));