Help me!该如何处理

Help me!
ClassB中能那样定义一个ClassC的变量,然后仅仅用哪个变量就可以调用ClassC中的实例方法而非成员方法?!
NullPointerException是因为什么引起的呢?

Java code

class ClassA {    public void methodA() {        ClassB classB = new ClassB();        classB.getValue();    }}class ClassB {    public ClassC classC;    public String getValue() {        return classC.getValue();    }}class ClassC {    public String value;    public String getValue() {        value = "ClassB";        return value;    }}public class Question {    /**     * @param args     */    public static void main(String[] args) {        // TODO Auto-generated method stub        ClassA a = new ClassA();        a.methodA();    }}//Exception in thread "main" java.lang.NullPointerException//    at ClassB.getValue(Question.java:11)//    at ClassA.methodA(Question.java:4)//    at Question.main(Question.java:31)