Java中的数据成员继承的多态

public class PolymorphismTest {
public void show() {
    A a = new B();
    B b = new B();
    a.show();
    b.show();
    System.out.println("a.a: " + a.a);
    System.out.println("b.a: " + b.a);
}

public class A { 
    public int a = 5;
    public void show() {
        System.out.println("A show");
    }

}

public class B extends A {
    public int a = 6;
    public void show() {
        System.out.println("B show");
    }
}
}

为什么这里a.a的值是5而不是6呢?