java类的强制转换问题

Question

package lesson4;
class A
{
public void func1()
{
System.out.println("A func1 is calling");
}
public void func2()
{
func1();
}
}
class B extends A
{
public void
func1()
{
System.out.println("B func1 is calling");
}
public void func3()
{
System.out.println("B func3 is calling");
}
}
class C
{
public static void main(String [] args)
{
B b = new B();
callA(b);
A a = b;
callA(new A());
}
public static void callA(A a)
{
B b = (B)a;
b.func1();
b.func2();
b.func3();
}
}
以上为代码部分
B func1 is calling
B func1 is calling
B func3 is calling
Exception in thread "main" java.lang.ClassCastException: lesson4.A cannot be cast to lesson4.B
at lesson4.C.callA(A.java:36)
at lesson4.C.main(A.java:32)

以上为运行结果出错部分
望各位大神抽出时间予以解答

Answer 1

 callA(new A());这里的实例是A，B是继承A的，是A的子类，父类实例不能转成子类实例，反过来可以