java类的强制转换问题

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

 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

 public class TestExtend {

    public static void main(String[] args) {

        B b = new B();
        callA(b);
        A a = b;//b 能被强转 因为b是子类
        callA(new A());//但你传入一个父类，父类在下面的方法中就不能被转成子类了。
    }

    public static void callA(A a){
        B b = (B)a;
        b.func1();
        b.func2();
        b.func3();
    }
}

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");
    }
}