向上转型--Upcasting

The new class is a type of the existing class.

新类是已存在类的一种类型。

向上转型：有子类转型成父类。向上转型总是安全的，因为向上转型从一个较特殊类型到较通用类型的转换。

package upcastingdemo;
class A {
public A(){}
public void m1(){
System.out.println("A.m1()");
}
public void m2(){
System.out.println("A.m2()");
}
public void m3(){
System.out.println("A.m3()");
}
}
class B extends A {
public B(){}
}
public class UpCastingDemo {
public static void test(A a){
a.m1();
a.m2();
a.m3();
}
public static void main(String[] args){
A a = new B();
a.m1();
a.m2();
a.m3();
System.out.println();
B b = new B();
UpCastingDemo.test(b);
}
}

运行结果：

A.m1()
A.m2()
A.m3()

A.m1()
A.m2()
A.m3()

让类B覆盖类中的方法：

package upcastingdemo;
class A {
public A(){}
public void m1(){
System.out.println("A.m1()");
}
public void m2(){
System.out.println("A.m2()");
}
public void m3(){
System.out.println("A.m3()");
}
}
class B extends A {
public B(){}
public void m1(){
System.out.println("B.m1()");
}
public void m2(){
System.out.println("B.m2()");
}
public void m3(){
System.out.println("B.m3()");
}
}
public class UpCastingDemo {
public static void test(A a){
a.m1();
a.m2();
a.m3();
}
public static void main(String[] args){
A a = new B();
a.m1();
a.m2();
a.m3();
System.out.println();
B b = new B();
UpCastingDemo.test(b);
}
}

运行结果：

B.m1()
B.m2()
B.m3()

B.m1()
B.m2()
B.m3()