Java Object - 멀티태스킹

 -         
 -       

     ：     

class A2{
    public void tell1() {
        System.out.println("A2---tell1");
    }
    public void tell2() {
        System.out.println("A2---tell2");

    }
}
class B2 extends A2{
    public void tell1() {
        System.out.println("B2---tell1");
    }
    public void tell3() {
        System.out.println("B2--tell3");
    }
}
public class Demo22 {
    public static void main(String[] args) {
        //    
        B2 b = new B2();
        A2 a=b;
        a.tell1();//tell1    ，           
        a.tell2();
    }
}

  ：
B2---tell1
A2---tell2

    ：     

class A2{
    public void tell1() {
        System.out.println("A2---tell1");
    }
    public void tell2() {
        System.out.println("A2---tell2");

    }
}
class B2 extends A2{
    public void tell1() {
        System.out.println("B2---tell1");
    }
    public void tell3() {
        System.out.println("B2--tell3");
    }
}
public class Demo22 {
    public static void main(String[] args) {
        //    
        A2 a2 = new B2();
        B2 b2 = (B2)a2;
        b2.tell1();
        b2.tell2();
        b2.tell3();
    }
}

  ：
B2---tell1
A2---tell2
B2--tell3

     ：       

class A2{
    public void tell1() {
        System.out.println("A2---tell1");
    }
    public void tell2() {
        System.out.println("A2---tell2");

    }
}
class B2 extends A2{
    public void tell1() {
        System.out.println("B2---tell1");
    }
    public void tell3() {
        System.out.println("B2--tell3");
    }
}
public class Demo22 {
    public static void main(String[] args) {
        //    
        A2 a2 = new A2(); // B2  A2，   -       
        B2 b2 = (B2)a2;
        b2.tell1();
        b2.tell2();
        b2.tell3();
    }
}

  ：
Exception in thread "main" java.lang.ClassCastException: cn.sec.ch02.A2 cannot be cast to cn.sec.ch02.B2
    at cn.sec.ch02.Demo22.main(Demo22.java:35)

  

class A3{
    public void tell1(){
        System.out.println("A3---tell1");
    }
}
class B3 extends A3{
    public void tell2(){
        System.out.println("B3--tell2");
    }
}
class C3 extends A3{
    public void tell3() {
        System.out.println("C3--tell3");
    }
}
class D3 extends A3{
    
}
public class Demo23 {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        say(new B3());
        say(new C3());
        say(new D3());
    }
    /**
     *       B3  C3     tell1   
     */
    public static void say(A3 a) {
        a.tell1();
    }

}
  ：
A3---tell1
A3---tell1
A3---tell1

  
class A4{
    public void tell1() {
        System.out.println("A4---tell1");
    }
    public void tell2() {
        System.out.println("A4---tell2");

    }
}
class B4 extends A4{
    public void tell1() {
        System.out.println("B4---tell1");
    }
    public void tell3() {
        System.out.println("B4--tell3");
    }
}
public class Demo24 {
    public static void main(String[] args) {
        A4 a4  = new A4();
        System.out.println(a4 instanceof A4);
        System.out.println(a4 instanceof B4);
        
        A4 a = new B4(); //    
        System.out.println(a instanceof A4);
        System.out.println(a instanceof B4);
    }
}

  ：
true
false
true
true