Java — 面向对象（自测题）

1、

package package1;

class Test
{
  private static int x =  10;

  public void show(int x)  
  {
    x++;
    this.x++;
    System.out.println(x+" "+this.x);
  } 

  public static void main(String[] args)
  {
    int x = 20;  
    Test t = new Test();
    t.show(x);
  }
}

【答案】：21 11

2、

package package1;

class Fu{
  public int num = 10;
  public Fu(){
    System.out.println("fu");
  }
}
class Zi extends Fu{
  public int num = 20;
  public Zi(){
    System.out.println("zi");
  }
  public void show(){
    int num = 30;
    System.out.println(num);
    System.out.println(this.num);
    System.out.println(super.num);
  }
}
class Test {
  public static void main(String[] args) {
    Zi z = new Zi();
    z.show();
  }
}

【答案】：

fu

zi

30

20

10

3、

/*
 * 1、先加载类 -> 静态代码块中的代码
 * 2、构造器 -> 构造代码块+构造方法
 */
class Fu {
  static {
    System.out.println("静态代码块Fu");
  }

  {
    System.out.println("构造代码块Fu");
  }

  public Fu() {
    System.out.println("构造方法Fu");
  }
}

class Zi extends Fu {
  static {
    System.out.println("静态代码块Zi");
  }

  {
    System.out.println("构造代码块Zi");
  }

  public Zi() {
    System.out.println("构造方法Zi");
  }
}
class Test {
  public static void main(String[] args) {
    Zi z = new Zi();
    
  }
}

【答案】：

静态代码块Fu

静态代码块Zi

构造代码块Fu

构造方法Fu

构造代码块Zi

构造方法Zi

4、

package package1;
/*
 * 系统会自上而下寻找构造器，直到找到构造器，并调用构造器
 * YXYZ
 */
class X {
  Y b = new Y();
  X() {
    System.out.print("X");
  }
}
class Y {
  Y() {
    System.out.print("Y");
  }
}
public class Z extends X {
  Y y = new Y();
  Z() {
    System.out.print("Z");
  }
  public static void main(String[] args) {
    new Z(); 
  }
}

【答案】：YXYZ

5、

package package1;
class Fu
{
  public void show()
  {
    System.out.println("fu show");
  }
}

class Zi extends Fu
{
  public void show()
  {
    System.out.println("zi show");
  }

  public void method()
  {
    System.out.println("zi method");
  }
}
class Test
{
  public static void main(String[] args)
  {
    Fu f = new Zi();
    /*
     * 子到父，父中没有该方法，所以会报错
     */
    //f.method();
    
    /*
     * 父到子，将父类的引用转化成子类的引用，前提是子类还是子类
     * Zi z = ...;
     */
    Zi z = (Zi)f;
    z.method();
  }
}

【答案】：zi method

6、

package package1;

import java.nio.channels.ShutdownChannelGroupException;

class A
{
  public void show()
  {
    show2();
  }
  
  public void show2()
  {
    System.out.println("我");
  }
}
class B extends A 
{
//  public void show() {
//    System.out.println("B");
//  }
  public void show2()
  {
    System.out.println("爱");
  }
}
class C extends B 
{
  public void show()
  {
    super.show();
    //System.out.println("C");
  }
  
  public void show2()
  {
    System.out.println("你");
  }
}
public class Test 
{
  public static void main(String[] args) 
  {
    /*
     * 1.在执行一个方法（不是构造器）的时候一定先在实例（右边）中找方法，
     * 2.找不到，自下而上的找，
     * 3.重点，每个方法都要这样去找，执行
     */
    A a = new B();
    a.show();
    B b = new C();
    b.show();
  }
}

【答案】：

爱

你

7、

package package1;

class Animal{}
class Dog extends Animal{}
class Cat extends Animal{}

class Test{
  public static void main(String []args) {
    Animal p0 = new Animal();
    Dog p1 = new Dog ();
    Cat p2 = new Cat ();
    Animal p3 = new Dog ();
    Animal p4 = new Cat ();

    p0 = p1;  //向上造型
  //  p1 = p2;  //同类直接不可以直接转化
  //  p1 = (Dog)p2;
    p1 = (Dog)p3;  //向下造型
    p2 = (Cat)p4;
  }
}