java中的对象转换

曾经终败给现在 2023-10-17 10:42 36阅读 0赞

用例子来说明：

package com.converttest;

class Animal \{  
Animal()  
\{  
System.out.println("Animal constructor!");  
\}  
public void eat()  
\{  
System.out.println("Animal eat!");  
\}  
  
\}

class Bird extends Animal\{  
Bird()  
\{  
System.out.println("Bird constructor!");  
\}  
  
public void fly()  
\{  
System.out.println("Bird fly!");  
\}  
public void eat()  
\{  
System.out.println("Bird eat!");  
\}  
\}

class converttest \{  
public static void main(String\[\] args)\{  
System.out.println("test case 1: ---------------!");  
Animal a1 = new Animal();  
Bird b1 = new Bird();  
  
a1.eat();  
b1.eat();  
  
a1 = b1; //OK, 隐式转换，子类实例向父类转换,相当于a1 = (Animal)b1;。  
a1.eat(); //OK，调用的是子类的eat函数。  
//a1.fly(); // error, fly 不是a1类型的。  
  
//b1 = a1; //error, 父类转向子类，没用强转，所以编译错误。  
b1 = (Bird)a1; //OK, 父类转向子类，用强转，所以编译正确。  
b1.eat(); //OK, 调用的是子类的eat函数。

System.out.println("test case 2: ---------------!");  
Animal a2 = new Bird(); //OK, 隐式转换，子类实例向父类转换,相当于Animal a2 = (Animal)new Bird();;。  
a2.eat();  
//a2.fly(); // error,Animal无法访问子类的fly函数。

//Bird b2 = new Animal(); //error, 父类转向子类，没用强转，所以编译错误。  
Bird b2 = (Bird)new Animal(); //OK, 父类转向子类，必须用强转。  
b2.eat(); //Exception in thread "mainException in thread "main" java." java.lang.ClassCastException: com.interfacetest.Animal cannot be cast to com.interfacetest.Bird  
//at com.interfacetest.converttest.main(interfacetest1.java:101)  
b2.fly(); //Exception in thread "mainException"......  
\}  
\}

/\* result:  
test case 1: ---------------!  
Animal constructor!  
Animal constructor!  
Bird constructor!  
Animal eat!  
Bird eat!  
Bird eat!  
Bird eat!  
test case 2: ---------------!  
Animal constructor!  
Bird constructor!  
Bird eat!  
Animal constructor!  
Exception in thread "main" java.lang.ClassCastException: com.interfacetest.Animal cannot be cast to com.interfacetest.Bird  
at com.interfacetest.converttest.main(interfacetest1.java:101)  
\*/

说明： 1. 子类向父类会自动进行隐式转换，是类型安全的;

2. 父类向子类要用强制转换，但类型不安全.