java中Object.equals()简单用法

/*
equals（）方法默认的比较两个对象的引用！
*/
class Child {
    int num;
    public Child(int x){
        num = x;
    }
     
    //人文的抛出运行时异常的好处是：可以自定义错误信息！
     
    /*public boolean equals(Object o) throws ClassCastException{
         if(!(o instanceof Child)) throw new ClassCastException("中文提示：类型错误");
         Child ch = (Child) o;
         return num == ch.num ;
    }*/
     
    /*
        Exception in thread "main" java.lang.ClassCastException: 中文提示：类型错误
        at Child.equals(PC.java:8)
        at PC.main(PC.java:17)
    */
     
    public boolean equals(Object o){
         Child ch = (Child) o;
         return num == ch.num ;
    }
     
    /*
        Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot
        be cast to Child
        at Child.equals(PC.java:14)
        at PC.main(PC.java:22)
    */
}
 
public class PC{
     public static void main(String[] args){
        Child p1 = new Child(11);
        if(p1.equals(new Integer(11)))
           System.out.println("true");
            
        System.out.println("这句话是否执行？");// 如果异常得到了处理， 那么这句话就会执行，否则就不会被执行！
     }
}