重写equals和hashCode

equals()方法

1. 自反性：A.equals(A)要返回true.

2. 对称性：如果A.equals(B)返回true, 则B.equals(A)也要返回true.

3. 传递性：如果A.equals(B)为true, B.equals(C)为true, 则A.equals(C)也要为true. 说白了就是 A = B , B = C , 那么A = C.

4. 一致性：只要A,B对象的状态没有改变，A.equals(B)必须始终返回true.

5. A.equals(null) 要返回false.

//测试

public class Person {
    private int age;
    private String name;
    
　　
    @Override
    public boolean equals(Object obj){
        if(this == obj){
            return true;
        }else if(!(obj instanceof Person)){
            return false;
        }else{
            Person p = (Person) obj;
            return p.name.equals(name) && p.age == age;
        }
    }
    
    @Override
    public int hashCode(){
        int result = 17;
        result = result * 31 + age;
        result = result * 31 + name.hashCode();
        return result; 
    }
    
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    
}
//验证，如果不重写hashCode()，输出false；

   public static void main(String[] args) {
        Person p = new Person();
        p.setAge(3);
        p.setName("ff");
        Person p1 = new Person();
        p1.setAge(3);
        p1.setName("ff");
        Set<Person> set = new HashSet<Person>();  
        set.add(p);
        System.out.println(set.contains(p1));
    }