Java学习笔记_180717_hashCode问题(不懂)

package com.hxzy.demo001;

import java.util.HashSet;

class RectObject {
	public int x;
	public int y;

	public RectObject(int x, int y) {
		this.x = x;
		this.y = y;
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + x;
		result = prime * result + y;
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		final RectObject other = (RectObject) obj;
		if (x != other.x) {
			return false;
		}
		if (y != other.y) {
			return false;
		}
		return true;
	}
}

public class Example {
	public static void main(String[] args) {
		HashSet<RectObject> set = new HashSet<RectObject>();
		RectObject r1 = new RectObject(3, 3);
		RectObject r2 = new RectObject(5, 5);
		RectObject r3 = new RectObject(3, 6);
		set.add(r1);
		set.add(r2);
		set.add(r3);
		for (RectObject rectObject : set) {
			System.out.println(rectObject.hashCode());
		}
		r3.y=7;
		for (RectObject rectObject : set) {
			System.out.println(r3.equals(rectObject));
			System.out.println(rectObject.equals(r3));
		}
		System.out.println("删除前的大小size:" + set.size());
		System.out.println(set.contains(r3));
		System.out.println(set.remove(r3));
		System.out.println("删除后的大小size:" + set.size());
	}
}

根据contains的描述：

Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).

Specified by: contains(...) in Set, Overrides: contains(...) in AbstractCollection
Parameters:
o element whose presence in this set is to be tested
Returns:
true if this set contains the specified element

 既然System.out.println(r3.equals(rectObject)); System.out.println(rectObject.equals(r3));返回在rectObject为r3时返回值都为true；为何set.contains(r3)的返回值为false？

确定是否存在element是根据hashCode确定的，r3.y=7;之后r3的hashcode()返回值改变了，所以set.remove(r3);失败返回值为false；元素个数

不理解。

System.out.println("是否包含元素：");
		for (RectObject rectObject : set) {
			System.out.println(rectObject.x + ":"+rectObject.y+"--"+set.contains(rectObject));
                }    

 运行这个代码返回值竟然是：true false true;

看到一种说法，hashcode重写应该保证，同一对象的返回值不改变，暂时只能这么理解了。

https://blog.csdn.net/violet_echo_0908/article/details/50152915

 hashCode重写原则：

代码中明显违反了该原则