hashcode、equals、HashMap间的关系

1、从Object说起
package com.hallo.collection;

public class ObjectDemo {

public static void main(String[] args) {

Object o1 = new Object();
Object o2 = new Object();

//同一Object对象，hashCode永远相同
assert(o1.hashCode() == o1.hashCode());
//不同Object对象，hashCode永远不相同
assert(o1.hashCode() != o2.hashCode());

//不同对象equals返回false
assert(!o1.equals(o2));

System.out.println("end");

}
}
输出：end

2、String类Demo
package com.hallo.collection;

public class StringDemo {

public static void main(String[] args) {

String str1 = "hello";
String str2 = "hello";
String str3 = new String("hello");

//不同string对象（==不成立）保存相同字符串时，hashCode和equals返回相同
assert(str1.hashCode() == str2.hashCode());
assert(str1.equals(str2));
assert(str2.hashCode() == str3.hashCode());
assert(str2.equals(str3));

System.out.println("end");
}
}
输出：end

3、自定义类，重写hashcode、equals方法
package com.hallo.collection;

import java.util.HashMap;
import java.util.Map;

public class MyObject {

private String name;

public String getName() {
return name;
}

public MyObject(String name) {
this.name = name;
}

@Override
public int hashCode() {
//使用object类的方法
return super.hashCode();
}

@Override
public boolean equals(Object obj) {

if(this == obj)
return true;

if(obj instanceof MyObject) {
return ((MyObject) obj).getName(http://www.amjmh.com).equals(this.getName());
}

return false;
}

public static void main(String[] args) {

MyObject o1 = new MyObject("k1");
MyObject o2 = new MyObject("k1");
MyObject o3 = new MyObject("k3");

Map<MyObject, String> myObjectMap = new HashMap<MyObject, String>();
myObjectMap.put(o1, "o1");
myObjectMap.put(o1, "o11");
myObjectMap.put(o2, "o2");
myObjectMap.put(o3, "o3");

System.out.println(myObjectMap);
}

}
输出：

{com.hallo.collection.MyObject@7852e922=o11,

com.hallo.collection.MyObject@70dea4e=o3,

com.hallo.collection.MyObject@4e25154f=o2}
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