equals和hashcode

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手头上边的事情做完了，在看源码的时候想到了以前留的一个坑，就是String源码比较有意思的地
方，想了半天没想起来自己当时是怎么想的，看到了equals方法，决定看看以前老生常谈的一个问题，
为什么重写equals方法要重写hashcode方法

为了搞清这个问题，我自己写了一个测试类

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package top.shownmmp.quartz;

/**
 * @author wqh
 * @date 2019-7-18
 */
public class TestEqualsAndHashCode {

    public static void main(String[] args) {
        UserDemo userDemo = new UserDemo("1", 16);
        UserDemo userDemo1 = new UserDemo("2", 16);
        boolean equals = userDemo.equals(userDemo1);
        boolean b = userDemo.hashCode() == userDemo1.hashCode();
        System.out.println("equals:"+equals);
        System.out.println("hashCode:"+b);
    }


}
class UserDemo{
    public UserDemo(String userId, int age) {
        this.userId = userId;
        this.age = age;
    }

    private String userId;
    private int age;

    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj==this) return true;
        if (obj instanceof UserDemo) {
            UserDemo userDemo = (UserDemo) obj;
           return userDemo.getAge()==this.getAge();
        }
        return false;
    }
}

运行上面的代码输出是

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equals:true
hashCode:false

问题就出现了

先看object类中的hashCode方法

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/**
     * Returns a hash code value for the object. This method is
     * supported for the benefit of hash tables such as those provided by
     * {@link java.util.HashMap}.
     * <p>
     * The general contract of {@code hashCode} is:
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during
     *     an execution of a Java application, the {@code hashCode} method
     *     must consistently return the same integer, provided no information
     *     used in {@code equals} comparisons on the object is modified.
     *     This integer need not remain consistent from one execution of an
     *     application to another execution of the same application.
     * <li>If two objects are equal according to the {@code equals(Object)}
     *     method, then calling the {@code hashCode} method on each of
     *     the two objects must produce the same integer result.
     * <li>It is <em>not</em> required that if two objects are unequal
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
     *     method, then calling the {@code hashCode} method on each of the
     *     two objects must produce distinct integer results.  However, the
     *     programmer should be aware that producing distinct integer results
     *     for unequal objects may improve the performance of hash tables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by
     * class {@code Object} does return distinct integers for distinct
     * objects. (This is typically implemented by converting the internal
     * address of the object into an integer, but this implementation
     * technique is not required by the
     * Java&trade; programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    public native int hashCode();

上面其中一段解释了为什么需要重写

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* <li>If two objects are equal according to the {@code equals(Object)}
*     method, then calling the {@code hashCode} method on each of
*     the two objects must produce the same integer result.

换句话就是说

Java中规定如何两个对象通过equals方法得到的是true，那么返回的应该是同一个hashcode值，反过来如果两对象的hashcode值相同的话，这两个对象不一定相同，当然这两个对象都可能不是同一种类型，取决于怎么重写的hashcode。假设有这么一种情况，你往hashmap里面放了一个对象，但是你的这个对象里面只重写了equals方法但是没重写hashcode,因此我用另一个属性跟前面完全一样的对象，虽然equals方法返回是true，但是hashcode并没有重写，导致两个返回的是不同的整型值，而map的get方法会去判断key的hash值，不同的hashcode值必然导致这两个key不等，所以会拿到一个null值。

技术能力有限写不出来很高深的东西

成就感

但是比较有成就感的是这一次我竟然能够看懂别人写的ssm项目，第一个实习的时候好多地方都看不懂，可能是其他方面导致的比如项目需要所以看了redis的使用，redispool和redisconnectionfactory，用过了quartz，看完了mybatis文档，知道点security相关的东西,知道文件上传以及诸如此类的东西，看的东西越多，越来越感觉基础很重要，也知道了为什么招聘者会考一些基础知识。也是别人说的面试造飞机，上面拧螺丝。危机感很严重…..

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