Java_map的key为自定义对象

首先自定义Key对象

import lombok.AllArgsConstructor;

import lombok.Getter;

import lombok.Setter;

import java.util.Objects;

/**

 * @author AganRun

 * @date 2019/10/16

 */

@Getter

@Setter

@AllArgsConstructor

public class SelfKey {

    private String first;

    private String second;

    private String third;

    @Override

    public int hashCode() {

        return first.hashCode() + second.hashCode() + third.hashCode();

    }

    @Override

    public boolean equals(Object obj) {

//        super.equals(obj);

        SelfKey selfKey = (SelfKey) obj;

        boolean equals1 = Objects.equals(this.first, selfKey.getFirst());

        boolean equals2 = Objects.equals(this.second, selfKey.getSecond());

        boolean equals3 = Objects.equals(this.third, selfKey.getThird());

        return equals1 && equals2 && equals3;

    }

}

测试类

import lombok.extern.slf4j.Slf4j;

import org.junit.Test;

import java.util.HashMap;

import java.util.Map;

/**

 * @author AganRun

 * @date 2019/10/16

 */

@Slf4j

public class HashMapSelfKeyTest {

    @Test

    public void test(){

        SelfKey selfKey1 = new SelfKey("1","2","3");

        SelfKey selfKey2 = new SelfKey("3","2","1");

        Map<SelfKey, String> map = new HashMap<>();

        map.put(selfKey1, "value1");

        map.put(selfKey2, "value2");

        /**

         * 如果不重写hashcode和equals方法，至于put时的对象可以取出对应的值

         * 第一个输出：06:53:50.193 [main] INFO com.agan.map.HashMapSelfKeyTest - null

         * 第二个输出：06:53:50.195 [main] INFO com.agan.map.HashMapSelfKeyTest - value1

         *

         * 如果只重写hashCode方法。equals比较时依旧比较内存地址，导致不通过

         * 输出结果和第一个相同

         *

         * 自定义重写了hashCode和Equeals方法后

         * 07:02:47.469 [main] INFO com.agan.map.HashMapSelfKeyTest - value1

         * 07:02:47.472 [main] INFO com.agan.map.HashMapSelfKeyTest - value1

         */

        log.info(map.get(new SelfKey("1", "2", "3")));

        log.info(map.get(selfKey1));

    }

}

在这里再写两个其他的发现。

equals在程序执行时可能会调用多次，比如可以在equals中打印语句。

toString方法会调用这个方法的HashCode()方法

public String toString() {

    return getClass().getName() + "@" + Integer.toHexString(hashCode());

}

HashMap的判断

如果hash值mod之后的索引，索引冲突后会看key是否相同，如果相同则不再比较，如果不同再比较equals

public V put(K key, V value) {

    return putVal(hash(key), key, value, false, true);

}

static final int hash(Object key) {

    int h;

    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,

                   boolean evict) {

    Node<K,V>[] tab; Node<K,V> p; int n, i;

    if ((tab = table) == null || (n = tab.length) == 0)

        n = (tab = resize()).length;

    if ((p = tab[i = (n - 1) & hash]) == null)

        tab[i] = newNode(hash, key, value, null);

    else {

        Node<K,V> e; K k;

        if (p.hash == hash &&

            ((k = p.key) == key || (key != null && key.equals(k))))

            e = p;

        else if (p instanceof TreeNode)

            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);

        else {

            for (int binCount = 0; ; ++binCount) {

                if ((e = p.next) == null) {

                    p.next = newNode(hash, key, value, null);

                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st

                        treeifyBin(tab, hash);

                    break;

                }

                if (e.hash == hash &&

                    ((k = e.key) == key || (key != null && key.equals(k))))

                    break;

                p = e;

            }

        }

        if (e != null) { // existing mapping for key

            V oldValue = e.value;

            if (!onlyIfAbsent || oldValue == null)

                e.value = value;

            afterNodeAccess(e);

            return oldValue;

        }

    }

    ++modCount;

    if (++size > threshold)

        resize();

    afterNodeInsertion(evict);

    return null;

}