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;
}