Java 每次创建对象时,哈希代码都会更改
关于hashCode,有一些东西我没有意识到。我知道如果我超越 等于,那么我也必须重写哈希代码。我也知道如果两个物体 相等,则两个对象的哈希代码必须相等。如果在equals方法中检查的字段没有更改,那么hashCode就不应该更改,对吗 如果是这样,那么我就无法理解为什么每次创建下面对象的实例时,都会得到不同的哈希代码:Java 每次创建对象时,哈希代码都会更改,java,hashcode,Java,Hashcode,关于hashCode,有一些东西我没有意识到。我知道如果我超越 等于,那么我也必须重写哈希代码。我也知道如果两个物体 相等,则两个对象的哈希代码必须相等。如果在equals方法中检查的字段没有更改,那么hashCode就不应该更改,对吗 如果是这样,那么我就无法理解为什么每次创建下面对象的实例时,都会得到不同的哈希代码: public class Effect { private long timestamp; private int damage; private S
public class Effect {
private long timestamp;
private int damage;
private SquareType squareType;
public Effect(long timestamp, int damage, SquareType squareType) {
this.timestamp = timestamp;
this.damage = damage;
this.squareType = squareType;
}
public long getTimestamp() {
return timestamp;
}
public int getDamage() {
return damage;
}
public SquareType getSquareType() {
return squareType;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Effect effect = (Effect) o;
if (getDamage() != effect.getDamage()) return false;
return getSquareType() == effect.getSquareType();
}
@Override
public int hashCode() {
int result = getDamage();
result = 31 * result + (getSquareType() != null ? getSquareType().hashCode() : 0);
return result;
}
@Override
public String toString() {
String ret = "Effect hashcode: " + hashCode();
return ret;
}
}
在代码中,我随着时间的推移不断创建此类对象。每次更改的唯一字段是“timestamp”,但其他两个字段不会更改(除非有特定事件)。发生的情况是,hashCode值总是不同的,即使“损害”和“平方类型”是相同的。
我在equals和hashCode中没有使用“timestamp”,所以我不明白为什么会出现这种行为
更新
这是方形的:
public enum SquareType {
FIRE, WIND, WATER, EARTH
}
更新2
例如,如果我创建了10个Effect实例,我迭代它们,然后
打印它们(toString()返回hashCode值)我得到10个不同的值
如果效果的两个实例具有相同的“损害”和“平方类型”,则它们必须相等并具有相同的哈希代码
更新3
效果如下所示:
@Override
public void friendlyFire(BaseBullet bullet, BaseSquare square) {
square.notifyFriendlyFire(new Effect(TimeUtils.millis(),
square.getDamage(), square.getSquareType()), new MyKey(square.getUniqueID()));
}
唯一改变的效果字段是时间戳,我不在equals和hashCode中使用它
public void notifyFriendlyFire(Effect newEffect, MyKey key) {
// System.out.println("The map contains the key? " + effectMap.containsKey(key));
if(effectMap.containsKey(key)) {
Effect oldEffect = effectMap.get(key);
System.out.println(newEffect);
if(!oldEffect.equals(newEffect)) {
System.out.println("old effect changed!");
// remove the old effect
removeEffect(oldEffect);
// update the map with the new effect
effectMap.put(key, newEffect); //
// apply the new effect
applyEffect(newEffect);
}
}
else {
// new effect
effectMap.put(key, newEffect);
applyEffect(newEffect);
}
}
检查“if(!oldEffect.equals(newEffect))”始终为真,即使损坏和类型相同
更新4
我找到了窃听器。伤害持续增加。现在,我只想弄清楚为什么…以下是我对您的实现的近似尝试:
package cruft;
import java.util.Date;
/**
* Equals and hashCode test
* Creation date 1/16/2016.
* @link https://stackoverflow.com/questions/34826585/hashcode-changes-each-time-the-object-is-created
*/
public class OverrideDemo {
private long timestamp;
private int damage;
private SquareType squareType;
public OverrideDemo(int damage, SquareType squareType) {
this(damage, squareType, new Date().getTime());
}
public OverrideDemo(int damage, SquareType squareType, long timestamp) {
if (squareType == null) throw new IllegalArgumentException("square type cannot be null");
this.timestamp = timestamp;
this.damage = damage;
this.squareType = squareType;
}
public long getTimestamp() {
return timestamp;
}
public int getDamage() {
return damage;
}
public SquareType getSquareType() {
return squareType;
}
@Override
public boolean equals(Object o) {
if (this == o) { return true; }
if (o == null || getClass() != o.getClass()) { return false; }
OverrideDemo that = (OverrideDemo) o;
if (damage != that.damage) { return false; }
return squareType == that.squareType;
}
@Override
public int hashCode() {
int result = damage;
result = 31 * result + squareType.hashCode();
return result;
}
@Override
public String toString() {
return "OverrideDemo{" +
"timestamp=" + timestamp +
", damage=" + damage +
", squareType=" + squareType +
'}';
}
}
enum SquareType { FIRE, WIND, WATER, EARTH }
下面是一个Junit测试,它显示了这些方法的行为。所有测试均通过;我认为这个实现是正确的
package cruft;
import org.junit.Assert;
import org.junit.Test;
/**
* Junit test demonstrates testing equals and hashcode contract
* Created by Michael
* Creation date 1/16/2016.
* @link https://stackoverflow.com/questions/34826585/hashcode-changes-each-time-the-object-is-created
*/
public class OverrideDemoTest {
@Test
public void testEquals_Null() {
OverrideDemo x = new OverrideDemo(5, SquareType.EARTH);
Assert.assertFalse(x.equals(null));
}
@Test
public void testEquals_Reflexive() {
OverrideDemo x = new OverrideDemo(5, SquareType.EARTH);
Assert.assertTrue(x.equals(x));
}
@Test
public void testEquals_Symmetric() {
OverrideDemo x = new OverrideDemo(5, SquareType.EARTH);
OverrideDemo y = new OverrideDemo(5, SquareType.EARTH);
Assert.assertTrue(x.equals(y));
Assert.assertTrue(y.equals(x));
Assert.assertTrue(x.hashCode() == y.hashCode());
}
@Test
public void testEquals_Transitive() {
OverrideDemo x = new OverrideDemo(5, SquareType.EARTH);
OverrideDemo y = new OverrideDemo(5, SquareType.EARTH);
OverrideDemo z = new OverrideDemo(5, SquareType.EARTH);
Assert.assertTrue(x.equals(y));
Assert.assertTrue(y.equals(z));
Assert.assertTrue(z.equals(x));
Assert.assertTrue(x.hashCode() == y.hashCode());
Assert.assertTrue(y.hashCode() == z.hashCode());
Assert.assertTrue(z.hashCode() == x.hashCode());
}
@Test
public void testEquals_DifferentDamage_NotEqual() {
OverrideDemo x = new OverrideDemo(5, SquareType.EARTH);
OverrideDemo y = new OverrideDemo(10, SquareType.EARTH);
Assert.assertFalse(x.equals(y));
Assert.assertFalse(y.equals(x));
Assert.assertFalse(x.hashCode() == y.hashCode());
}
@Test
public void testEquals_DifferentSquareType_NotEqual() {
OverrideDemo x = new OverrideDemo(10, SquareType.EARTH);
OverrideDemo y = new OverrideDemo(10, SquareType.FIRE);
Assert.assertFalse(x.equals(y));
Assert.assertFalse(y.equals(x));
Assert.assertFalse(x.hashCode() == y.hashCode());
}
}
你能发布
SquareType
吗?它可能不会覆盖hashCode。SquareType的hashCode是如何计算的?您可以发布一个完整的代码来演示这个问题吗?为什么不按照Tunaki的要求发布演示代码呢?也许您正在做一些愚蠢的事情,比如创建带有损坏和时间戳参数changedJust的效果对象,以确保:在toString方法中,print还返回损坏和方形类型,以确保对象与预期的相等