为什么重写 equals() 要重写 hashCode()? hashCode 值相等，两个对象不一定相等？

Love The Way You Lie 2021-10-14 03:00 311阅读 0赞

### 为什么重写 equals() 要重写 hashCode()? hashCode 值相等，两个对象不一定相等？ ###

hashCode 方法，如果不重写的话，返回的实际上是该对象在 jvm 的堆上的地址，而不同对象的地址肯定不同，所以这个 hashCode 也就肯定不同了。如果重写了的话，由于采用的算法的问题，有可能导致两个不同对象的 hashCode 相同。

equals 方法，如果不重写的话，是严格判断一个对象是否相等的方法（object1 == object2）

> 看了一些博客，感觉不靠谱，虽然我的可能也会出现小问题，但会尽量追求证实，来看源码。

#### java.lang.Object.equals() 方法 ####

原版：

/** * Indicates whether some other object is "equal to" this one. * * The {@code equals} method implements an equivalence relation * on non-null object references: * <ul> * <li>It is reflexive: for any non-null reference value * {@code x}, {@code x.equals(x)} should return * {@code true}. * <li>It is symmetric: for any non-null reference values * {@code x} and {@code y}, {@code x.equals(y)} * should return {@code true} if and only if * {@code y.equals(x)} returns {@code true}. * <li>It is transitive: for any non-null reference values * {@code x}, {@code y}, and {@code z}, if * {@code x.equals(y)} returns {@code true} and * {@code y.equals(z)} returns {@code true}, then * {@code x.equals(z)} should return {@code true}. * <li>It is consistent: for any non-null reference values * {@code x} and {@code y}, multiple invocations of * {@code x.equals(y)} consistently return {@code true} * or consistently return {@code false}, provided no * information used in {@code equals} comparisons on the * objects is modified. * <li>For any non-null reference value {@code x}, * {@code x.equals(null)} should return {@code false}. * </ul> * * The {@code equals} method for class {@code Object} implements * the most discriminating possible equivalence relation on objects; * that is, for any non-null reference values {@code x} and * {@code y}, this method returns {@code true} if and only * if {@code x} and {@code y} refer to the same object * ({@code x == y} has the value {@code true}). * * Note that it is generally necessary to override the {@code hashCode} * method whenever this method is overridden, so as to maintain the * general contract for the {@code hashCode} method, which states * that equal objects must have equal hash codes. * * @param obj the reference object with which to compare. * @return {@code true} if this object is the same as the obj * argument; {@code false} otherwise. * @see #hashCode() * @see java.util.HashMap */
 public boolean equals(Object obj) { 
 return (this == obj);
 }

翻译版：

/** * 表明其他对象是否“等于”此对象. * * equals 方法对非空对象引用实现等价关系: * <ul> * <li> * 自反的: 对于任何非空引用值 x, 对于 x.equals(x) 应该 return true * <li> * 对称的: 对于任何非空引用值 x, y, 对于 x.equals(y) 语句 return true, 当且仅当 y.equals(x) 也 returns true 是成立时， * <li> * 可传递的: 对于任何非空引用值 x, y, z, 如果 * x.equals(y) returns true 并且 y.equals(z) returns true，则 * x.equals(z) 应该 return true * <li> * 一致的: 对于任何非空引用值 x, y, 多次调用 x.equals(y) 始终如一地 return true * 或者始终如一地 return false, 如果在 equals 方法在对象比较中未使用任何信息，则将修改这些对象的比较 * <li> * 对于任何非空引用值 x, 对于 x.equals(null) 应该 return false * </ul> * * Object 类的 equals 方法在对象上实现最具辨别力的可能等价关系；即, 对于任何非空引用值 x, y, 该方法 returns true 当且仅当 x, y 指向相同的 object。此时 x == y 的值也是 true * * !!!请注意，每当重写此 equals() 方法时, 重写 hashCode() 方法通常是必要的， * 以保持 hashcode() 方法的常规的约定，其中规定相等的对象必须具有相等的哈希代码 * * @param obj 要与之比较的引用对象 * @return 如果是同一个对象返回 true 否则返回 false * @see #hashCode() * @see java.util.HashMap */
 public boolean equals(Object obj) { 
 return (this == obj);
 }

最后一句话说明了

#### java.lang.Object.equals() 方法 ####

原版：

/** * 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}. * * 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 not 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> * * 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™ 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();

翻译版：

/** * 返回对象的哈希代码值。此方法支持哈希表，例如 java.util.HashMap * * 关于 hashCode() 常规的约定是: * <ul> * <li> * 每当在 Java 应用程序执行过程中在同一对象上多次调用它时, hashCode() 必须始终如一地 return 相同的整数, * 如果在 equals() 在对象比较中未使用任何信息，则该对象的比较将被修改。 * 从应用程序的一次执行到同一应用程序的另一次执行，这个整数不需要保持一致。 * <li> * 如果两个对象根据 equals(object) 方法相等，则对两个对象中的每个对象调用 hashcode() 方法 * 都必须产生相同的整数结果。 * <li>如果 java.lang.Object#equals(java.lang.Object) 方法中两个对象不相等，则调用两个对象，然后调用两个对象中的每一个对象的 HASCODE() 方法必须产生不同的整数结果。但是，程序员应该知道，为不同的对象生成不同的整数结果可能会提高哈希表的性能。 * </ul> * * 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™ 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();