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

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.
     * <p>
     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
     * <li>It is <i>symmetric</i>: 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 <i>transitive</i>: 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 <i>consistent</i>: 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>
     * <p>
     * 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}).
     * <p>
     * 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.
     * <p>
     *  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>
     * <p>
     * 	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
     * <p>
     *	 !!!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}.
     * <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();

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
     * <p>
     * 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>
     * <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();