Java HasCode equals == 的區別

==

用來判斷兩個值，或者兩個對象的內存地址是否一樣。

equals

equals 方法用來判斷兩個對象是否相等。equals 是Object 類的方法，默認情況下，比較兩個對象是否是同一個對象，內部通過 == 實現。如果想比較兩個對象的其他內容，則可以通過重寫equals 方法。比如String 就重寫了equals 方法。

equals是Object類的方法，默認情況下比較兩個對象是否是同一個對象，內部實現是通過“==”來實現的。

如果想比較兩個對象的其他內容，則可以通過重寫equals方法，

例如：String類就重寫了equals方法，改成了對象的內容是否相等。

hasCode方法：

    /**
     * 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 int hashCode() {
        return identityHashCode(this);
    }

hasCode 這個方法會在HasMap 裏面使用到。
通常來說，在一次程序運行期間，一個對象的hasCode 是不變的。在不同的程序運行期間可以不同。
如果兩個對象的equals的方法返回的值是相等的，那麼hasCode 就應該返回一樣的值。

不同的對象的hasCode可能是一樣的。但是不同對象返回不同hasCode 能夠提高has 表的性能。

同一個類，new 出來兩個實例，他們倆的hasCode 是不一樣的。