HashCode和Equals的区别

我们首先来看一下HashCode的源码：

    /**
     * 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();

刨到这种地步，也差不多了，我们看一下上面的注释是啥意思？

返回对象的Hash Code值，支持此方法的好处是可以使用{@link java.util.HashMap}.提供的哈希表

每当在Java应用程序执行期间在同一个对象上多次调用该方法时，
{@code hashCode}方法必须一致地返回相同的整数，
前提是不修改对象上的{@code =}比较中使用的信息。
从应用程序的一次执行到同一应用程序的另一次执行，该整数不必保持一致。

String string = new String();
string="bbbb";
System.out.println(string.hashCode());
string="dddd";
System.out.println(string.hashCode());

如果根据{@code equals(Object)}方法两个对象相等，
那么在两个对象上调用{@code hashCode}方法必须产生相同的整数结果。

如果根据{@link java.lang.Object#equals(java.lang.Object)}方法两个对象不相等，
那么在两个对象上调用{@code hashCode}方法必须产生不同的整数结果，
这是不必要的。但是，程序员应该意识到，为不相等的对象生成不同的整数结果可能会提高哈希表的性能。

在相当实际的情况下，类{@code Object}定义的hashCode方法确实为不同的对象返回不同的整数。
(这通常是通过将对象的内部地址转换为整数来实现的，
但是Java&trade并不需要这种实现技术;编程语言)。

/**
     * 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);
    }

1、{@code =}方法在非空对象引用上实现等价关系:
2、它是自反的:对于任何非空引用值{@code x}，
  {@code x.equals(x)}应该返回{@code true}。
3、它是对称的:对于任何非空的引用值{@code x}和{@code y}，
  当且仅当{@code y = (x)}返回{@code true}时，
  {@code x.equals(y)}应该返回{@code true}。
  


4、 它是传递的:对于任何非空的参考值{@code x}， {@code y}， {@code z}，如果{@code x.equals(y)}返回{@code true}，
{@code y.equals(z)}返回{@code true}，那么{@code x.equals(z)}应该返回{@code true}。

5、它是一致的:对于任何非空引用值{@code x}和{@code y}，对{@code x.equals(y)}的多次调用
一致返回{@code true}或一致返回{@code false}，但在对象被修改。
6、对于任何非空引用值{@code x}， {@code x.equals(null)}应该返回{@code false}。

7、{@code eauals}方法类{@codeObject}实现最歧视可能等价关系对象,也就是说,对于任何非空引用值{@code x}和
{@code y},该方法返回{@code true}当且仅当{@code x}和{@code y}引用同一个对象({@code x = = y}
{@code true})的值。

8、注意，通常需要在重写该方法时重写{@code hashCode}方法，以便维护{@code hashCode}方法的通用契约，
该契约声明相等的对象必须具有相等的散列代码。