Java equals 方法與hashcode 方法的深入解析

面試時經常會問起字符串比較相關的問題,比如：字符串比較時用的什麼方法,內部實現如何?hashcode的作用,以及重寫equal方法,為什麼要重寫hashcode方法?以下就為大家解答，需要的朋友可以參考下  

PS:本文使用jdk1.7
解析
1.Object類 的equals 方法

復制代碼 代碼如下:
   /**
     * 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);
    }

看代碼,Object的equals方法,采用== 進行比較,只是比較對象的引用,如果引用的對象相同,那麼就返回true.
看注釋,Object的equals方法,具有如下特性
1.reflexive-自反性 
 x.equals(x)  return true
2.symmetric-對稱性
x.equals(y)  return true
y.equals(x)  return true
3.transitive-傳遞性
x.equals(y)  return true
y.equals(z)  return true
x.equals(z)  return true
4.consistent-一致性
x.equals(y)  return true //那麼不管調用多少次,肯定都是返回true
5.與null的比較
x.equals(null) return false //對於none-null的x對象,每次必然返回false
6.於hashcode的關系
     * 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.
需要注意的是,一般來說,如果重寫了equals方法,都必須要重寫hashcode方法,
來確保具有相同引用的對象,能夠具有同樣的hashcode值
好了,看到這裡,我們就明白了,為什麼重寫了equals方法,一般來說就需要重寫hashcode方法,
雖然這個不是強制性的,但是如果不能保證相同的引用對象,沒有相同的hashcode,會對系統留下很大隱患
2.String類的equals方法

復制代碼 代碼如下:
   /**
     * Compares this string to the specified object.  The result is {@code
     * true} if and only if the argument is not {@code null} and is a {@code
     * String} object that represents the same sequence of characters as this
     * object.
     *
     * @param  anObject
     *         The object to compare this {@code String} against
     *
     * @return  {@code true} if the given object represents a {@code String}
     *          equivalent to this string, {@code false} otherwise
     *
     * @see  #compareTo(String)
     * @see  #equalsIgnoreCase(String)
     */
    public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String) anObject;
            int n = value.length;
            if (n == anotherString