Java equals 方法与hashcode 方法的深入解析

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Java equals 方法与hashcode 方法的深入解析

来源:互联网 作者:脚本宝典 时间:2015-07-29 17:40 【

PS:本文使用jdk1.7 解析 1.Object类 的equals 方法 复制代码 代码如下: /** * Indicates whether some other object is equal to this one. * p * The [email protected] equals} method implements an equivalence relation * on non-null object ref

PS:本文使用jdk1.7
解析
1.Object类 的equals 方法

复制代码代码如下:


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


看代码,Object的equals方法,采用== 进行比较,只是比较对象的引用,如果引用的对象相同,那么就返回true. 
看注释,Object的equals方法,具有如下特性
1.reflexive-自反性  
 x.equals(x)  return true
2.symmetric-对称性
x.equals(y)  return true
y.equals(x)  return true
3.transitive-传递性
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 //那么不管调用多少次,肯定都是返回true
5.与null的比较
x.equals(null) return false //对于none-null的x对象,每次必然返回false
6.于hashcode的关系
     * Note that it is generally necessary to override the [email protected] hashCode}
     * method whenever this method is overridden, so as to maintain the
     * general contract for the [email protected] hashCode} method, which states
     * that equal objects must have equal hash codes.
需要注意的是,一般来说,如果重写了equals方法,都必须要重写hashcode方法,
来确保具有相同引用的对象,能够具有同样的hashcode值
好了,看到这里,我们就明白了,为什么重写了equals方法,一般来说就需要重写hashcode方法,
虽然这个不是强制性的,但是如果不能保证相同的引用对象,没有相同的hashcode,会对系统留下很大隐患
2.String类的equals方法

复制代码代码如下:


   /**
     * Compares this string to the specified object.  The result is [email protected]
     * true} if and only if the argument is not [email protected] null} and is a [email protected]
     * String} object that represents the same sequence of characters as this
     * object.
     *
     * @param  anObject
     *         The object to compare this [email protected] String} against
     *
     * @return  [email protected] true} if the given object represents a [email protected] String}
     *          equivalent to this string, [email protected] 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.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                            return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }


看源码,我们可以发现,这个比较分为两部分
1.先比较是否引用同一对象
2.如果引用对象不同,是否两个String的content相同
3,String 类的hashcode 方法

复制代码代码如下:


    /**
     * Returns a hash code for this string. The hash code for a
     * <code>String</code> object is computed as
     * <blockquote><pre>
     * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
     * </pre></blockquote>
     * using <code>int</code> arithmetic, where <code>s[i]</code> is the
     * <i>i</i>th character of the string, <code>n</code> is the length of
     * the string, and <code>^</code> indicates exponentiation.
     * (The hash value of the empty string is zero.)
     *
     * @return  a hash code value for this object.
     */
    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;
            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }


可以看到hashcode的计算公式为:s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] 
因此,对于同一个String,得出的hashcode必然是一致的
另外,对于空的字符串,hashcode的值是0


小结
至此,我们可以对本文开头的疑问做一个小结.
1.字符串比较时用的什么方法,内部实现如何?
使用equals方法,先比较引用是否相同,后比较内容是否一致.

2.hashcode的作用,以及重写equal方法,为什么要重写hashcode方法?
hashcode是系统用来快速检索对象而使用,equals方法是用来判断引用的对象是否一致,所以,当引用对象一致时,必须要确保其hashcode也一致,因此需要重写hashcode方法来确保这个一致性 


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