【Java】Hashcode

定义

在java.lang包下的Object类中注释了HashCode的定义：

/**
 * 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}.
 * 

 * The general contract of {@code hashCode} is:
 * 

 * 
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.
 * 
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.
 * 
It is not 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.
 * 
 * 

 * 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™ 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();

从上面的注释中我们可以知道：

1. HashCode的存在主要是为了查找的快捷性；
2. 如果两个对象相同（equals相等），那么他们的HashCode一定相同；
3. 如果重写了类的equals()方法，建议同时重写类的hashCode()方法，务必不要违反第 2 点；
4. HashCode相同的两个对象并不一定相同，只能断定他们存放在散列储存结构中的相同位置。

例子

针对第 4 点，就拿常用的String类举一个例子。

在String类中，hashCode()方法是这样的：

/**
 * Returns a hash code for this string. The hash code for a
 * {@code String} object is computed as
 * 
 * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
 * 
 * using {@code int} arithmetic, where {@code s[i]} is the
 * ith character of the string, {@code n} is the length of
 * the string, and {@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;
}

照着上面的HashCode计算方式，我们可以很简单地举出反例：

package com.vingyun;

public class HashCodeTest {

    public static void main(String[] args) {
        // TODO 自动生成的方法存根

        String vingStr = "Ving";
        String mirrorStr = "WKPH";

        System.out.println("vingStr.equals(mirrorStr): " + vingStr.equals(mirrorStr));

        System.out.print("vingStr.hashCode() == mirrorStr.hashCode(): ");
        System.out.println(vingStr.hashCode() == mirrorStr.hashCode());

    }
}

用途

主要在散列储存结构中使用，例如：HashSet、HashMap等。