覆盖equals时总要覆盖hasCode

最新推荐文章于 2022-09-14 09:28:53 发布

原创最新推荐文章于 2022-09-14 09:28:53 发布 · 639 阅读

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覆盖equals时总要覆盖hasCode

在每个覆盖了equals方法的类中，也必须覆盖hashCode方法。如果不这样做的话，违反Object hashCode的通用约定，从而导致该类无法结合所有基于散列的集合一起正常运作，这样的集合包括HashMap，HashSet，Hashtable。

例如：

import java.util.*;

public final class PhoneNumber {
    private final short areaCode;
    private final short prefix;
    private final short lineNumber;

    public PhoneNumber(int areaCode, int prefix,
                       int lineNumber) {
        rangeCheck(areaCode,    999, "area code");
        rangeCheck(prefix,      999, "prefix");
        rangeCheck(lineNumber, 9999, "line number");
        this.areaCode  = (short) areaCode;
        this.prefix  = (short) prefix;
        this.lineNumber = (short) lineNumber;
    }

    private static void rangeCheck(int arg, int max,
                                   String name) {
        if (arg < 0 || arg > max)
           throw new IllegalArgumentException(name +": " + arg);
    }

    @Override public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof PhoneNumber))
            return false;
        PhoneNumber pn = (PhoneNumber)o;
        return pn.lineNumber == lineNumber
            && pn.prefix  == prefix
            && pn.areaCode  == areaCode;
    }

    // Broken - no hashCode method!

    // A decent hashCode method - Page 48
//  @Override public int hashCode() {
//      int result = 17;
//      result = 31 * result + areaCode;
//      result = 31 * result + prefix;
//      result = 31 * result + lineNumber;
//      return result;
//  }


    // Lazily initialized, cached hashCode - Page 49
//  private volatile int hashCode;  // (See Item 71)
//
//  @Override public int hashCode() {
//      int result = hashCode;
//      if (result == 0) {
//          result = 17;
//          result = 31 * result + areaCode;
//          result = 31 * result + prefix;
//          result = 31 * result + lineNumber;
//          hashCode = result;
//      }
//      return result;
//  }

    public static void main(String[] args) {
        Map<PhoneNumber, String> m
            = new HashMap<PhoneNumber, String>();
        m.put(new PhoneNumber(707, 867, 5309), "Jenny");
        System.out.println(m.get(new PhoneNumber(707, 867, 5309)));
    }
}

console控制台如下：

Connected to the target VM, address: '127.0.0.1:51509', transport: 'socket'
null
Disconnected from the target VM, address: '127.0.0.1:51509', transport: 'socket'

Process finished with exit code 0

很奇怪，为什么

m.get(new PhoneNumber(707, 867, 5309))  拿不到  "Jenny"

hasMap put 进去的是什么 get的时候是什么

或者说 get的时候，根据get的值对象，没有找到，返回一个null

Object.java

 /**
     * Returns a hash code value for the object. This method is 
     * supported for the benefit of hashtables such as those provided by 
     * <code>java.util.Hashtable</code>. 
     * <p>
     * The general contract of <code>hashCode</code> is: 
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during 
     *     an execution of a Java application, the <tt>hashCode</tt> method 
     *     must consistently return the same integer, provided no information 
     *     used in <tt>equals</tt> 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 <tt>equals(Object)</tt>
     *     method, then calling the <code>hashCode</code> 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 <tt>hashCode</tt> 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 hashtables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by 
     * class <tt>Object</tt> 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<font size="-2"><sup>TM</sup></font> programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.util.Hashtable
     */
    public native int hashCode();

打开覆盖的hasCode，

console控制台：

Connected to the target VM, address: '127.0.0.1:51736', transport: 'socket'
Jenny
Disconnected from the target VM, address: '127.0.0.1:51736', transport: 'socket'

Process finished with exit code 0

很土的理解：散性函数返回结果不一样。导致get拿到的是空。

重写覆盖后，我们能正确的拿到自己想要的Value

相等的实例是不是具有相等的散列码？