理解 equals，hashcode

Effective Java:第九条：覆盖equals时总要覆盖hashcode.

如果不这样做的话，会违反Objcet.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工作。这样的集合包括HashMap，HashSet，HashTable。

下面是Object的约定规范:

1. 在应用程序执行期间，只要equals方法所用到的信息没有修改，那个对同一个对象多次调用，hashCode方法都必须始终如一的返回同一个整数。在同一个程序的多次执行过程中，每次执行所返回的整数可以不一致。
2. 如果两个对象根据equals（object）方法比较是相等的，那么调用这两个对象的任意一个对象的hashCode方法都必须产生同样的整数值。
3.  如果两个对象根据equals（object）方法比较是不相等的，那么调用这两个对象的任意一个对象的hashCode方法，则不一定要产生不同的整数值。但是程序员应该知道，给不相等的对象产生不同的hashcode，有可能提高散列表的性能。

Object hascode:=========

    /**
     * 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">^TM</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();


exmaple code :========= package com.kvmk.basic; 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; } 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))); // print null. } } </phonenumber></phonenumber>