package cn.xf.cp.ch02.item9;import java.util.HashMap;import java.util.Map;public 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;    }        /*    @Override    //至于为什么使用31，这个是推荐值，研究表明这个数字用起来性能比较好    public int hashCode()    {        int result = 17;        result = 31 * result + areaCode;        result = 31 * result + prefix;        result = 31 * result + lineNumber;        return result;    }    */        //如果一个对象不是经常变动，而且开销比较大的话，就要考虑吧散列码缓存在对象内部    //用volatile修饰的变量，线程在每次使用变量的时候，都会读取变量修改后的最的值。    private volatile int hashcode;        @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");        //这里不会返回jenny哦，会返回null，这个是因为put对象吧他们放到不同的散列桶中        System.out.println(m.get(new PhoneNumber(707, 867, 5309)));    }}