Java 覆盖equals时总要覆盖hashcode

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)));
  }
}