Item 9 覆盖equals时总要覆盖hashCode

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;
  }

  @Override
  public int hashCode() {
    return new HashCodeBuilder(17, 37).append(lineNumber).append(prefix ).append(areaCode)
        .toHashCode();
  }

  // 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)));
  }
}