/**toString()方法的约定：结果应是一个简明但易于读懂。建议所有子类都重写此方法。
* 当一个println,字符串连接(+)操作符或assert的时候，toString方法会自动被调用。
*
* 在实际应用中，toString方法应该返回对象中包含的所有令人感兴趣的信息或摘要信息。
* 不管你是否决定指定返回值的格式，都应该在文档中明确地表明你的意图。
* 另外，为toString返回值中包含的所有信息都提供一种编程访问途径是一个好的做法，
* 这样可以让程序直接得到特定的数据，则无需要费力来解析这个字符串来获得。
*
*/
public class PhoneNumber {
private final short areaCode;
private final short exchange;
private final short extension;
public PhoneNumber(int areaCode, int exchange,
int extension) {
rangeCheck(areaCode, 999, "area code");
rangeCheck(exchange, 999, "exchange");
rangeCheck(extension, 9999, "extension");
this.areaCode = (short) areaCode;
this.exchange = (short) exchange;
this.extension = (short) extension;
}
private static void rangeCheck(int arg, int max,
String name) {
if (arg < 0 || arg > max)
throw new IllegalArgumentException(name +": " + arg);
}
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof PhoneNumber))
return false;
PhoneNumber pn = (PhoneNumber)o;
return pn.extension == extension &&
pn.exchange == exchange &&
pn.areaCode == areaCode;
}
// Page 39
public int hashCode() {
int result = 17;
result = 37*result + areaCode;
result = 37*result + exchange;
result = 37*result + extension;
return result;
}
// Page 43
/**
* Returns the string representation of this phone number.
* The string consists of fourteen characters whose format
* is "(XXX) YYY-ZZZZ", where XXX is the area code, YYY is
* the extension, and ZZZZ is the exchange. (Each of the
* capital letters represents a single decimal digit.)
*
* If any of the three parts of this phone number is too small
* to fill up its field, the field is padded with leading zeros.
* For example, if the value of the exchange is 123, the last
* four