第九条：覆盖equals时总要覆盖hashCode

文章40页有错误，m.get(new PhoneNumber(408,867,5309))应该改为m.get(new PhoneNumber(707,867,5309))，与文章上面的HashMap例子一致，对于文章中的代码，我自己手敲整理了下。

首先是PhoneNumber类，如下代码

public classPhoneNumber {

private shortareaCode;

private shortprefix;

private shortlineNumber;

publicPhoneNumber(intareaCode, intprefix, intlineNumber){

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 voidrangeCheck(intarg, intmax,String name){

if(arg <0|| arg > max){

throw newIllegalArgumentException(name +":"+ arg);

}

}

@Override

public booleanequals(Object o) {

if(o ==this){

return true;

}

if(!(oinstanceofPhoneNumber)){

return false;

}

PhoneNumber pn = (PhoneNumber)o;

returnpn.lineNumber==lineNumber&& pn.prefix==prefix&& pn.areaCode==areaCode;

}

private volatile inthashCode;// 轻量级线程安全

@Override

public inthashCode() {

intresult =hashCode;

if(result ==0){

result =17;// 可以任意不等于0的值

intprimeNumber =31;// 奇素数

result = primeNumber * result +areaCode;

result = primeNumber * result +prefix;

result = primeNumber * result +lineNumber;

hashCode= result;

}

returnresult;

}

@Override

publicStringtoString() {

returnString.format("(%03d) %03d-%04d",areaCode,prefix,lineNumber);

}

}

调用如上代码，如下

Map m =newHashMap();

m.put(newPhoneNumber(707,867,5309),"Jenny");// 存储到map中

m.get(newPhoneNumber(707,867,5309));// 从map中取出