1:HashMap可以存放键值对,如果要以对象(自己创建的类等)作为键,实际上是以对象的散列值(以hashCode方法计算得到)作为键。hashCode计算的hash值默认是对象的地址值。
这样就会忽略对象的内容,不是以对象的内容来判断。如果要以对象的内容进行判断,就要覆盖掉对象原有的hashCode方法。
另外HashMap是以equals方法判断当前的键是否与表中存在的键是否相同,所以覆盖hashCode方法之后,还不能正常运行。还要覆盖equals方法
先判断hash(hash函数)值是否相同,相同了说明某个位置上有多个元素,再用equals(线性查找)方法判断。
来看下面一个天气预报系统的例子:
2:创建一个作为HashMap的键的对象Groundhog(土拔鼠)与Prediction(预报)对象联系起来,
package chapter17.nine; public class Groundhog { protected int number; public Groundhog(int n) { // TODO Auto-generated constructor stub number=n; } @Override public String toString() { // TODO Auto-generated method stub return "Groundhog #"+number; } } package chapter17.nine; import java.util.Random; public class Prediction { private static Random random=new Random(); //传递进参数,创建的随机数将会相同 private boolean shadow=random.nextDouble()>0.5; @Override public String toString() { // TODO Auto-generated method stub if(shadow){ return "Six more weeks of Winter!"; }else{ return "Early Spring"; } } }
3:创建一个测试类
public class SpringDetector { public static <T extends Groundhog> void detectSpring(Class<T> type) throws Exception, IllegalAccessException, IllegalArgumentException, InvocationTargetException{ Constructor<T> ghog=type.getConstructor(int.class); //含有int参数的构造函数 Map<Groundhog, Prediction> map=new HashMap<Groundhog, Prediction>(); for (int i = 0; i < 10; i++) { map.put(ghog.newInstance(i), new Prediction()); } System.out.println("map="+map); Groundhog gh=ghog.newInstance(3); System.out.println("Looking up prediction for "+gh); if(map.containsKey(gh)){ //map是根据对象生成的hascode地址判断对象是否相同的, //如果想让对象作为map的键,来判断对象是否相同。那么对象应该覆盖hashcode和equals方法。生成对象自己的hashcode吗。 //例如按照对象包含的int属性生成hashcode 吗等。HashMap使用equals判断当前的键是否与表中存在的键相同。所以要覆盖equals方法 System.out.println(map.get(gh)); }else{ System.out.println("Key not found"+gh); } } public static void main(String[] args) { try { detectSpring(Groundhog.class); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
测试结果:
map={Groundhog #1=Six more weeks of Winter!, Groundhog #4=Six more weeks of Winter!, Groundhog #5=Six more weeks of Winter!, Groundhog #3=Early Spring, Groundhog #8=Six more weeks of Winter!, Groundhog #7=Six more weeks of Winter!, Groundhog #0=Early Spring, Groundhog #2=Early Spring, Groundhog #9=Six more weeks of Winter!, Groundhog #6=Early Spring} Looking up prediction for Groundhog #3 Key not foundGroundhog #3
我们发现,map中并不包含Groundhog gh=ghog.newInstance(3);的键,可是实际上map中又添加了ghog.newInstance(3)的键。这是为什么呢?
因为实际上判断的对象的hash值。所有造成:第一个ghog.newInstance(3)的hash值不等于第二个ghog.newInstance(3)的hash值;
4:重写Groundhog并覆盖hashCode方法和equals方法
package chapter17.nine; public class Groundhog2 extends Groundhog{ public Groundhog2(int n) { super(n); // TODO Auto-generated constructor stub } @Override public int hashCode() { // TODO Auto-generated method stub return number; } @Override public boolean equals(Object obj) { // TODO Auto-generated method stub return obj instanceof Groundhog2 && number==((Groundhog2)obj).number; } }
5:测试。
package chapter17.nine; import java.lang.reflect.InvocationTargetException; public class SpringDetector2 { public static void main(String[] args) { try { SpringDetector.detectSpring(Groundhog2.class); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
6:测试结果。
map={Groundhog #0=Six more weeks of Winter!, Groundhog #1=Early Spring, Groundhog #2=Early Spring, Groundhog #3=Early Spring, Groundhog #4=Early Spring, Groundhog #5=Six more weeks of Winter!, Groundhog #6=Early Spring, Groundhog #7=Early Spring, Groundhog #8=Early Spring, Groundhog #9=Six more weeks of Winter!} Looking up prediction for Groundhog #3 Early Spring
第一个ghog.newInstance(3)的hash值等于第二个ghog.newInstance(3)的hash值。定义作为HashMap键的对象成功。