使用对象作为hashMap的键，需要覆盖hashcode和equals方法

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。