在HashMap或HashTable，实现自定义类型做Key

在HashMap和HashTable中的key值是不允许重复的，否则新的value会覆盖旧的value，那么是如何判断key值是否重复的。我们先来看一下存值的put（）函数

public V put(K key, V value) {
    if (key == null)
        return putForNullKey(value);
    int hash = hash(key);
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        Object k;
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }

    modCount++;
    addEntry(hash, key, value, i);
    return null;
}

　　判断key是否存在的时候，是先比较key的hashcode，然后再比较相等或equals的，如果原本已经存在对应的key，则直接改变对应的value，并返回旧的value。而如果对应的key原本不存在的话将调用addEntry将对应的key-value添加到Map中。addEntry传递的参数hash就是对应key的hashCode。 

所以当用自定义类做key时，需要重写hashCode()和equals()方法才可以实现自定义键在HashMap中的查找。

例子如下：

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;

//没有重写hashcode和equals
class Person {
    String id;
    String name;
    public Person(String id, String name) {
        this.id = id;
        this.name = name;
    }
    public String toString() {
        return "id = " + id + " , name = " + name;
    }
}
//重写了hashcode和equals
class Student {
    String id;
    String name;
    public Student(String id, String name) {
        this.id = id;
        this.name = name;
    }
    public int hashCode() {
        return id.hashCode();
    }
    public boolean equals(Object ob) {
        Student student = (Student)ob;
        if(student.id.equals(this.id)) {
            return true;
        }else {
            return false;
        }
    }
    public String toString() {
        return "id = " + id + " , name = " + name;
    }
}
public class HashMapTest {
    public static void main(String[] args) {
       testHashMapWithoutEquals();
       testHashMapWithEquals();
    }
    public static void testHashMapWithoutEquals() {
        Map<Person, String> hMap = new HashMap<Person, String>();
        Person person1 = new Person("123", "Tom");
        Person person2 = new Person("123", "Tom");
        hMap.put(person1, "address");
        hMap.put(person2, "address");
        Iterator iterator = hMap.entrySet().iterator();
        while(iterator.hasNext()) {
            Map.Entry entry = (Map.Entry) iterator.next();
            Person key = (Person) entry.getKey();
            String val = (String) entry.getValue();
            System.out.println("key = " + key + "  value = " + val);
        }
    }
    public static void testHashMapWithEquals() {
        System.out.println("*********************");
        Map<Student, String> hMap = new HashMap<Student, String>();
        Student student1 = new Student("123", "Tom");
        Student student2 = new Student("123", "Tom");
        hMap.put(student1, "address");
        hMap.put(student2, "address");
        Iterator iterator = hMap.entrySet().iterator();
        while(iterator.hasNext()) {
            Map.Entry entry = (Map.Entry) iterator.next();
            Student key = (Student) entry.getKey();
            String val = (String) entry.getValue();
            System.out.println("key = " + key + "  value = " + val);
        }
    }
}

例子转载自：http://blog.csdn.net/achiberx/article/details/73655737