Map集合

Map接口主要实现类有HashMap,HashTable,TreeMap.

键要唯一，值可以一样，若是键一致，后面存入的会覆盖前面的。

HashMap,HashTable区别：HashMap可以键值都可以是null，线程是不同步的；而HashTable的键值都不可以为null，线程是同步的。

Hash比较唯一性是hashCode();equals()方法；map有两种迭代取出，entrySet()，keySet.

entrySet()返回的是Set<Map.entry<K,V>>,存到set中的是KEY 和Value的对应关系。通过迭代器取出；取出的方法是getkey(),getValue();

keySet返回的是Key的Set集合；再通过get(key),返回值。

而TreeMap要键元素自身具有比较性，Comparable接口和comparaTo方法。或者集合有Comparator接口的compare方法。

public class HashMapTest {
public static void main(String[] args){
HashMap<Student,String> ha = new HashMap<Student,String>();

ha.put(new Student("lisi",23), "tianjing");
ha.put(new Student("wangwu",23), "beijian");
ha.put(new Student("zhaoliu",23), "nanjing");
ha.put(new Student("lisi",23), "shanghai");

/*
* 方式一：keySet
* */
/*
Set<Student> keySet = ha.keySet();
Iterator<Student> it = keySet.iterator();
while(it.hasNext()){
Student stu = it.next();
String add = ha.get(stu);
System.out.println("stu:"+stu.getName()+"---stu_add--"+add);
}
*/

/*
* 方式2：entrySet
* */
Set<Map.Entry<Student, String>> entry = ha.entrySet();
Iterator<Map.Entry<Student, String>> it = entry.iterator();
while(it.hasNext()){
Map.Entry<Student, String> entry1 = it.next();
System.out.println("---Key--"+entry1.getKey().getName()+"---value---"+entry1.getValue());
}
}
}

class Student{
private String name;
private int age;

public Student(String name, int age) {
super();
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public int hashCode(){
System.out.println("调用hashcode--------");
return age;
}
public boolean equals(Object obj){
if(!(obj instanceof Student)){
throw new ClassCastException("不是学生类型");
}
System.out.println("调用equals----------");
Student stu = (Student)obj;
return this.name.equals(stu.getName());
}
}