Java---29---Map中元素的两种取出方式

Map中是没有迭代器的，那么Map中的元素是如何取出的呢？

有两种方法：

一种是通过Set <k> keySet ()

一种是通过 Set <Map.Entry<k,v>> entrySet ()

keySet:： 将map中所有的键存入到Set集合中。在通过map的getKey （）方法即可获得 value的值

entrySet: 返回此映射中包含的映射关系的Set视图。既包含键也包含值。 这个映射关系的数据类型就是Map.Entry.

public class MapDemo {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		keySet_method();
		entrySet_method();
	}

	public static void entrySet_method() {
		Map<String, String> map = new HashMap<String, String>();

		map.put("01", "a5");
		map.put("02", "a1");
		map.put("03", "a2");
		map.put("04", "a3");
		map.put("05", "a4");

		Set<Map.Entry<String, String>> entryset = map.entrySet();

		Iterator<Map.Entry<String, String>> it = entryset.iterator();

		while (it.hasNext()) {
			Map.Entry<String, String> me = it.next();

			String key = me.getKey();
			String value = me.getValue();

			System.out.println("key : " + key + "    Value : " + value);
		}
	}

	public static void keySet_method() {
		Map<String, String> map = new HashMap<String, String>();

		map.put("01", "a5");
		map.put("02", "a1");
		map.put("03", "a2");
		map.put("04", "a3");
		map.put("05", "a4");

		Set<String> keyset = map.keySet();

		Iterator<String> it = keyset.iterator();

		while (it.hasNext()) {
			// 获取键
			String key = it.next();

			String value = map.get(key);
			System.out.println("key : " + key + "    Value : " + value);
		}

	}

}

存入自定义元素，通过两种方式来取出：

class Stu implements Comparable<Stu> {
	private String name;
	private int age;

	public Stu(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;
	}

	@Override
	public boolean equals(Object obj) {
		// TODO Auto-generated method stub
		if (!(obj instanceof Stu))
			throw new ClassCastException("不是本类的对象");
		Stu s = (Stu) obj;
		return this.name.equals(s.name) && this.age == s.age;
	}

	@Override
	public int hashCode() {
		// TODO Auto-generated method stub

		return name.hashCode() + age * 27;
	}

	public int compareTo(Stu o) {
		// TODO Auto-generated method stub

		Integer num = new Integer(this.age).compareTo(new Integer(o.age));

		if (num == 0)
			return this.name.compareTo(o.name);

		return num;
	}

}

public class MapDemo2 {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Map<Stu, String> map = new HashMap<Stu, String>();

		map.put(new Stu("01", 10), "beijing");
		map.put(new Stu("02", 9), "shanghai");
		map.put(new Stu("03", 8), "huangzhou");
		map.put(new Stu("04", 15), "tianjin");

		Set<Stu> keySet = map.keySet();

		Iterator<Stu> it = keySet.iterator();
		while (it.hasNext()) {
			Stu key = it.next();
			String value = map.get(key);

			System.out.println("key : " + key.getName() + "---" + key.getAge()
					+ "    Value : " + value);
		}
		/*************************************************/
		Set<Map.Entry<Stu, String>> entrySet = map.entrySet();

		Iterator<Map.Entry<Stu, String>> iterator = entrySet.iterator();

		while (iterator.hasNext()) {
			Map.Entry<Stu, String> me = iterator.next();

			Stu key = me.getKey();
			String value = me.getValue();

			System.out.println("key : " + key.getName() + "---" + key.getAge()
					+ "    Value : " + value);
		}
	}

}