Java的Map集合常用方法

import java.util.*;
public class Main {
	public static void main(String[]args) {
		HashMap<Integer,Double> map=new HashMap<>();
		map.put(1, 2.4);
		map.put(6, 2.5);
		map.put(3, 6.8);
		System.out.println("①"+map);
		//遍历根据主键输出值
		Set<Integer> set=map.keySet();
		System.out.println("②"+map.keySet());
		for(Integer a:set) {
			Double value=map.get(a);
			System.out.println("③"+value);
		}
		//遍历值集合
		for ( Object value : map.values() ) {
			System.out.println(value);
			}

		//遍历输出键和值
		Set<Map.Entry<Integer, Double>> entries=map.entrySet();
		System.out.println(entries);
		for(Map.Entry<Integer, Double> entry:entries) {
			Integer key=entry.getKey();
			Double value=entry.getValue();
			System.out.println("④"+"key=" + key + " value=" + value);
		}
		//Iterator遍历
		Iterator it=entries.iterator();
		while(it.hasNext()) {
			Map.Entry me = (Map.Entry) it.next(); // ③ 获得键值对对象
			Object key = me.getKey(); // ④ 从键值对对象中获取key值
			Object value = me.getValue(); // ⑤ 根据key获得对应的value值
			System.out.println("key=" + key + " value=" + value);
		}
		//判断是否含义键
		System.out.println("⑤"+map.containsKey(1));
		//判断是否含义值
		System.out.println("⑥"+map.containsValue(2.4));
		//map.put(key,value) 可以添加 map.remove(key) 可以移除，map.clear() 可以清空 map.size()获取map长度
		
		//排序使用TreeMap
		//根据键值key
		Map<Integer,Double> tree=new TreeMap<>(
			new Comparator<Integer>() {
				@Override
				public int compare(Integer o1,Integer o2) {
					return o2-o1;//降序，升序不用写默认o1-o2 Map<Integer,Double> tree=new TreeMap();
				}
			});
		tree.putAll(map);
		System.out.println(tree);
		//根据value排序
		List<Map.Entry<Integer, Double>> list=new ArrayList<>(map.entrySet());
		Collections.sort(list,(o1,o2)->(int) (o1.getValue()-o2.getValue()));//o1.getValue()-o2.getValue()升序，反正降序
		for(Map.Entry i:list) {
			System.out.println("key   "+i.getKey()+" value "+i.getValue());
		}
		
	}
}

输出：
①{1=2.4, 3=6.8, 6=2.5}
②[1, 3, 6]
③2.4
③6.8
③2.5
2.4
6.8
2.5
[1=2.4, 3=6.8, 6=2.5]
④key=1 value=2.4
④key=3 value=6.8
④key=6 value=2.5
key=1 value=2.4
key=3 value=6.8
key=6 value=2.5
⑤true
⑥true
{6=2.5, 3=6.8, 1=2.4}
key 1 value 2.4
key 6 value 2.5
key 3 value 6.8