Java中Map的相关用法

把一个Map转化为List

map转List有以下三种转化方式：

• 把map的键key转化为list
• 把map的值value转化为list
• 把map的键值key-value转化为list

public class Test {
    public static void main(String[] args) {
        Map<Integer, String> map = new HashMap<>();
        map.put(2, "jay");
        map.put(1, "whx");
        map.put(3, "huaxiao");
        //把一个map的键转化为list
        List<Integer> keyList = new ArrayList<>(map.keySet());
        System.out.println(keyList);
        //把map的值转化为list
        List<String> valueList = new ArrayList<>(map.values());
        System.out.println(valueList);
        把map的键值转化为list
        List entryList = new ArrayList(map.entrySet());
        System.out.println(entryList);

    }
}

遍历一个Map

entrySet+for实现遍历

ublic class EntryMapTest {
    public static void main(String[] args) {
        Map<Integer, String> map = new HashMap<>();
        map.put(2, "jay");
        map.put(1, "whx");
        map.put(3, "huaxiao");

        for(Map.Entry entry: map.entrySet()) {
            // get key
            Integer key = (Integer) entry.getKey();
            // get value
            String value = (String) entry.getValue();

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

Iterator+while实现遍历

public class IteratorMapTest {
    public static void main(String[] args) {
        Map<Integer, String> map = new HashMap<>();
        map.put(2, "jay");
        map.put(1, "whx");
        map.put(3, "huaxiao");

        Iterator itr = map.entrySet().iterator();
        while(itr.hasNext()) {
            Map.Entry entry = (Map.Entry) itr.next();
            // get key
            Integer key = (Integer) entry.getKey();
            // get value
            String value = (String) entry.getValue();

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

根据Map的keys进行排序

把Map.Entry放进list，再用Comparator对list进行排序

# 伪代码
List list = new ArrayList(map.entrySet());
Collections.sort(list, (Entry e1, Entry e2)-> {
    return e1.getKey().compareTo(e2.getKey());
});

public class SortKeysMapTest {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("2010", "jay");
        map.put("1999", "whx");
        map.put("3010", "huaxiao");

        List<Map.Entry<String,String>> list = new ArrayList<>(map.entrySet());
        Collections.sort(list, (Map.Entry e1, Map.Entry e2)-> {
                return e1.getKey().toString().compareTo(e2.getKey().toString());
        });

        for (Map.Entry entry : list) {
            System.out.println("key:" + entry.getKey() + ",value:" + entry.getValue());
        }

    }
}

使用SortedMap+TreeMap+Comparator实现

#伪代码
SortedMap sortedMap = new TreeMap(new Comparator() {
  @Override
  public int compare(K k1, K k2) {
    return k1.compareTo(k2);
  }
});
sortedMap.putAll(map);

public class SortKeys2MapTest {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("2010", "jay");
        map.put("1999", "whx");
        map.put("3010", "huaxiao");

        SortedMap sortedMap = new TreeMap(new Comparator<String>() {
            @Override
            public int compare(String k1, String k2) {
                return k1.compareTo(k2);
            }
        });
        sortedMap.putAll(map);

        Iterator itr = sortedMap.entrySet().iterator();
        while(itr.hasNext()) {
            Map.Entry entry = (Map.Entry) itr.next();
            // get key
            String key = (String) entry.getKey();
            // get value
            String value = (String) entry.getValue();

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

对Map的values进行排序

#伪代码
List list = new ArrayList(map.entrySet());
Collections.sort(list, (Entry e1, Entry e2) ->{
    return e1.getValue().compareTo(e2.getValue());
  });

public class SortValuesMapTest {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("2010", "jay");
        map.put("1999", "whx");
        map.put("3010", "huaxiao");

        List <Map.Entry<String,String>>list = new ArrayList<>(map.entrySet());
        Collections.sort(list, (Map.Entry e1, Map.Entry e2)-> {
                return e1.getValue().toString().compareTo(e2.getValue().toString());
            }
        );

        for (Map.Entry entry : list) {
            System.out.println("key:" + entry.getKey() + ",value:" + entry.getValue());
        }
    }
}

结果：
key:3010,value:huaxiao
key:2010,value:jay
key:1999,value:whx

Map各种类型区别

	HashMap	TreeMap	Hashtable	ConcurrentHashMap
有序性	否	是	否	否
null k-v	是-是	否-是	否-否	否-否
线性安全	否	否	是	是
时间复杂度	O（1）	O（log n）	O(1)	O（log n）
底层结构	数组+链表+红黑树	红黑树	数组+链表	数组+链表+红黑树