1.putIfAbsent

// 如果这个key不存在，就put进去

import java.util.HashMap;
import java.util.Map;

/** * @author bincai, bincai@mobvoi.com * @date Oct 08 , 2018 */
public class Run {

  public static void main(String[] args) throws Exception {

    Map<Integer, String> map = new HashMap<>();
    map.put(1,"bincai");
    for (int i = 0; i < 3; i++) {
      map.putIfAbsent(i, "val" + i);
    }
    map.forEach((id, val) -> System.out.println(val));
  }
}

输出：

val0
bincai
val2

2.computeIfAbsent、computeIfPresent、compute

⑴compute

对已经存在和未存在的key都进行操作。

⑵computeIfPresent

computeIfPresent 的方法,对 指定的 在map中已经存在的key的value进行操作。
统计单词sed和magna出现的次数:

public static void main(String[] args) {

        Map<String, Integer> wordCounts = new ConcurrentHashMap<>(10);
        String s =
                "Lorem ipsum dolor sit amet consetetur iam nonumy sadipscing " +
                        " elitr, sed diam nonumy eirmod tempor invidunt ut erat sed " +
                        "labore et dolore magna dolor sit amet aliquyam erat sed diam";

        wordCounts.put("sed", 0);
        wordCounts.put("magna",0);
        for (String t : s.split(" ")) {
            wordCounts.computeIfPresent(t, (k, v) -> v + 1);
        }
        System.out.println(wordCounts);
    }

⑶computeIfAbsent

computeIfAbsent 的方法,对 指定的 在map中未存在的key的value进行操作。
加入我们有几个学生需要根据男女分组：

 public static void main(String[] args) {

        //学生的集合
        List<Student> students = new ArrayList<>();
        students.add(new Student("张三","男",18));
        students.add(new Student("李四","男",20));
        students.add(new Student("韩梅梅","女",18));
        students.add(new Student("小红","女",45));

        //声明接收结果的 map
        Map<String, List<Student>> resultMap = new HashMap<>();
        for (Student student : students) {
            List<Student> s = resultMap.computeIfAbsent(student.getSex(), k -> new ArrayList<>());
            s.add(student);
        }

        System.out.println(resultMap);
    }

3.remove

根据value移除

private static void testRemove(){
    Map<Integer, String> map = new HashMap<>();
    for (int i = 0; i < 3; i++) {
      map.putIfAbsent(i, "val" + i);
    }
    map.remove(0,"val1");
    map.remove(2,"val2");
    // 可见,key为2的被移除
    map.forEach((id, val) -> System.out.println(val));
  }

输出：

val0
val1

4.getOrDefault

取不到就取默认值

   private static void testGetOrDefault(){
    Map<Integer, String> map = new HashMap<>();
    for (int i = 0; i < 3; i++) {
      map.putIfAbsent(i, "val" + i);
    }
    System.out.println(map.getOrDefault(1,"bincai"));
    System.out.println(map.getOrDefault(10,"bincai"));
  }

输出:

val1
bincai

5.getOrDefault

  private static void testMerge(){
    Map<Integer, String> map = new HashMap<>();
    map.merge(9, "val9", (value, newValue) -> value.concat(newValue));
    System.out.println(map.get(9));             // val9

    map.merge(9, "concat", (value, newValue) -> value.concat(newValue));
    System.out.println(map.get(9));               // val9concat
  }

输出:

val9
val9concat

6.遍历map

  private static void bianlimap(){
    Map<String, Object> map = new HashMap<>();
    map.put("key1", "value1");
    map.put("key2", "value2");
    map.put("key3", "value3");
    map.put("key4", 4);
    map.put("key5", 5);
    map.put("key5", 'h');
    map.keySet().forEach(key -> System.out.println("map.get(" + key + ") = " + map.get(key)));
    map.entrySet().iterator().forEachRemaining(item -> System.out.println("key:value=" + item.getKey() + ":" + item.getValue()));
    // 推荐使用
    map.entrySet().forEach(entry -> System.out.println("key:value = " + entry.getKey() + ":" + entry.getValue()));
    map.values().forEach(System.out::println); // 等价于map.values().forEach(value -> System.out.println(value));
    map.forEach((k, v) -> System.out.println("key:value = " + k + ":" + v));
  }

输出:

map.get(key1) = value1
map.get(key2) = value2
map.get(key5) = h
map.get(key3) = value3
map.get(key4) = 4
key:value=key1:value1
key:value=key2:value2
key:value=key5:h
key:value=key3:value3
key:value=key4:4
key:value = key1:value1
key:value = key2:value2
key:value = key5:h
key:value = key3:value3
key:value = key4:4
value1
value2
h
value3
4
key:value = key1:value1
key:value = key2:value2
key:value = key5:h
key:value = key3:value3
key:value = key4:4