# Map 集åˆ
Java8 针对 map æ“ä½œå¢žåŠ äº†ä¸€äº›æ–¹æ³•ï¼Œéžå¸¸æ–¹ä¾¿
1ã€åˆ é™¤å…ƒç´ ä½¿ç”¨`removeIf()`方法。
```java
/**
* @description:
* @author: 程åºå‘˜å¤§å½¬
* @time: 2021-09-07 00:03
*/
public class MapTest {
public static void main(String[] args) {
Map map = new HashMap<>();
map.put(1, "dabin1");
map.put(2, "dabin2");
//åˆ é™¤value没有å«æœ‰1的键值对
map.values().removeIf(value -> !value.contains("1"));
System.out.println(map);
}
/**
* output
* {1=dabin1}
*/
}
```
2ã€`putIfAbsent(key, value) ` 如果指定的 key ä¸å˜åœ¨ï¼Œåˆ™ put 进去。
```java
/**
* @description:
* @author: 程åºå‘˜å¤§å½¬
* @time: 2021-09-07 00:08
*/
public class MapTest1 {
public static void main(String[] args) {
Map map = new HashMap<>();
map.put(1, "大彬1");
for (int i = 0; i < 3; i++) {
map.putIfAbsent(i, "大彬" + i);
}
map.forEach((id, val) -> System.out.print(val + ", "));
}
/**
* output
* 大彬0, 大彬1, 大彬2
*/
}
```
3ã€map 转æ¢ã€‚
```java
/**
* @author: 程åºå‘˜å¤§å½¬
* @time: 2021-09-07 08:15
*/
public class MapTest2 {
public static void main(String[] args) {
Map map = new HashMap<>();
map.put("1", 1);
map.put("2", 2);
Map newMap = map.entrySet().stream()
.collect(Collectors.toMap(e -> e.getKey(), e -> "大彬" + String.valueOf(e.getValue())));
newMap.forEach((key, val) -> System.out.print(val + ", "));
}
/**
* output
* 大彬1, 大彬2,
*/
}
```
4ã€mapé历。
```java
/**
* @author: 程åºå‘˜å¤§å½¬
* @time: 2021-09-07 08:31
*/
public class MapTest3 {
public static void main(String[] args) {
Map map = new HashMap<>();
map.put(1, "大彬1");
map.put(2, "大彬2");
//æ–¹å¼1
map.keySet().forEach(k -> {
System.out.print(map.get(k) + ", ");
});
//æ–¹å¼2
map.entrySet().iterator().forEachRemaining(e -> System.out.print(e.getValue() + ", "));
//æ–¹å¼3
map.entrySet().forEach(entry -> {
System.out.print(entry.getValue() + ", ");
});
//æ–¹å¼4
map.values().forEach(v -> {
System.out.print(v + ", ");
});
}
}
```