Java8常用Lamda表达式(持续更新)
package lamda;import lombok.Builder;
import lombok.Data;
import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;
/**
* @describe: Lamda 表达式
* @author:（willdas）
**/
public class Demo {
public static void main(String[] args) {
listToMap();
mapToList();
listSort();
}
/**
* 1、List转Map
*/
public static void listToMap() {
//(1)
List<String> stringList = new ArrayList<>();
stringList.add("1");
stringList.add("2");
stringList.add("3");
Map<String, String> stringMap = stringList.stream().collect(Collectors.toMap(k -> k, k -> k));
//{1=1, 2=2, 3=3}
System.out.println(stringMap);
//(2)
List<People> entityList = new ArrayList<>();
entityList.add(People.builder().userId(1).userAge(21).userName("张三").build());
entityList.add(People.builder().userId(2).userAge(22).userName("李四").build());
entityList.add(People.builder().userId(3).userAge(23).userName("王五").build());
Map<Integer, People> peopleMap1 = entityList.stream().collect(Collectors.toMap(People::getUserId, p -> p));
Map<Integer, People> peopleMap2 = entityList.stream().collect(Collectors.toMap(p -> p.getUserId(), p -> p));
//避免键重复
Map<Integer, People> peopleMap3 = entityList.stream().collect(Collectors.toMap(p -> p.getUserId(), Function.identity(), (k, v) -> k));
//{1=People(userId=1, userAge=21, userName=张三), 2=People(userId=2, userAge=22, userName=李四), 3=People(userId=3, userAge=23, userName=王五)}
System.out.println(peopleMap1);
System.out.println(peopleMap2);
System.out.println(peopleMap3);
Map<Integer, String> userNameMap = entityList.stream().collect(Collectors.toMap(People::getUserId, People::getUserName));
//{1=张三, 2=李四, 3=王五}
System.out.println(userNameMap);
}
/**
* 2、Map转List
*/
public static void mapToList() {
Map<String, Integer> stringMap = new HashMap<>(4, 0.75f);
stringMap.put("1", 21);
stringMap.put("2", 22);
stringMap.put("3", 23);
List<Map.Entry<String, Integer>> stringMapList = stringMap.entrySet().stream().collect(Collectors.toList());
//[1=21, 2=22, 3=23]
System.out.println(stringMapList);
List<String> stringKeyList = stringMap.keySet().stream().collect(Collectors.toList());
//[1, 2, 3]
System.out.println(stringKeyList);
List<Integer> stringValueList = stringMap.values().stream().collect(Collectors.toList());
//[21, 22, 23]
System.out.println(stringValueList);
Map<Integer, People> peopleMap = new HashMap<>(4, 0.75f);
peopleMap.put(1, People.builder().userId(1).userAge(21).userName("张三").build());
peopleMap.put(2, People.builder().userId(2).userAge(22).userName("李四").build());
peopleMap.put(3, People.builder().userId(3).userAge(23).userName("王五").build());
List<Map.Entry<Integer, People>> peopleMapList = peopleMap.entrySet().stream().collect(Collectors.toList());
//[1=People(userId=1, userAge=21, userName=张三), 2=People(userId=2, userAge=22, userName=李四), 3=People(userId=3, userAge=23, userName=王五)]
System.out.println(peopleMapList);
List<Integer> peopleKeyList = peopleMap.keySet().stream().collect(Collectors.toList());
//[1, 2, 3]
System.out.println(peopleKeyList);
List<People> peopleValueList = peopleMap.values().stream().collect(Collectors.toList());
//[People(userId=1, userAge=21, userName=张三), People(userId=2, userAge=22, userName=李四), People(userId=3, userAge=23, userName=王五)]
System.out.println(peopleValueList);
}
/**
* 3、排序
*/
public static void listSort() {
List<Integer> integerList = new ArrayList<>();
integerList.add(1);
integerList.add(3);
integerList.add(2);
//升序 [1, 2, 3]
List<Integer> integerAscSortList = integerList.stream().sorted(Comparator.comparing(k -> k)).collect(Collectors.toList());
System.out.println(integerAscSortList);
//降序 [3, 2, 1]
integerList.sort(Comparator.comparing(t -> t.toString()).reversed());
System.out.println(integerList);
//升序 [People(userId=1, userAge=21, userName=张三), People(userId=2, userAge=22, userName=李四), People(userId=3, userAge=23, userName=王五)]
List<People> peopleList = new ArrayList<>();
peopleList.add(People.builder().userId(1).userAge(21).userName("张三").build());
peopleList.add(People.builder().userId(2).userAge(22).userName("李四").build());
peopleList.add(People.builder().userId(3).userAge(23).userName("王五").build());
List<People> peopleAscSortList = peopleList.stream().sorted(Comparator.comparing(People::getUserId)).collect(Collectors.toList());
System.out.println(peopleAscSortList);
//降序 [People(userId=3, userAge=23, userName=王五), People(userId=2, userAge=22, userName=李四), People(userId=1, userAge=21, userName=张三)]
List<People> peopleDescSortList = peopleList.stream().sorted(Comparator.comparing(People::getUserId).reversed()).collect(Collectors.toList());
System.out.println(peopleDescSortList);
}
}
@Data
@Builder
class People {
private Integer userId;
private Integer userAge;
private String userName;
}
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