Java流（stream）的用法

package src;

import java.util.*;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

public class MapTest {
    public static void test(){
        //stream
        Map<Integer,String> map=new HashMap<Integer,String>();
        for (int i = 1; i < 20; i++) {
            map.put(i,"a"+i);
        }
        //1.转换key列表
        List<String> valuesList=map.values().stream().collect(Collectors.toList());
        for (String a : valuesList) System.out.println(a);
        System.out.println("-------------------");
        //2.转换为value列表
        List key_list=map.keySet().stream().collect(Collectors.toList());
        for (Object b:key_list) {
            System.out.println(b);
        }
        //3.按要求过滤
        System.out.println("-------------------");
        List Filter_key_list=map.keySet().stream().filter(m->m>10).collect(Collectors.toList());
        for (Object a:Filter_key_list) {
            System.out.println(a);
        }
        System.out.println("-------------------");
        //4.按要求过滤--过滤后仍然是map
        Map<Integer,String> ChildMap=map.entrySet().stream().filter(m->m.getKey()>10).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        for(Map.Entry a:ChildMap.entrySet()) System.out.println(a.getKey()+"-"+a.getValue());
        System.out.println("-------------------");
        //5.求和
        int total=map.keySet().stream().mapToInt(Integer::intValue).sum();
        System.out.println(total);
        //6.排序键值对
        System.out.println("-------------------");
        List<Integer> collect1 = map.keySet().stream().sorted().collect(Collectors.toList());
        for (Integer a:collect1) System.out.println(a);
        //降序
        List<Map.Entry<Integer,String>> collect2 = map.entrySet().stream()
                .sorted(Map.Entry.<Integer,String>comparingByKey().reversed())
                .collect(Collectors.toList());
        collect2.forEach(m-> System.out.println(m.getKey()+"-"+m.getValue()));
    }
    public static void test2(){
        //7.分组统计
        Map<String,String> map=new HashMap();
        map.put("张三","1");
        map.put("李四","2");
        map.put("王五","1");
        map.put("赵六","2");
        map.put("刘七","3");
        Map<String,Long> childmap = map.values().stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
        childmap.forEach((m,n)-> System.out.println(m+"-"+n));
    }
    public static void test3(){
        //8.联合操作
        Map<String,Integer> salaries=new HashMap<>();//假设这是一个员工薪水的Map
        salaries.put("张三",5000);
        salaries.put("李四",3000);
        salaries.put("王五",5000);
        salaries.put("赵六",8000);
        salaries.put("刘七",6000);
        Map<String,String> departments=new HashMap<>();//假设这是一个员工部门的Map
        departments.put("张三","1");
        departments.put("李四","2");
        departments.put("王五","1");
        departments.put("赵六","2");
        departments.put("刘七","3");
        //输出高新员工，及其部门
        Map<String, String> collect = salaries.entrySet().stream().filter(e -> e.getValue() > 5000).map(m -> new AbstractMap.SimpleEntry<>(m.getKey(), departments.get(m.getKey())))
                .collect(Collectors.toMap(
                        Map.Entry::getKey, Map.Entry::getValue
                ));
        collect.forEach((m,n)-> System.out.println(m+"-"+n));
    }
    public static void main(String[] args){
        test3();
    }
}