逍遥阁

学习java的Stream

关键词: java stream

学习java的Stream

```
package dry.module.example.test;

import dry.module.example.pojo.User;

import java.util.*;
import java.util.stream.Collectors;

public class StreamTest {

public static void main(String[] args) {
        run08();
    }

public static void run01() {
        List<String> list = Arrays.asList("abc", "", "123", "def", "456", "", "789");
        List<String> listNew = list.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
        System.out.println(listNew);
        /*
            [abc, 123, def, 456, 789]
        */
    }

public static void run02() {
        List<String> list = Arrays.asList("abc", "", "123", "def", "456", "", "789");
        long count = list.stream().filter(String::isEmpty).count();
        System.out.println(count);
        /*
            2 空的个数
        */
    }

public static void run03() {
        Random r = new Random();
        r.ints().limit(10).forEach(System.out::println);
        /*
            10个随机整数
        */
    }

public static void run04() {
        Random r = new Random();
        r.ints().limit(10).sorted().forEach(System.out::println);
        /*
            10个排好序的随机整数
        */
    }

public static void run05() {
        List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
        List<Integer> listNew = list.stream().map(i -> i * i).collect(Collectors.toList());
        System.out.println(listNew);
        /*
            [1, 4, 9, 16, 25]
         */
    }

public static void run06() {
        List<String> list = Arrays.asList("abc", "", "123", "def", "456", "", "789");
        String data = list.stream().filter(string -> !string.isEmpty()).collect(Collectors.joining("-"));
        System.out.println(data);
        /*
            abc-123-def-456-789
        */
    }

public static void run07() {
        List<Integer> list = Arrays.asList(5, 2, 3, 4, 1);
        IntSummaryStatistics s = list.stream().mapToInt(x -> x).summaryStatistics();
        String data = String.format("个数=%d 最小=%d 最大=%d 平均值=%f 和=%d", s.getCount(), s.getMin(), s.getMax(), s.getAverage(), s.getSum());
        System.out.println(data);
    }

public static void run08() {
        List<User> list = makeData();
        /*
             每一个元素的id组成的列表
        */
        List<Long> userIds = list.stream().map(User::getId).collect(Collectors.toList());
        System.out.println(userIds);
        /*
            每一个元素的id为键，name为值
        */
        Map<Long, String> map1 = list.stream().collect(Collectors.toMap(User::getId, User::getName));
        System.out.println(map1);
        /*
             每一个元素的id为键，元素自己为值
        */
        Map<Long, User> map2 = list.stream().collect(Collectors.toMap(User::getId, user -> user));
        System.out.println(map2);
        /*
            调用函数，可以进行复杂计算
        */
        List<String> nameList = list.stream().map(StreamTest::handle).collect(Collectors.toList());
        System.out.println(nameList);
    }

public static String handle(User user) {
        return user.getName();
    }

public static List<User> makeData() {
        List<User> list = new ArrayList<>();
        list.add(new User().setId(1L).setName("a").setAge(10));
        list.add(new User().setId(2L).setName("b").setAge(20));
        list.add(new User().setId(3L).setName("c").setAge(30));
        return list;
    }

}
```

admin
2021-07-24 00:43:33
phantoscope

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学习java的Stream