Java Stream 特性

package test.demo;

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.TypeReference;
import lombok.extern.log4j.Log4j2;
import org.testng.annotations.Test;

import java.util.Arrays;
import java.util.Comparator;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.stream.Collectors;

@Log4j2
public class JavaDemoTest extends TestCase {

    @Test
    public void testListStreamSorted(){

        String s = "[{\"rank\":1,\"team\":\"Liverpool\",\"points\":100}," +
                "{\"rank\":2,\"team\":\"Arsenal\",\"points\":98},\n" +
                "{\"rank\":4,\"team\":\"Chelsea\",\"points\":190},\n" +
                "{\"rank\":3,\"team\":\"Manchester Utd\",\"points\":97}]";

        // string 转 List对象
        List<PremierLeagueTable> premierLeagueTableList = JSON.parseObject(s, new TypeReference<List<PremierLeagueTable>>() {
        });
        // 按 rank字段顺序排序
        System.out.println("sort rank");
        premierLeagueTableList = premierLeagueTableList.stream().sorted(Comparator.comparing(PremierLeagueTable::getRank)).collect(Collectors.toList());
        for(int i=0;i<premierLeagueTableList.size();i++){
            System.out.println("rank:"+premierLeagueTableList.get(i).getRank()+",team:"+premierLeagueTableList.get(i).getTeam()+",points:"+premierLeagueTableList.get(i).getPoints());
        }
        // 按 rank字段倒序排序
        System.out.println("sort reversed rank");
        premierLeagueTableList = premierLeagueTableList.stream().sorted(Comparator.comparing(PremierLeagueTable::getRank).reversed()).collect(Collectors.toList());
        for(int i=0;i<premierLeagueTableList.size();i++){
            System.out.println("rank:"+premierLeagueTableList.get(i).getRank()+",team:"+premierLeagueTableList.get(i).getTeam()+",points:"+premierLeagueTableList.get(i).getPoints());
        }
        // 按 points字段顺序排序
        System.out.println("sort points");
        premierLeagueTableList = premierLeagueTableList.stream().sorted(Comparator.comparing(PremierLeagueTable::getPoints)).collect(Collectors.toList());
        for(int i=0;i<premierLeagueTableList.size();i++){
            System.out.println("rank:"+premierLeagueTableList.get(i).getRank()+",team:"+premierLeagueTableList.get(i).getTeam()+",points:"+premierLeagueTableList.get(i).getPoints());
        }
    }


    @Test
    public void testListStreamMap(){

        String s = "[{\"rank\":1,\"team\":\"Liverpool\",\"points\":100}," +
                "{\"rank\":2,\"team\":\"Arsenal\",\"points\":98},\n" +
                "{\"rank\":4,\"team\":\"Chelsea\",\"points\":190},\n" +
                "{\"rank\":3,\"team\":\"Manchester Utd\",\"points\":97}]";

        // string 转 List对象
        List<PremierLeagueTable> premierLeagueTableList = JSON.parseObject(s, new TypeReference<List<PremierLeagueTable>>() {
        });

        System.out.println("n->n");
        premierLeagueTableList.stream().map(n->n).forEach(n-> System.out.println(n));

        // 对 list 对象的字段操作
        System.out.println("n->n.getRank():");
        premierLeagueTableList.stream().map(n->n.getRank()).forEach(n-> System.out.println(n));

        System.out.println("n->n.getRank()*10:");
        premierLeagueTableList.stream().map(n->n.getRank()*10).forEach(n-> System.out.println(n));

        System.out.println("n->n.getTeam():");
        premierLeagueTableList.stream().map(n->n.getTeam()).forEach(n-> System.out.println(n));

    }

    @Test
    public void testListStreamFilter(){

        List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
        // 获取空字符串的数量
        long count = strings.stream().filter(string -> string.isEmpty()).count();
        System.out.println("获取空字符串的数量:"+count);

        // 含有bc字符串的数量
        count = strings.stream().filter(string -> string.contains("bc")).count();
        System.out.println("含有bc字符串的数量:"+count);
    }

    @Test
    public void testListStreamStatistics(){

        List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);

        IntSummaryStatistics stats = numbers.stream().mapToInt((x) -> x).summaryStatistics();

        System.out.println("列表中最大的数 : " + stats.getMax());
        System.out.println("列表中最小的数 : " + stats.getMin());
        System.out.println("所有数之和 : " + stats.getSum());
        System.out.println("总数 : " + stats.getCount());
        System.out.println("平均数 : " + stats.getAverage());
        
    }

}

package test.demo;

import io.swagger.annotations.ApiModelProperty;
import lombok.Data;

@Data
public class PremierLeagueTable {

    @ApiModelProperty("rank")
    private int rank;

    @ApiModelProperty("team")
    private String team;

    @ApiModelProperty("points")
    private int points;

}

streams 特性
https://www.runoob.com/java/java8-streams.html