Java中Collector

用法:
直接上代码

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

import java.util.*;
import java.util.function.BinaryOperator;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * Created by macro on 2022/3/19
 *
 * @author hhoa
 **/
public class TestCollector {
    private List<Double> doubleList;
    private List<Integer> integerList;
    private List<String> stringList;

    @DisplayName("测试返回不可变数组")
    @Test
    public void testToUnmodifiableList(){
        Stream<Integer> stream = integerList.stream();
        //使用Collectors的toUnmodifiableList()方法
        List<Integer> collect = stream.collect(Collectors.toUnmodifiableList());
        Assertions.assertThrows(Exception.class,()-> collect.add(1), "没有抛出异常");
        System.out.println("抛出了异常");
    }

    /**
     * 测试Collectors中的AveragingDouble
     * 同理:
     *  averagingDouble、averagingInt、averagingLong一样
     */
    @DisplayName("测试返回平均值")
    @Test
    public void testAveragingDouble(){
        Stream<Double> stream = doubleList.stream();
        Stream<Integer> integerStream = integerList.stream();
        //求double数组的平均值
        Double collect = stream.collect(Collectors.averagingDouble((a) -> a));
        System.out.println("double数组的平均值: " +collect);
        //把Integer转化为double再求平均值
        Double collect1 = integerStream.collect(Collectors.averagingDouble(Integer::doubleValue));
        System.out.println("integer数组的平均值: " + collect1);
    }

    /**
     * 经过一个collector转换后再执行一个Function
     */
    @DisplayName("测试CollectingAndThen")
    @Test
    public void testCollectingAndThen(){
        Stream<Integer> integerStream = integerList.stream();
        //把Integer转化为double再求平均值
        Boolean collect = integerStream.collect(Collectors.collectingAndThen(Collectors.averagingInt((a) -> a), (a) -> a > 5));
        testAveragingDouble();
        System.out.println("integer数组求平均值之后再判断是否>5: " + collect);
    }

    /**
     * 获取个数
     */
    @DisplayName("测试Counting")
    @Test
    public void testCounting(){
        Stream<Integer> integerStream = integerList.stream();
        Long collect = integerStream.collect(Collectors.counting());
        System.out.println("数组个数: " + collect);
    }

    /**
     * 分类分类结果保存在Map中
     */
    @DisplayName("测试GroupBy")
    @Test
    public void testGroupBy(){
        Stream<Integer> integerStream = integerList.stream();
        Map<Object, List<Integer>> collect = integerStream.collect(Collectors.groupingBy((t)->t > 5));
        System.out.println("根据是否>5分类");
        for (Object o : collect.entrySet()) {
            System.out.println(o);
        }
        integerStream = integerList.stream();
        Map<Boolean, Long> collect1 = integerStream.collect(Collectors.groupingBy((t) -> t > 5, Collectors.counting()));
        System.out.println("获取大于5和不大于的5的数的个数");
        for (Object o : collect1.entrySet()) {
            System.out.println(o);
        }
    }

    /**
     * 连接容器里面的字符串，并做修饰操作
     */
    @DisplayName("测试Joining")
    @Test
    public void testJoining(){
        Stream<String> stringStream = stringList.stream();
        String collect = stringStream.collect(Collectors.joining());
        System.out.println("将所有字符串串联起来");
        System.out.println(collect);

        stringStream = stringList.stream();
        collect = stringStream.collect(Collectors.joining("-"));
        System.out.println("用-将所有字符串串联起来");
        System.out.println(collect);

        stringStream = stringList.stream();
        collect = stringStream.collect(Collectors.joining("-", "[", "]"));
        System.out.println("用-将所有字符串串并用[]将修饰他们联起来");
        System.out.println(collect);
    }

    /**
     * 映射
     */
    @DisplayName("测试Mapping")
    @Test
    public void testMapping(){
        Stream<Integer> stream = integerList.stream();
        //先把原来的数映射为两倍再求平均值
        Double collect = stream.collect(Collectors.mapping((t) -> t * 2, Collectors.averagingInt((t) -> t)));
        System.out.println(collect);
    }

    /**
     * 通过比较器获取最大值
     */
    @DisplayName("测试MaxBy")
    @Test
    public void testMaxBy(){
        Stream<Integer> stream = integerList.stream();
        Optional<Integer> collect = stream.collect(Collectors.maxBy((a, b) -> a - b));
        System.out.println(collect.orElse(null));
    }

    /**
     * 通过比较器获取最小值
     */
    @DisplayName("测试minBy")
    @Test
    public void testMinBy(){
        Stream<Integer> stream = integerList.stream();
        Optional<Integer> collect = stream.collect(Collectors.minBy((a, b) -> a - b));
        System.out.println(collect.orElse(null));
    }

    /**
     * 类似于GroupBy
     */
    @DisplayName("测试PartitionBy")
    @Test
    public void testPartitionBy(){
        Stream<Integer> stream = integerList.stream();
        Map<Boolean, List<Integer>> collect = stream.collect(Collectors.partitioningBy((t) -> t > 5));
        System.out.println(collect);
    }

    /**
     * 类似maxBy或minBy
     * maxBy和minBy的底层也是用BinaryOperator.minBy或BinaryOperator.maxBy来写的
     */
    @DisplayName("测试Reducing")
    @Test
    public void testReducing(){
        Stream<Integer> stream = integerList.stream();
        Optional<Integer> collect = stream.collect(Collectors.reducing(BinaryOperator.minBy((a, b)->a - b)));
        System.out.println(collect.orElse(null));
    }

    /**
     * 获取一个对数组转化为int型之后的统计资料
     * return Statistics - 里面包含转成的int值，还包括min, max, average等
     *
     * 同理还有summarizingDouble, summarizingLong
     *
     */
    @DisplayName("测试summarizingInt")
    @Test
    public void testSummarizingInt(){
        Stream<String> stream = stringList.stream();
        IntSummaryStatistics collect = stream.collect(Collectors.summarizingInt((t) -> {
            switch (t) {
                case "zhangsan":
                    return 1;
                case "lisi":
                    return 2;
                case "wangwu":
                    return 3;
                default:
                    return 4;
            }
        }));
        double average = collect.getAverage();
        System.out.println(collect);
        System.out.println(average);
    }

    /**
     * 返回流内数据转化为Int后的和
     * 同理还有summingDouble, summingInt
     */
    @DisplayName("测试SummingInt")
    @Test
    public void testSummingInt(){
        Stream<Integer> stream = integerList.stream();
        Integer collect = stream.collect(Collectors.summingInt((t) -> t));
        System.out.println(collect);
    }

    /**
     * 返回源数组
     */
    @DisplayName("测试ToList")
    @Test
    public void testToList(){
        Stream<Integer> stream = integerList.stream();
        List<Integer> collect = stream.collect(Collectors.toList());
        System.out.println(collect);
    }


    /**
     * 根据Function获取key,和value并注入一个Map中,最后返回map
     *
     */
    @DisplayName("测试toMap")
    @Test
    public void testToMap(){
        Stream<Integer> stream = integerList.stream();
        Map<Integer, Integer> collect = stream.collect(Collectors.toMap((t) -> t*10, (t) -> t));
        System.out.println(collect);
    }

    /**
     * 把数组转化为set集合，里面相同的元素只会保留一个
     */
    @DisplayName("测试toSet")
    @Test
    public void testToSet(){
        Stream<Integer> stream = integerList.stream();
        Set<Integer> collect = stream.collect(Collectors.toSet());
        System.out.println(collect);
    }

    @DisplayName("----------------------------------分界线-------------------------------------------------")
    @BeforeEach
    public void getStringList(){
        List<String> list = Arrays.asList("zhangsan", "lisi", "wangwu");
        System.out.print("字符串数组:[");
        this.stringList = list;
        for (String str: list){
            System.out.print(str+ " ");
        }
        System.out.print("]\n");
    }
    @BeforeEach
    public void getIntegerList(){
        List<Integer> list = new ArrayList<>();
        System.out.print("整型数组:[");
        for (int i = 0; i < 4; i++){
            int num = Math.abs(new Random().nextInt() % 10);
            list.add(num);
            System.out.print(num+ "  ");
        }
        this.integerList = list;
        System.out.print("]\n");
    }
    @BeforeEach
    public void getDoubleList(){
        List<Double> list = new ArrayList<>();
        System.out.print("Double数组:[ ");
        for (int i = 0; i < 4; i++){
            double num = Math.abs(new Random().nextDouble() % 10);
            list.add(num);
            System.out.print(num+ "  ");
        }
        this.doubleList = list;
        System.out.print("]\n");
    }
}