JDK8流式处理常用例子

public class Apple {

    private Integer id;
    private String name;

    private BigDecimal money;
    private Integer num;

    public Apple(Integer id, String name, BigDecimal money, Integer num) {
        this.id = id;
        this.name = name;
        this.money = money;
        this.num = num;
    }

    @Override
    public String toString() {

        return this.name + " " + this.money + " " + this.num;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public BigDecimal getMoney() {
        return money;
    }

    public void setMoney(BigDecimal money) {
        this.money = money;
    }

    public Integer getNum() {
        return num;
    }

    public void setNum(Integer num) {
        this.num = num;
    }
}

@RunWith(JUnit4.class)
public class CollectionTest {

    static List<Apple> appleList;

    static {

        appleList = new ArrayList<>();// 存放apple对象集合

        Apple apple1 = new Apple(1, "苹果1", new BigDecimal("3.25"), 10);
        Apple apple12 = new Apple(1, "苹果2", new BigDecimal("1.35"), 20);
        Apple apple2 = new Apple(2, "香蕉", new BigDecimal("2.89"), 30);
        Apple apple3 = new Apple(3, "荔枝", new BigDecimal("9.99"), 40);

        appleList.add(apple1);
        appleList.add(apple12);
        appleList.add(apple2);
        appleList.add(apple3);
    }
    
    @Test
    public void practiceGrouping() {
        Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));

        System.out.println("groupBy:" + groupBy);
    }

    @Test
    public void practiceMapping() {

        /**
         * List -> Map 需要注意的是： toMap 如果集合对象有重复的key，会报错Duplicate key .... apple1,apple12的id都为1。 可以用 (k1,k2)->k1
         * 来设置，如果有重复的key,则保留key1,舍弃key2
         */
        Map<Integer, Apple> appleMap = appleList.stream()
                .collect(Collectors.toMap(Apple::getId, a -> a, (k1, k2) -> k1));
    }

    @Test
    public void practiceSum() {

        BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
        System.err.println("totalMoney:" + totalMoney); // totalMoney:17.48　　　　　
　　　　 //上面是bigdecimal的 如果是Integer　　　　

　　　　 Integer totalMoney2 = appleList.stream().map(Apple::getNum).reduce(0, Integer::sum);　　　　
        System.err.println("totalMoney:" + totalMoney2); // totalMoney:500

  }

    @Test
    public void practiceMaxMin() {

        Optional<Apple> maxDish = appleList.stream().collect(Collectors.maxBy(Comparator.comparing(Apple::getMoney)));
        maxDish.ifPresent(System.out::println);

        Optional<Apple> minDish = appleList.stream().collect(Collectors.minBy(Comparator.comparing(Apple::getMoney)));
        minDish.ifPresent(System.out::println);

　　　　　　Optional<Apple> maxDish2 = appleList.stream().max(Comparator.comparing(Apple::getMoney));
　　　　　　maxDish.ifPresent(System.out::println);

　　　　　　Optional<Apple> minDish2 = appleList.stream().min(Comparator.comparing(Apple::getMoney));
　　　　　　minDish.ifPresent(System.out::println);

    }

    @Test
    public void practiceRemoveDup() {
        List<Apple> unique = appleList.stream().collect(
                Collectors.collectingAndThen(
                        Collectors.toCollection(() -> new TreeSet<>(Comparator.comparingLong(Apple::getId))), ArrayList::new)
        );
        
        for (Apple apple : unique) {
            System.out.println(apple);
        }
    }
}

 更新一组List怎么转数组

List<Integer> list2 = new ArrayList<>();
        for (int i = 1; i <= 10; ++i) {
            list2.add(i);
        }
        int[] array = list2.stream().mapToInt(Integer::valueOf).toArray();

而调用List本身的toArray只能返回包装类型

Integer[] tempArray = new Integer[0];

        list2.toArray(tempArray);

如果tempArray是 int[]直接报错

====================== 2021-10-26 =================================

最大最小值

例子1：

List<String> list = Arrays.asList("adnm", "admmt", "pot", "xbangd", "weoujgsd");
        Optional<String> max = list.stream().max(Comparator.comparing(String::length));
        System.out.println("最长的字符串：" + max.get());

例子2

List<Integer> list = Arrays.asList(7, 6, 9, 4, 11, 6);
        // 自然排序
        Optional<Integer> max = list.stream().max(Integer::compareTo);
        // 自定义排序
        Optional<Integer> max2 = list.stream().max(new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1.compareTo(o2);
            }
        });
        System.out.println("自然排序的最大值：" + max.get());
        System.out.println("自定义排序的最大值：" + max2.get());

count方法计数

List<Integer> list = Arrays.asList(7, 6, 4, 8, 2, 11, 9);
        long count = list.stream().filter(x -> x > 6).count();
        System.out.println("list中大于6的元素个数：" + count);

reduce

List<Integer> list = Arrays.asList(1, 3, 2, 8, 11, 4);
        // 求和方式1
        Optional<Integer> sum = list.stream().reduce(Integer::sum);
        // 求和方式2
        Optional<Integer> sum2 = list.stream().reduce(Integer::sum);
        // 求和方式3
        Integer sum3 = list.stream().reduce(0, Integer::sum);
        // 求乘积
        Optional<Integer> product = list.stream().reduce((x, y) -> x * y);
        // 求最大值方式1
        Optional<Integer> max = list.stream().reduce((x, y) -> x > y ? x : y);
        // 求最大值写法2
        Integer max2 = list.stream().reduce(1, Integer::max);

归集(toList/toSet/toMap)

　　　　 List<Integer> list = Arrays.asList(1, 6, 3, 4, 6, 7, 9, 6, 20);
        List<Integer> listNew = list.stream().filter(x -> x % 2 == 0).collect(Collectors.toList());
        Set<Integer> set = list.stream().filter(x -> x % 2 == 0).collect(Collectors.toSet());
        List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
        personList.add(new Person("Anni", 8200, 24, "female", "New York"));
        Map<?, Person> map = personList.stream().filter(p -> p.getSalary() > 8000)
                .collect(Collectors.toMap(Person::getName, p -> p));

统计(count/averaging) 求和 

　　　　 List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
        // 求总数
        long count = personList.size();
        // 求平均工资
        Double average = personList.stream().collect(Collectors.averagingDouble(Person::getSalary));
        // 求最高工资
        Optional<Integer> max = personList.stream().map(Person::getSalary).max(Integer::compare);
        // 求工资之和
        int sum = personList.stream().mapToInt(Person::getSalary).sum();
        // 一次性统计所有信息
        DoubleSummaryStatistics collect = personList.stream().collect(Collectors.summarizingDouble(Person::getSalary));

分组(partitioningBy/groupingBy)、

List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Tom", 8900, 23, "male", "Washington"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "New York"));
        personList.add(new Person("Anni", 8200, 24, "female", "New York"));
        // 将员工按薪资是否高于8000分组
        Map<Boolean, List<Person>> part = personList.stream().collect(Collectors.partitioningBy(x -> x.getSalary() > 8000));
        // 将员工按性别分组
        Map<String, List<Person>> group = personList.stream().collect(Collectors.groupingBy(Person::getSex));
        // 将员工先按性别分组，再按地区分组
        Map<String, Map<String, List<Person>>> group2 = personList.stream().collect(Collectors.groupingBy(Person::getSex, Collectors.groupingBy(Person::getArea)));

join

List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
        String names = personList.stream().map(Person::getName).collect(Collectors.joining(","));
        System.out.println("所有员工的姓名：" + names);
        List<String> list = Arrays.asList("A", "B", "C");
        String string = list.stream().collect(Collectors.joining("-"));
        System.out.println("拼接后的字符串：" + string);

sort

List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Sherry", 9000, 24, "female", "New York"));
        personList.add(new Person("Tom", 8900, 22, "male", "Washington"));
        personList.add(new Person("Jack", 9000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 8800, 26, "male", "New York"));
        personList.add(new Person("Alisa", 9000, 26, "female", "New York"));
        // 按工资升序排序（自然排序）
        List<String> newList = personList.stream().sorted(Comparator.comparing(Person::getSalary)).map(Person::getName)
                .collect(Collectors.toList());
        // 按工资倒序排序
        List<String> newList2 = personList.stream().sorted(Comparator.comparing(Person::getSalary).reversed())
                .map(Person::getName).collect(Collectors.toList());
        // 先按工资再按年龄升序排序
        List<String> newList3 = personList.stream()
                .sorted(Comparator.comparing(Person::getSalary).thenComparing(Person::getAge)).map(Person::getName)
                .collect(Collectors.toList());
        // 先按工资再按年龄自定义排序（降序）
        List<String> newList4 = personList.stream().sorted((p1, p2) -> {
            if (p1.getSalary() == p2.getSalary()) {
                return p2.getAge() - p1.getAge();
            } else {
                return p2.getSalary() - p1.getSalary();
            }
        }).map(Person::getName).collect(Collectors.toList());

提取/组合

　　　　 String[] arr1 = { "a", "b", "c", "d" };
        String[] arr2 = { "d", "e", "f", "g" };
        Stream<String> stream1 = Stream.of(arr1);
        Stream<String> stream2 = Stream.of(arr2);
        // concat:合并两个流 distinct：去重
        List<String> newList = Stream.concat(stream1, stream2).distinct().collect(Collectors.toList());
        // limit：限制从流中获得前n个数据
        List<Integer> collect = Stream.iterate(1, x -> x + 2).limit(10).collect(Collectors.toList());
        // skip：跳过前n个数据
        List<Integer> collect2 = Stream.iterate(1, x -> x + 2).skip(1).limit(5).collect(Collectors.toList());