[Java知识库]Java Stream流语法总结

Java 8 提供的Lambda+Stream流让人感受到了什么叫函数式编程的快乐!

这里目前只讲Stream流，那什么是Stream流呢?

Stream将要处理的元素集合看作一种流，在流的过程中，借助Stream API对流中的元素进行操作，比如：筛选、排序、聚合等。

我们来对Stream流的功能做下区分：

1：循环和匹配（foreach/find/match）：

package com.company.Test;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class DebugTest {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(7, 6, 9, 3, 8, 2, 1);

        // 遍历出所有元素
        list.stream().forEach(System.out::println);
        // 匹配第一个元素
        Optional<Integer> findFirst = list.stream().findFirst();
        // 是否包含符合特定条件的元素
        boolean anyMatch = list.stream().anyMatch(x -> x < 6);
        System.out.println("匹配第一个值：" + findFirst.get());
        
        System.out.println("是否存在大于6的值：" + anyMatch);
    }
}

结果：

?7
6
9
3
8
2
1
匹配第一个值：7
是否存在大于6的值：true

筛选（filter）

筛选就是设置条件挑选出自己想要的元素。

案列一：简单整数集合筛选

package com.company.Test;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;

public class DebugTest {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(6, 7, 3, 8, 1, 2, 9);
        Stream<Integer> stream = list.stream();
        stream.filter(x -> x > 7).forEach(System.out::println);

    }
}

结果：

8
9

案列二：针对对象属性进行筛选

package com.company.Test;

        import com.company.pojo.Person;

        import java.util.ArrayList;
        import java.util.Arrays;
        import java.util.List;
        import java.util.stream.Collectors;
        import java.util.stream.Stream;

public class DebugTest {
    public static void main(String[] args) {
        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"));
        personList.add(new Person("Owen", 9500, 25, "male", "New York"));
        personList.add(new Person("Alisa", 7900, 26, "female", "New York"));

        List<Person> fiterList = personList.stream().filter(x -> x.getSalary() > 8000).collect(Collectors.toList());
        System.out.println("高于8000的员工姓名：" );
        fiterList.stream().forEach(x->{
            System.out.println(x.getName());
        });
    }
}

结果：

高于8000的员工姓名：
Tom
Anni
Owen

聚合（max/min/count)

个人理解就是用来计算的

案列一：算出最长的那个字段

package com.company.Test;

        import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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());

    }
}

结果：

最长的字符串：weoujgsd

案列二：获取Integer集合中的最大值。

package com.company.Test;

        import com.company.pojo.Person;

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

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

结果：

自然排序的最大值：11
自定义排序的最大值：11

案列三：从对象中取值进行判断

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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"));
        personList.add(new Person("Owen", 9500, 25, "male", "New York"));
        personList.add(new Person("Alisa", 7900, 26, "female", "New York"));
        Optional<Person> max = personList.stream().max(Comparator.comparingInt(Person::getSalary));
        System.out.println("员工工资最大值：" + max.get().getSalary());
    }
}

结果：

员工工资最大值：9500

案列四：计算Integer集合中大于6的元素的个数

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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);

    }
}

结果：

list中大于6的元素个数：4

映射(map/flatMap)

map:接收一个函数作为参数，该函数会被应用到每个元素上，并将其映射成一个新的元素。

flatMap:接收一个函数作为参数，将流中的每个值都换成另一个流，然后把所有流连接成一个流。

案例一：英文字符串数组的元素全部改为大写。整数数组每个元素+3。

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        String[] strArr = { "abcd", "bcdd", "defde", "fTr" };
        List<String> strList = Arrays.stream(strArr).map(String::toUpperCase).collect(Collectors.toList());

        List<Integer> intList = Arrays.asList(1, 3, 5, 7, 9, 11);
        List<Integer> intListNew = intList.stream().map(x -> x + 3).collect(Collectors.toList());

        System.out.println("每个元素大写：" + strList);
        System.out.println("每个元素+3：" + intListNew);


    }
}

结果：

每个元素大写：[ABCD, BCDD, DEFDE, FTR]
每个元素+3：[4, 6, 8, 10, 12, 14]

案例二：将对象的属性全部增加1000。

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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"));
        personList.add(new Person("Owen", 9500, 25, "male", "New York"));
        personList.add(new Person("Alisa", 7900, 26, "female", "New York"));

        // 不改变原来员工集合的方式
        List<Person> personListNew = personList.stream().map(person -> {
            Person personNew = new Person(person.getName(), 0, 0, null, null);
            personNew.setSalary(person.getSalary() + 10000);
            return personNew;
        }).collect(Collectors.toList());
        System.out.println("一次改动前：" + personList.get(0).getName() + "-->" + personList.get(0).getSalary());
        System.out.println("一次改动后：" + personListNew.get(0).getName() + "-->" + personListNew.get(0).getSalary());

        // 改变原来员工集合的方式
        List<Person> personListNew2 = personList.stream().map(person -> {
            person.setSalary(person.getSalary() + 10000);
            return person;
        }).collect(Collectors.toList());
        System.out.println("二次改动前：" + personList.get(0).getName() + "-->" + personListNew.get(0).getSalary());
        System.out.println("二次改动后：" + personListNew2.get(0).getName() + "-->" + personListNew.get(0).getSalary());


    }
}

案例三：将两个字符数组合并成一个新的字符数组

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("m,k,l,a", "1,3,5,7");
        List<String> listNew = list.stream().flatMap(s -> {
            // 将每个元素转换成一个stream
            String[] split = s.split(",");
            Stream<String> s2 = Arrays.stream(split);
            return s2;
        }).collect(Collectors.toList());

        System.out.println("处理前的集合：" + list);
        System.out.println("处理后的集合：" + listNew);
    }
}

结果：

处理前的集合：[m,k,l,a, 1,3,5,7]
处理后的集合：[m, k, l, a, 1, 3, 5, 7]

归约(reduce)

归约，也称缩减，顾名思义，是把一个流缩减成一个值，能实现对集合求和、求乘积和求最值操作

案列一：求Integer集合的元素之和、乘积和最大值

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1, 3, 2, 8, 11, 4);
        // 求和方式1
        Optional<Integer> sum = list.stream().reduce((x, y) -> x + y);
        // 求和方式2
        Optional<Integer> sum2 = list.stream().reduce(Integer::sum);
        // 求和方式3 前面这个0是设置默认值，加了这个0后返回的类型就变了不在是Optional，而是Integer类型
        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);

        System.out.println("list求和：" + sum.get() + "," + sum2.get() + "," + sum3);
        System.out.println("list求积：" + product.get());
        System.out.println("list求和：" + max.get() + "," + max2);

    }
}

结果：

list求和：29,29,29
list求积：2112
list求和：11,11

案列二：对象属性求和。

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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"));
        personList.add(new Person("Owen", 9500, 25, "male", "New York"));
        personList.add(new Person("Alisa", 7900, 26, "female", "New York"));

        // 求工资之和方式1：
        Optional<Integer> sumSalary = personList.stream().map(Person::getSalary).reduce(Integer::sum);
        // 求工资之和方式2： 自己定义求和方法，(sum1, sum2) -> 0声明求和的类型这个求和的类型为Integer
        Integer sumSalary2 = personList.stream().reduce(0, (sum, p) -> sum += p.getSalary(), (sum1, sum2) -> 0);
        // 求工资之和方式3：
        Integer sumSalary3 = personList.stream().reduce(0, (sum, p) -> sum += p.getSalary(), Integer::sum);

        // 求最高工资方式1：
        Integer maxSalary = personList.stream().reduce(0, (max, p) -> max > p.getSalary() ? max : p.getSalary(),Integer::max);
        // 求最高工资方式2：
        Integer maxSalary2 = personList.stream().reduce(0, (max, p) -> max > p.getSalary() ? max : p.getSalary(),
                (max1, max2) -> max1 > max2 ? max1 : max2);

        System.out.println("工资之和：" + sumSalary.get() + "," + sumSalary2 + "," + sumSalary3);
        System.out.println("最高工资：" + maxSalary + "," + maxSalary2);


    }
}

结果：

工资之和：49300,49300,49300
最高工资：9500,9500

收集(collect)

把流组合成一个值或者一个集合

归集(toList/toSet/toMap)

案列：三种集合的生成

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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));
        System.out.println("toList:" + listNew);
        System.out.println("toSet:" + set);
        System.out.println("toMap:" + map);
    }
}

结果：

toList:[6, 4, 6, 6, 20]
toSet:[4, 20, 6]
toMap:{Tom=com.company.pojo.Person@67b64c45, Anni=com.company.pojo.Person@4411d970}

?统计(count/averaging)

案列：求数量，求平均，求和，求最高，一次性统计其他值

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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.stream().collect(Collectors.counting());
        // 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).collect(Collectors.maxBy(Integer::compare));
        // 求工资之和
        Integer sum = personList.stream().collect(Collectors.summingInt(Person::getSalary));
        // 一次性统计所有信息
        DoubleSummaryStatistics collect = personList.stream().collect(Collectors.summarizingDouble(Person::getSalary));

        System.out.println("员工总数：" + count);
        System.out.println("员工平均工资：" + average);
        System.out.println("员工工资总和：" + sum);
        System.out.println("员工工资所有统计：" + collect);

    }
}

结果：

员工总数：3
员工平均工资：7900.0
员工工资总和：23700
员工工资所有统计：DoubleSummaryStatistics{count=3, sum=23700.000000, min=7000.000000, average=7900.000000, max=8900.000000}

分组(partitioningBy/groupingBy)

• 分区：将stream按条件分为两个Map，满足条件的和不满足条件的。
• 分组：将集合分为多个Map，类似于MySQL中的分组语句。

案列：将员工按薪资是否高于8000分为两部分；将员工按性别和地区分组

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Tom", 8900, "male", "New York"));
        personList.add(new Person("Jack", 7000, "male", "Washington"));
        personList.add(new Person("Lily", 7800, "female", "Washington"));
        personList.add(new Person("Anni", 8200, "female", "New York"));
        personList.add(new Person("Owen", 9500, "male", "New York"));
        personList.add(new Person("Alisa", 7900, "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)));
        System.out.println("员工按薪资是否大于8000分组情况：" + part);
        System.out.println("员工按性别分组情况：" + group);
        System.out.println("员工按性别、地区：" + group2);


    }
}

结果

员工按薪资是否大于8000分组情况：{false=[com.company.pojo.Person@7e0ea639, com.company.pojo.Person@3d24753a, com.company.pojo.Person@59a6e353], true=[com.company.pojo.Person@7a0ac6e3, com.company.pojo.Person@71be98f5, com.company.pojo.Person@6fadae5d]}
员工按性别分组情况：{female=[com.company.pojo.Person@3d24753a, com.company.pojo.Person@71be98f5, com.company.pojo.Person@59a6e353], male=[com.company.pojo.Person@7a0ac6e3, com.company.pojo.Person@7e0ea639, com.company.pojo.Person@6fadae5d]}
员工按性别、地区：{female={New York=[com.company.pojo.Person@71be98f5, com.company.pojo.Person@59a6e353], Washington=[com.company.pojo.Person@3d24753a]}, male={New York=[com.company.pojo.Person@7a0ac6e3, com.company.pojo.Person@6fadae5d], Washington=[com.company.pojo.Person@7e0ea639]}}
?

?接合(joining)

joining可以将stream中的元素用特定的连接符（没有的话，则直接连接）连接成一个字符串。

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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(p -> p.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);

    }
}

结果：

所有员工的姓名：Tom,Jack,Lily
拼接后的字符串：A-B-C

归约(reducing)

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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"));

        // 每个员工减去起征点后的薪资之和（这个例子并不严谨，但一时没想到好的例子）
        Integer sum = personList.stream().collect(Collectors.reducing(0, Person::getSalary, (i, j) -> (i + j - 5000)));
        System.out.println("员工扣税薪资总和：" + sum);

        // stream的reduce
        Optional<Integer> sum2 = personList.stream().map(Person::getSalary).reduce(Integer::sum);
        System.out.println("员工薪资总和：" + sum2.get());
    }
}

结果：

员工扣税薪资总和：8700
员工薪资总和：23700

?排序(sorted)

sorted，中间操作。有两种排序：

• sorted()：自然排序，流中元素需实现Comparable接口
• sorted(Comparator com)：Comparator排序器自定义排序

案例：将员工按工资由高到低（工资一样则按年龄由大到小）排序?

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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());

        System.out.println("按工资升序排序：" + newList);
        System.out.println("按工资降序排序：" + newList2);
        System.out.println("先按工资再按年龄升序排序：" + newList3);
        System.out.println("先按工资再按年龄自定义降序排序：" + newList4);

    }
}

结果

按工资升序排序：[Lily, Tom, Sherry, Jack, Alisa]
按工资降序排序：[Sherry, Jack, Alisa, Tom, Lily]
先按工资再按年龄升序排序：[Lily, Tom, Sherry, Jack, Alisa]
先按工资再按年龄自定义降序排序：[Alisa, Jack, Sherry, Tom, Lily]

提取/组合

流也可以进行合并、去重、限制、跳过等操作。

package com.company.Test;

import com.company.pojo.Person;

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

public class DebugTest {
    public static void main(String[] args) {
        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());

        System.out.println("流合并：" + newList);
        System.out.println("limit：" + collect);
        System.out.println("skip：" + collect2);


    }
}

结果：

流合并：[a, b, c, d, e, f, g]
limit：[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
skip：[3, 5, 7, 9, 11]