JAVA stream 技巧练习汇总

长期汇总Stream的使用技巧。

创建stream的常用方法

1. 通过Collection.stream()来生成

List<String> list = Arrays.asList(new String[] {"ab", "bc", "ab", "cd", "bc", "ed", "ae"});
list.stream().sorted().forEach(System.out::println);

2. 通过数组来生成，Arrays.stream()或者Stream.of()

if (Arrays.stream(new String[] {"ab", "bc", "ab", "cd", "bc", "ed", "ae"})
                .anyMatch(n -> n == "ab"))
                System.out.println("exists");

和

System.out.println(
    Stream.of(new Integer[] {1, 2, 3, 4, 5, 6, 7, 8, 9}).reduce(0, (a, b) -> a + b));

3. 使用stream builder来生成

Stream<String> b = Stream.<String>builder().add("ef").add("fg").add("eg").add("gh").build();
b.forEach(System.out::println);

4. 使用stream iterate来生成

Stream.iterate(1, i -> i + 1).limit(5).forEach(System.out::println);

5. 使用stream generate来生成

Stream.generate(UUID::randomUUID).limit(5).forEach(System.out::println);

6. 使用IntStream.range(..)来生成

        IntStream.range(1, 20).forEach(i -> {
            i = i + 10;
            System.out.println(i);
        });

7. 读取文件来生成stream

文件是

Code,firstName,lastName,age
1,Chris,Riley,35  
2,Harold,Campbell,50  
3,Jessica,Nichols,30  
4,Catherine,Brown,40 
5,Kelly,Frazier,60
6,Dennis,Howard,35

        try {
            BufferedReader br = Files.newBufferedReader(Paths.get("c:\\temp\\1.txt"));
            Stream<String> lines = br.lines();

            List<Person> persons = lines.skip(1).map(line -> {
                String[] arr = line.split(",");
                return new Person(arr[1], arr[2], Integer.parseInt(arr[3].trim()));
            }).collect(Collectors.toList());
            persons.forEach(System.out::println);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

Person类

/**
 * Copyright: 2021 Hswfit. All rights reserved.
 * 
 * @Title: Person.java
 * @Prject: hellotest
 * @Package: com.test.hello
 * @Description: TODO
 * @author: Jay
 * @version: V1.0
 */
package com.test.hello;

/**
 * @ClassName: Person
 * @Description: TODO
 * @author: Jay
 * @date: 2021-10-19
 */
public class Person {

    @Override
    public String toString() {
        return "Person [firstName=" + firstName + ", lastName=" + lastName + ", age=" + age + "]";
    }

    /**
     * @Title:Person
     * @Description:TODO
     * @param firstName
     * @param lastName
     * @param age
     */
    public Person(String firstName, String lastName, Integer age) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }

    /**
     * @return the firstName
     */
    public String getFirstName() {
        return firstName;
    }

    /**
     * @param firstName the firstName to set
     */
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    /**
     * @return the lastName
     */
    public String getLastName() {
        return lastName;
    }

    /**
     * @param lastName the lastName to set
     */
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    /**
     * @return the age
     */
    public Integer getAge() {
        return age;
    }

    /**
     * @param age the age to set
     */
    public void setAge(Integer age) {
        this.age = age;
    }

    private String firstName;
    private String lastName;
    private Integer age;

}

输出结果为

Person [firstName=Chris, lastName=Riley, age=35]
Person [firstName=Harold, lastName=Campbell, age=50]
Person [firstName=Jessica, lastName=Nichols, age=30]
Person [firstName=Catherine, lastName=Brown, age=40]
Person [firstName=Kelly, lastName=Frazier, age=60]
Person [firstName=Dennis, lastName=Howard, age=35]

Stream.reduce技巧

1. 查找最大值

System.out.println(Stream.of(new Integer[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10})
     .reduce(0,(a, c) -> a > c ? a : c));

10

2. 查找最长字符串

System.out.println(Stream.of(new String[] {"abc", "cdfe", "abcdefg"}).reduce("",
                (String a, String c) -> a.length() > c.length() ? a : c));

abcdefg

3. 在字符串数组中，查找字符串是否存在

if (Arrays.stream(new String[] {"ab", "bc", "ab", "cd", "bc", "ed", "ae"})
                .anyMatch(n -> n == "ab"))
            System.out.println("exists");

exists

4. 字符串数组合并成一个字符串

        System.out.println(Stream.of(new String[] {"ab", "cd", "ef", "gh", "lm"}).reduce("",
                (a, b) -> a + "|" + b));

|ab|cd|ef|gh|lm

5. 统计所有人员的年龄合计

System.out.println(personList.stream().
    map(x -> x.getAge()).
    reduce(0, (a, b) -> a + b));

Stream.collect技巧

1. 将所有元素放入不同容器

将元素分别归纳进可变容器 List、Map、Set、Collection 或者ConcurrentMap 。

    Collectors.toList();
    Collectors.toMap();
    Collectors.toSet();
    Collectors.toCollection();
    Collectors.toConcurrentMap();

2. 巧用Collectors.joining

        //abcdefghlm
        System.out.println(Stream.of(new String[] {"ab", "cd", "ef", "gh", "lm"})
                .collect(Collectors.joining()));
        //ab,cd,ef,gh,lm
        System.out.println(Stream.of(new String[] {"ab", "cd", "ef", "gh", "lm"})
                .collect(Collectors.joining(",")));
        //[ab,cd,ef,gh,lm]
        System.out.println(Stream.of(new String[] {"ab", "cd", "ef", "gh", "lm"})
                .collect(Collectors.joining(",", "[", "]")));