在这篇教程中我将向你展示 filter(), collect(), findAny() 和 orElse() 的用法

1. Streams filter() 和 collect()

1.1 在 Java 8 以前, 像下面这样过滤一个 List:

BeforeJava8.java

package com.mkyong.java8;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

public class BeforeJava8 {

public static void main(String[] args) {

List lines = Arrays.asList("spring", "node", "mkyong");

List result = getFilterOutput(lines, "mkyong");

for (String temp : result) {

System.out.println(temp); //output : spring, node

}

}

private static List getFilterOutput(List lines, String filter) {

List result = new ArrayList<>();

for (String line : lines) {

if (!"mkyong".equals(line)) { // we dont like mkyong

result.add(line);

}

}

return result;

}

}

Output

spring

node

1.2 Java 8里面的等价写法: 使用collect()将一个List转化为steam(流),然后使用stream.filter() 来过滤它.

NowJava8.java

package com.mkyong.java8;

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

public class NowJava8 {

public static void main(String[] args) {

List lines = Arrays.asList("spring", "node", "mkyong");

List result = lines.stream() // 转化为一个流

.filter(line -> !"mkyong".equals(line)) // 排除 'mkyong'

.collect(Collectors.toList()); // 吧输出流收集回List中

result.forEach(System.out::println); //输出 : spring, node

}

}

Output

spring

node

2. Streams filter(), findAny() 和 orElse()

Person.java

package com.mkyong.java8;

public class Person {

private String name;

private int age;

public Person(String name, int age) {

this.name = name;

this.age = age;

}

//gettersm setters, toString

}

2.1 在Java 8以前, 你想通过name获取一个Person通常像这样 :

BeforeJava8.java

package com.mkyong.java8;

import java.util.Arrays;

import java.util.List;

public class BeforeJava8 {

public static void main(String[] args) {

List persons = Arrays.asList(

new Person("mkyong", 30),

new Person("jack", 20),

new Person("lawrence", 40)

);

Person result = getStudentByName(persons, "jack");

System.out.println(result);

}

private static Person getStudentByName(List persons, String name) {

Person result = null;

for (Person temp : persons) {

if (name.equals(temp.getName())) {

result = temp;

}

}

return result;

}

}

Output

Person{name='jack', age=20}

2.2 在Java 8里面的等价操作是使用 stream.filter()来过滤 List,使用 .findAny().orElse (null) 来返回指定的对象.

NowJava8.java

package com.mkyong.java8;

import java.util.Arrays;

import java.util.List;

public class NowJava8 {

public static void main(String[] args) {

List persons = Arrays.asList(

new Person("mkyong", 30),

new Person("jack", 20),

new Person("lawrence", 40)

);

Person result1 = persons.stream() // 转化为流

.filter(x -> "jack".equals(x.getName())) // 只过滤出"jack"

.findAny() // 如果找到了就返回

.orElse(null); // 如果找不到就返回null

System.out.println(result1);

Person result2 = persons.stream()

.filter(x -> "ahmook".equals(x.getName()))

.findAny()

.orElse(null);

System.out.println(result2);

}

}

Output

Person{name='jack', age=20}

null

2.3 多重条件指定.

NowJava8.java

package com.mkyong.java8;

import java.util.Arrays;

import java.util.List;

public class NowJava8 {

public static void main(String[] args) {

List persons = Arrays.asList(

new Person("mkyong", 30),

new Person("jack", 20),

new Person("lawrence", 40)

);

Person result1 = persons.stream()

.filter((p) -> "jack".equals(p.getName()) && 20 == p.getAge())

.findAny()

.orElse(null);

System.out.println("result 1 :" + result1);

//or like this

Person result2 = persons.stream()

.filter(p -> {

if ("jack".equals(p.getName()) && 20 == p.getAge()) {

return true;

}

return false;

}).findAny()

.orElse(null);

System.out.println("result 2 :" + result2);

}

}

Output

result 1 :Person{name='jack', age=20}

result 2 :Person{name='jack', age=20}

3. Streams filter() 和 map()

NowJava8.java

package com.mkyong.java8;

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

public class NowJava8 {

public static void main(String[] args) {

List persons = Arrays.asList(

new Person("mkyong", 30),

new Person("jack", 20),

new Person("lawrence", 40)

);

String name = persons.stream()

.filter(x -> "jack".equals(x.getName()))

.map(Person::getName) //吧流转化为String

.findAny()

.orElse("");

System.out.println("name : " + name);

List collect = persons.stream()

.map(Person::getName)

.collect(Collectors.toList());

collect.forEach(System.out::println);

}

}

Output

name : jack

mkyong

jack

lawrence

注意

Highly recommend this Streams tutorial – Processing Data with Java SE 8 Streams

References