最常用的 Java 8 中的 Lambda 函数(项目中实用笔记)

简介

Java 8 中的新特性，虽然现在都出到了Java14版本，不过在日常的开发过程中，8的版本是足够使用了，再说现在的8以上的版本也都面向商业收费了，很多新手，我所接触到的，像我那时候一样，追求船新版本，一上来就去学java14的东西，当成一个爱好还行，重心还是要放在实用上

过滤

需求：我需要过滤高考分数大于500的人

首先，新建一个内部类

static class Student{

private String name;

private Integer score;

public String getName() {

return name;

}

public void setName(String name) {

this.name = name!=null ? name.trim() : null;

}

public Integer getScore() {

return score;

}

public void setScore(Integer score) {

this.score = score;

}

public Student(String name, Integer score) {

this.name = name;

this.score = score;

}

@Override

public String toString() {

return "Student{" +

"name='" + name + '\'' +

", score=" + score +

"}\n";

}

}

使用IntStream遍历快速初始化一批值

public static void main(String[] args) {

List studentList = IntStream.rangeClosed(0,20)

.mapToObj(i -> new Student("Java Pro"+i,490+i))

.collect(Collectors.toList());

}

过滤出分数大于500的并输出

List studentGiao = studentList.stream()

.filter(student -> student.score > 500)

.collect(Collectors.toList());

System.out.println(studentGiao.toString());

输出：

[Student{name='Java Pro11', score=501}

, Student{name='Java Pro12', score=502}

, Student{name='Java Pro13', score=503}

, Student{name='Java Pro14', score=504}

, Student{name='Java Pro15', score=505}

, Student{name='Java Pro16', score=506}

, Student{name='Java Pro17', score=507}

, Student{name='Java Pro18', score=508}

, Student{name='Java Pro19', score=509}

, Student{name='Java Pro20', score=510}

]

日常求和

需要考虑到为空和为0的情况

package com.github.gleans;

import java.util.Arrays;

import java.util.List;

import java.util.Objects;

public class LambdaLearning {

public static void main(String[] args) {

List nums = Arrays.asList(1.01, 2.11, 3.23, 4.222, null, 5.6);

double resNum = nums.stream()

.map(num -> Objects.isNull(num) ? 0 : num)

.mapToDouble(num -> num)

.sum();

System.out.println(resNum);

}

}

map是重新指向一个对象，把->右侧的对象赋予，此处判断若num为null则赋予0值

注意，这里不可给null使用filter过滤掉，否则全为null的情况，会报空指针异常

扩展计算

public static void testTwo(){

List nums = Arrays.asList(1.01, 2.11, 3.23, 4.222, null, 5.6);

DoubleSummaryStatistics number = nums.stream()

.map(num -> Objects.isNull(num) ? 0 : num)

.mapToDouble(num -> num)

.summaryStatistics();

System.out.println("最大值:"+number.getMax());

System.out.println("最小值:"+number.getMin());

System.out.println("平均值:"+number.getAverage());

}

输出

最大值:5.6

最小值:0.0

平均值:2.6953333333333336

reduce简单使用

public static void main(String[] args) {

testOne();

}

public static void testOne(){

List numbers = Arrays.asList(1, 2, 3, 4, 5, 6);

// 这里的 10 相当于初始值

int sum = numbers

.stream()

.reduce(10, Integer::sum);

System.out.println(sum);

}

Collectors.groupingBy

根据年龄分组

package com.github.gleans;

import java.math.BigDecimal;

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

public class SumWage {

public static void main(String[] args) {

List userDemoList = new ArrayList() {{

add(new UserDemo(20, "jason", BigDecimal.valueOf(1000000)));

add(new UserDemo(22, "yasuo", BigDecimal.valueOf(2000000)));

add(new UserDemo(22, "ekko", BigDecimal.valueOf(100)));

}};

Map> UserDemoMapByAge = userDemoList.stream()

.collect(Collectors.groupingBy(UserDemo::getAge));

System.out.println(UserDemoMapByAge.toString());

}

static class UserDemo {

private int age;

private String username;

private BigDecimal wage;

public UserDemo(int age, String username, BigDecimal wage) {

this.age = age;

this.username = username;

this.wage = wage;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public String getUsername() {

return username;

}

public void setUsername(String username) {

this.username = username;

}

public BigDecimal getWage() {

return wage;

}

public void setWage(BigDecimal wage) {

this.wage = wage;

}

@Override

public String toString() {

return "UserDemo{" +

"age=" + age +

", username='" + username + '\'' +

", wage=" + wage +

'}';

}

}

}

输出

{20=[UserDemo{age=20, username='jason', wage=1000000}], 22=[UserDemo{age=22, username='yasuo', wage=2000000}, UserDemo{age=22, username='ekko', wage=100}]}

json化观看观看更为直观

{

20:[

{

"age":20,

"username":"jason",

"wage":1000000

}

],

22:[

{

"age":22,

"username":"yasuo",

"wage":2000000

},

{

"age":22,

"username":"ekko",

"wage":100

}

]

}

进阶计算 Collectors.summarizingDouble

Map userAvgWageByAge = userDemoList.stream()

.collect(Collectors.groupingBy(UserDemo::getAge, Collectors.summarizingDouble(s -> s.getWage().doubleValue())));

userAvgWageByAge.forEach((k, v) -> System.out.println(String.format("年龄:%d,平均工资:%f", k, v.getAverage())));

数组快速转为List

Stream.of(1, 2, 3, 4).collect(Collectors.toList())

结论

后续会学习更多关于Lambda的操作，日积月累...一定会成为一个秃头的程序猿