文章目录

• 概述
• Case 1 : A List of Strings to Uppercase
• Case 2 : List of objects -> List of String
• Case 3 : List of objects -> List of other objects
• Test

概述

Stream.map()是Stream最常用的一个转换方法，它把一个Stream转换为另一个Stream

map()方法用于将一个Stream的每个元素映射成另一个元素并转换成一个新的Stream；

可以将一种元素类型转换成另一种元素类型。

    /*** Returns a stream consisting of the results of applying the given* function to the elements of this stream.** 
This is an intermediate* operation.** @param  The element type of the new stream* @param mapper a non-interfering,*               stateless*               function to apply to each element* @return the new stream*/ Stream map(Function mapper);

map()方法接收的对象是Function接口对象，它定义了一个apply()方法，负责把一个T类型转换成R类型。

其中，Function的定义为

@FunctionalInterface
public interface Function {/*** Applies this function to the given argument.** @param t the function argument* @return the function result*/R apply(T t);  //   将T类型转换为R:............}

Case 1 : A List of Strings to Uppercase

/*** A List of Strings to Uppercase*/public static void StringsListToUppercase() {List list = Arrays.asList("a", "b", "c", "d");// before java 8List newList = new ArrayList<>();for (String str : list) {newList.add(str.toUpperCase());}System.out.println(newList);// java 8List collect = list.stream().map(String::toUpperCase).collect(Collectors.toList());System.out.println(collect);// Extra, streams apply to any data type.List integers = Arrays.asList(1, 2, 3, 4);List collect1 = integers.stream().map(t -> t * 2).collect(Collectors.toList());System.out.println(collect1);}

Case 2 : List of objects -> List of String

 /*** List of objects -> List of String*/public static void objectsListToStringList() {List artisanList = Arrays.asList(new Artisan("小A", 18, new BigDecimal(100)),new Artisan("小B", 19, new BigDecimal(200)),new Artisan("小C", 20, new BigDecimal(300)));// Before Java 8List nameList = new ArrayList<>();for (Artisan artisan : artisanList) {nameList.add(artisan.getName());}log.info(nameList.toString());// Java 8List collect = artisanList.stream().map(Artisan::getName).collect(Collectors.toList());log.info(collect.toString());}

Case 3 : List of objects -> List of other objects

/*** List of objects -> List of other objects*/public static void objectsListToOtherObjectsList() {List artisanList = Arrays.asList(new Artisan("小A", 18, new BigDecimal(100)),new Artisan("小B", 19, new BigDecimal(200)),new Artisan("小C", 20, new BigDecimal(300)));// Before Java 8List convert2OtherList = new ArrayList<>();for (Artisan artisan : artisanList) {ArtisanOther artisanOther = new ArtisanOther();artisanOther.setAge(artisan.getAge());artisanOther.setName(artisan.getName());if ("小C".equals(artisan.getName())) {artisanOther.setOtherInfo("Only For 小C");}convert2OtherList.add(artisanOther);}log.info(convert2OtherList.toString());// Java 8artisanList.stream().map(artisan -> {ArtisanOther artisanOther = new ArtisanOther();artisanOther.setAge(artisan.getAge());artisanOther.setName(artisan.getName());if ("小C".equals(artisan.getName())) {artisanOther.setOtherInfo("Only For 小C");}return artisanOther;}).collect(Collectors.toList());log.info(convert2OtherList.toString());}

Test

    public static void main(String[] args) {// A List of Strings to UppercaseStringsListToUppercase();// List of objects -> List of StringobjectsListToStringList();// List of objects -> List of other objectsobjectsListToOtherObjectsList();}// 俩内部类 @Data@NoArgsConstructor@AllArgsConstructorstatic class Artisan {private String name;private int age;private BigDecimal salary;}@Data@NoArgsConstructor@AllArgsConstructorstatic class ArtisanOther {private String name;private int age;private String otherInfo;}

[A, B, C, D]
[A, B, C, D]
[2, 4, 6, 8]
21:57:13.914 [main] INFO com.artisan.java8.stream2.StreamMap - [小A, 小B, 小C]
21:57:13.918 [main] INFO com.artisan.java8.stream2.StreamMap - [小A, 小B, 小C]
21:57:13.919 [main] INFO com.artisan.java8.stream2.StreamMap - [StreamMap.ArtisanOther(name=小A, age=18, otherInfo=null), StreamMap.ArtisanOther(name=小B, age=19, otherInfo=null), StreamMap.ArtisanOther(name=小C, age=20, otherInfo=Only For 小C)]
21:57:13.920 [main] INFO com.artisan.java8.stream2.StreamMap - [StreamMap.ArtisanOther(name=小A, age=18, otherInfo=null), StreamMap.ArtisanOther(name=小B, age=19, otherInfo=null), StreamMap.ArtisanOther(name=小C, age=20, otherInfo=Only For 小C)]