【Java 8 新特性】Java 8 flatMap 使用示例
文章目录
- 1.在List中使用`Stream.flatMap`方法
- 2.在List列表中使用`Stream.flatMap`方法
- 3.在Array中使用`Stream.flatMap`方法
- 4.在对象数组中使用`Stream.flatMap`方法
- 5.`Files.lines()`中使用`Stream.flatMap`方法
- 6.`Optional flatMap`
- 代码下载
- 参考文献
在本页中，我们将提供Java8 FlatMap示例。我们在这里讨论Stream.flatMap以及Optional.flatMap()方法。
Stream.flatMap()返回流，该流将包含通过映射函数替换源流的每个元素而获得的元素，并展平结果。映射函数将生成流，并且在应用映射后关闭每个映射流。在对象流上应用统计函数是有用的，并且可以在一行中进行编码。
Optional.flatMap()如果存在值，则对其应用可选的方位映射函数。下面是不同场景下的flatMap示例。
在Java文档，Stream.flatMap()方法的定义如下
<R> Stream<R> flatMap(Function<? super T,? extends Stream<? extends R>> mapper)
Optional.flatMap()的定义如下
public <U> Optional<U> flatMap(Function<? super T,Optional<U>> mapper)
对于原始数据类型，java提供了flatMapToInt()、flatMapToLong()和flatMapToDouble()的接口。
1.在List中使用Stream.flatMap方法
这里我们有一份作家名单。每个作家都有一份书单。使用Stream.flatMap()我们将从所有作家那里得到所有书籍。然后我们会找到价格最高的那本书。
1.作家流对象.
{
{
"Mohan",
{
{
10,"AAA"}, {
20,"BBB"}
}
},
{
"Sohan",
{
{
30,"XXX"}, {
15,"ZZZ"}
}
}
}
2.使用flatMap(writer -> writer.getBooks().stream())方法后，得到书单的流对象。
{
{
10,"AAA"},
{
20,"BBB"},
{
30,"XXX"},
{
15,"ZZZ"}
}
这里的结果被flatMap()方法展开。
3.使用max(new BookComparator())方法后，得到最贵的书籍。
{
30,"XXX"}
下面我们来看代码
FlatmapWithList.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class FlatmapWithList {
public static void main(String[] args) {
List<Book> books = Arrays.asList(new Book(10, "AAA"), new Book(20, "BBB"));
Writer w1 = new Writer("Mohan", books);
books = Arrays.asList(new Book(30, "XXX"), new Book(15, "ZZZ"));
Writer w2 = new Writer("Sohan", books);
List<Writer> writers = Arrays.asList(w1, w2);
Book book = writers.stream().flatMap(writer -> writer.getBooks().stream())
.max(new BookComparator()).get();
System.out.println("Name:"+book.getName()+", Price:"+ book.getPrice() );
}
}
Writer.java
package com.concretepage;
import java.util.List;
public class Writer {
private String name;
private List<Book> books;
public Writer(String name, List<Book> books) {
this.name = name;
this.books = books;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Book> getBooks() {
return books;
}
public void setBooks(List<Book> books) {
this.books = books;
}
}
Book.java
package com.concretepage;
public class Book {
private int price;
private String name;
public Book(int price, String name) {
this.price = price;
this.name = name;
}
public int getPrice() {
return price;
}
public void setPrice(int price) {
this.price = price;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
BookComparator.java
package com.concretepage;
import java.util.Comparator;
public class BookComparator implements Comparator<Book> {
@Override
public int compare(Book b1, Book b2) {
if (b1.getPrice() > b2.getPrice()) {
return 1;
} else if (b1.getPrice() == b2.getPrice()) {
return 0;
} else {
return -1;
}
}
}
输出
Name:XXX, Price:30
2.在List列表中使用Stream.flatMap方法
在这里，我们将使用带有List列表的flatMap方法。我们正在创建两个列表，每个列表都包含图书的对象。最后，我将这两个列表添加到第三个列表中。我们将以最低价格买到这本书。
FlatmapWithListOfList.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class FlatmapWithListOfList {
public static void main(String[] args) {
List<Book> list1 = Arrays.asList(new Book(10, "AAA"), new Book(20, "BBB"));
List<Book> list2 = Arrays.asList(new Book(30, "XXX"), new Book(15, "ZZZ"));
List<List<Book>> finalList = Arrays.asList(list1, list2);
Book book = finalList.stream().flatMap(list -> list.stream()).min(new BookComparator()).get();
System.out.println("Name:"+book.getName()+", Price:"+ book.getPrice() );
}
}
输出
Name:AAA, Price:10
3.在Array中使用Stream.flatMap方法
在这里，我们将使用带数组的flatMap。我正在用整数数据创建一个二维数组。最后我们将找出偶数。
1.数组样本
{
{
1,2},{
3,4},{
5,6}}
2.使用flatMap(row -> Arrays.stream(row))方法后
{
1,2,3,4,5,6}
3.使用filter(num -> num%2 == 0)方法后
{
2,4,6}
下面来看具体的代码示例
FlatMapWithArray.java
package com.concretepage;
import java.util.Arrays;
public class FlatMapWithArray {
public static void main(String[] args) {
Integer[][] data = {
{
1,2},{
3,4},{
5,6}};
Arrays.stream(data).flatMap(row -> Arrays.stream(row)).filter(num -> num%2 == 0).
forEach(System.out::println);
}
}
输出
2
4
6
4.在对象数组中使用Stream.flatMap方法
这里我们将提供一个带有对象数组的flatMap示例。我们将创建Writer的二维数组。此类将包含书籍列表。我们会找到最高价的书。
FlatMapWithArrayOfObject.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class FlatMapWithArrayOfObject {
public static void main(String[] args) {
List<Book> books = Arrays.asList(new Book(10, "AAA"), new Book(20, "BBB"));
Writer w1 = new Writer("Mohan", books);
books = Arrays.asList(new Book(30, "CCC"), new Book(15, "DDD"));
Writer w2 = new Writer("Sohan", books);
books = Arrays.asList(new Book(45, "EEE"), new Book(25, "FFF"));
Writer w3 = new Writer("Vikas", books);
books = Arrays.asList(new Book(5, "GGG"), new Book(15, "HHH"));
Writer w4 = new Writer("Ramesh", books);
Writer[][] writerArray = {
{
w1,w2},{
w3,w4}};
Book book = Arrays.stream(writerArray).flatMap(row -> Arrays.stream(row)).
flatMap(writer -> writer.getBooks().stream()).max(new BookComparator()).get();
System.out.println("Name:"+book.getName()+", Price:"+ book.getPrice() );
}
}
输出
Name:EEE, Price:45
5.Files.lines()中使用Stream.flatMap方法
Files.lines()已在Java8中引入。它以流的形式读取文件的所有行。在我们的示例中，我们有一个包含一些行的文件。我们将把所有单词存储在一个列表中并打印出来。
info.txt
My name is Mohan
Country is India
FlatMapWithFile.java
package com.concretepage;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;
public class FlatMapWithFile {
public static void main(String[] args) {
Stream<String> lines = null;
try {
lines = Files.lines(Paths.get("D:/cp/info.txt"), StandardCharsets.UTF_8);
} catch (IOException e) {
e.printStackTrace();
}
Stream<String> stream = lines.flatMap(line -> Stream.of(line.split(" +")));
List<String> words = new ArrayList<>();
stream.forEach(w->words.add(w));
words.forEach(w -> System.out.println(w));
}
}
输出
My
name
is
Mohan
Country
is
India
6.Optional flatMap
java8中引入了Optional。它的行为类似于一个可以保持非空值的容器。它处理NullPointerException。flatMap仅当值存在时才应用。找一个例子。
OptionalflatMap.java
package com.concretepage;
import java.util.Optional;
public class OptionalflatMap {
public static void main(String[] args) {
Optional<PrimeMinister> primeMinister = Optional.of(new PrimeMinister("Narendra Modi", 65));
Optional<Country> country = Optional.of(new Country(primeMinister));
Optional<Person> person = Optional.of(new Person(country));
String pmName= person.flatMap(Person::getCountry).flatMap(Country::getPrimeMinister)
.map(PrimeMinister::getName).orElse("None");
System.out.println(pmName);
}
}
Country.java
package com.concretepage;
import java.util.Optional;
public class Country {
Optional<PrimeMinister> primeMinister;
public Country(){
}
public Country(Optional<PrimeMinister> primeMinister){
this.primeMinister = primeMinister;
}
public Optional<PrimeMinister> getPrimeMinister() {
return primeMinister;
}
public void setPrimeMinister(Optional<PrimeMinister> primeMinister) {
this.primeMinister = primeMinister;
}
}
Person.java
package com.concretepage;
import java.util.Optional;
public class Person {
Optional<Country> country;
public Person(){
}
public Person(Optional<Country> country){
this.country = country;
}
public Optional<Country> getCountry() {
return country;
}
public void setCountry(Optional<Country> country) {
this.country = country;
}
}
输出
Narendra Modi
代码下载
java-8-flatmap-example.zip
参考文献
【1】Java 8 flatMap Example