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Java 8のflatMapの例

Java 8 flatMapの例

Java 8では、Streamは次のようなさまざまなデータ型を保持できます。 

Stream
Stream>
Stream>
Stream>

ただし、Stream操作（filter、sum、distinct…）とコレクターはそれをサポートしていないため、次の変換を行うにはflatMap()が必要です。 

Stream      -> flatMap -> Stream
Stream> -> flatMap -> Stream
Stream>    -> flatMap -> Stream
Stream>    -> flatMap -> Stream





flatMap()のしくみ： 








{ {1,2}, {3,4}, {5,6} } -> flatMap -> {1,2,3,4,5,6}

{ {'a','b'}, {'c','d'}, {'e','f'} } -> flatMap -> {'a','b','c','d','e','f'}








1. ストリーム+ String [] + flatMap 




1.1 The below example will print an empty result, because filter() has no idea how to filter a stream of String[].






TestExample1.java








package com.example.java8;

import java.util.Arrays;
import java.util.stream.Stream;

public class TestExample1 {

    public static void main(String[] args) {

        String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};

        //Stream
        Stream temp = Arrays.stream(data);

        //filter a stream of string[], and return a string[]?
        Stream stream = temp.filter(x -> "a".equals(x.toString()));

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

    }

}








出力 








//empty...








1.2 In above example, we should use flatMap() to convert Stream<String[]> to Stream<String>.






TestExample1.java








package com.example.java8;

import java.util.Arrays;
import java.util.stream.Stream;

public class TestExample1 {

    public static void main(String[] args) {

        String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};

        //Stream
        Stream temp = Arrays.stream(data);

        //Stream, GOOD!
        Stream stringStream = temp.flatMap(x -> Arrays.stream(x));

        Stream stream = stringStream.filter(x -> "a".equals(x.toString()));

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

        /*Stream stream = Arrays.stream(data)
                .flatMap(x -> Arrays.stream(x))
                .filter(x -> "a".equals(x.toString()));*/

    }

}








出力 








a










2. ストリーム+セット+ flatMap 




2.1 A student POJO.






Student.java








package com.example.java8;

import java.util.HashSet;
import java.util.Set;

public class Student {

    private String name;
    private Set book;

    public void addBook(String book) {
        if (this.book == null) {
            this.book = new HashSet<>();
        }
        this.book.add(book);
    }
    //getters and setters

}








2.2 flatMap() and Set example.






TestExample2.java








package com.example.java8;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class TestExample2 {

    public static void main(String[] args) {

        Student obj1 = new Student();
        obj1.setName("example");
        obj1.addBook("Java 8 in Action");
        obj1.addBook("Spring Boot in Action");
        obj1.addBook("Effective Java (2nd Edition)");

        Student obj2 = new Student();
        obj2.setName("zilap");
        obj2.addBook("Learning Python, 5th Edition");
        obj2.addBook("Effective Java (2nd Edition)");

        List list = new ArrayList<>();
        list.add(obj1);
        list.add(obj2);

        List collect =
                list.stream()
                        .map(x -> x.getBook())      //Stream>
                        .flatMap(x -> x.stream())   //Stream
                        .distinct()
                        .collect(Collectors.toList());

        collect.forEach(x -> System.out.println(x));
    }

}








出力 








Spring Boot in Action
Effective Java (2nd Edition)
Java 8 in Action
Learning Python, 5th Edition








flatMap(x -> x.stream())は、Setオブジェクトのストリームを収集する方法がわからないため、Collectors.toList()はコンパイラエラーを要求するコメントを試してください。 








3. ストリーム+プリミティブ+ flatMapToInt 




3.1 For primitive type, you can use flatMapToInt.






TestExample3.java








package com.example.java8;

import java.util.Arrays;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class TestExample3 {

    public static void main(String[] args) {

        int[] intArray = {1, 2, 3, 4, 5, 6};

        //1. Stream
        Stream streamArray = Stream.of(intArray);

        //2. Stream -> flatMap -> IntStream
        IntStream intStream = streamArray.flatMapToInt(x -> Arrays.stream(x));

        intStream.forEach(x -> System.out.println(x));

    }

}








出力 








1
2
3
4
5
6










参考文献 




    

  1. 

    Stream＃flatMap JavaDoc 
    

    2. 

  2. 

    Stackoverflow – Java8のmapメソッドとflatMapメソッドの違い 
    

    3. 

  3. 

    Java 8 –配列を印刷する方法 
    

    4. 

  4. 

    Java 8 –コレクターのgroupingByとマッピングの例 
    

    5. 
























































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