Table of Contents

Java Collectors toList Method

使用所提供的靜態方法轉換成 List ， ToList Java Collectors 介紹常見的 toList 等方法，本篇增加了範例，並透過單元測試來驗證產出結果。

public static <T> Collector<T, ?, List<T>> toList() {
}

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- collector
       |                   +- CollectorsToListTest.java

單元測試

Collectors ToList Java 提供設定鍵與值轉換成 List。

toList

Collectors ToList Java 建立一個 Stream ，增加五個元素，轉換成 List 。

	@Test
	public void toList() {
		int expectedSize = 5;
		Stream<String> stream = Stream.of("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
		List<String> result = stream.collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

[Blueberry, Melon, Fig, Blueberry, Melon]

toCollection

Collectors ToList Java 建立一個 Stream ，增加五個元素，轉換成 List 。

	@Test
	public void toCollection() {
		int expectedSize = 5;
		Stream<String> stream = Stream.of("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
		List<String> result = stream.collect(Collectors.toCollection(LinkedList::new));
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

[Blueberry, Melon, Fig, Blueberry, Melon]

integerToList

Collectors ToList Java 建立一個 Stream ，增加五個元素，轉換成 List 。

	@Test
	public void integerToList() {
		int expectedSize = 5;
		Stream<Integer> stream = Stream.of(1, 2, 3, 1, 2);
		List<Integer> result = stream.collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

[1, 2, 3, 1, 2]

arrayToList

Java Collectors ToList 建立一個陣列，增加五個元素，轉換成 Stream ，再轉換成 List 。

	@Test
	public void arrayToList() {
		int expectedSize = 5;
		int[] array = { 1, 2, 3, 1, 2 };
		Stream<int[]> stream = Stream.of(array);
		List<Integer> result = stream.flatMapToInt(Arrays::stream).boxed().collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

[1, 2, 3, 1, 2]

stringToList

Java Collectors ToList 建立一個字串，取得 IntStream ，轉換成 List 。

	@Test
	public void stringToList() {
		int expectedSize = 9;
		String value = "Blueberry";
		IntStream stream = value.chars();
		List<Character> result = stream.mapToObj(e -> (char) e).collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

[B, l, u, e, b, e, r, r, y]

CollectorsToListTest.java

Collectors ToList Example 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.collector;

import static org.junit.Assert.*;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import org.junit.Test;

public class CollectorsToListTest {

	@Test
	public void toList() {
		int expectedSize = 5;
		Stream<String> stream = Stream.of("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
		List<String> result = stream.collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

	@Test
	public void toCollection() {
		int expectedSize = 5;
		Stream<String> stream = Stream.of("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
		List<String> result = stream.collect(Collectors.toCollection(LinkedList::new));
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

	@Test
	public void integerToList() {
		int expectedSize = 5;
		Stream<Integer> stream = Stream.of(1, 2, 3, 1, 2);
		List<Integer> result = stream.collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

	@Test
	public void arrayToList() {
		int expectedSize = 5;
		int[] array = { 1, 2, 3, 1, 2 };
		Stream<int[]> stream = Stream.of(array);
		List<Integer> result = stream.flatMapToInt(Arrays::stream).boxed().collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

	@Test
	public void stringToList() {
		int expectedSize = 9;
		String value = "Blueberry";
		IntStream stream = value.chars();
		List<Character> result = stream.mapToObj(e -> (char) e).collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}
}