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Java ArrayList Initialization - Java 147

Java ArrayList Initialization – Java 147

Java ArrayList Initialization

實作 List 接口，初始化集合可以存入重複元素，介紹常見的 constructor 、 immutableList 、 stream 等方法，了解初始化集合的不同操作和方法， ArrayList Java Initialization 初始化集合，會隨著元素增加或移除，大小自動增長或縮小，本篇增加了範例，並透過單元測試來驗證產出結果。

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- list
       |                   +- arraylist
       |                       +- ArrayListInitializationTest.java

單元測試

Initialization of ArrayList 提供初始化操作列表中的元素。

constructor

建立一個 ArrayList ，使用建構子，初始化三個元素。

	@Test
	public void constructor() {
		int expectedSize = 3;
		List<String> list = Arrays.asList("Apple", "Banana", "Cherry");
		List<String> newList = new ArrayList<>(list);
		System.out.println(newList);
		assertEquals(expectedSize, newList.size());

		List<Integer> intList = Arrays.asList(1, 2, 3);
		List<Integer> newIntList = new ArrayList<>(intList);
		System.out.println(newIntList);
		assertEquals(expectedSize, newIntList.size());
	}
[Apple, Banana, Cherry]
[1, 2, 3]

Stream_of

Initialization of ArrayList 建立一個 ArrayList ，使用 Stream of 初始化三個元素。

	@Test
	public void Stream_of() {
		int expectedSize = 3;
		List<String> list = Stream.of("Apple", "Banana", "Cherry").collect(Collectors.toList());
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = Stream.of(1, 2, 3).collect(Collectors.toList());
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());

		List<Integer> intArrayList = Stream.of(new int[] { 1, 2, 3 }).flatMapToInt(e -> Arrays.stream(e)).boxed()
				.collect(Collectors.toList());
		System.out.println(intArrayList);
		assertEquals(expectedSize, intArrayList.size());
	}
[Apple, Banana, Cherry]
[1, 2, 3]

Collections_addAll

Initialization of ArrayList 建立一個 ArrayList ，使用 Collections addAll 初始化三個元素。

	@Test
	public void Collections_addAll() {
		int expectedSize = 3;
		List<String> list = new ArrayList<>();
		Collections.addAll(list, "Apple", "Banana", "Cherry");
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = new ArrayList<>();
		Collections.addAll(intList, 1, 2, 3);
		System.out.println(intList);
		assertEquals(expectedSize, list.size());
	}
[Apple, Banana, Cherry]
[1, 2, 3]

Collections_unmodifiableList

Initialization of ArrayList 建立一個 List ，使用 Collections unmodifiableList 初始化三個元素，為不可變的集合，若操作新增、刪除等方法，會拋出例外。

	@Test
	public void Collections_unmodifiableList() {
		List<String> list = new ArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		List<String> result = Collections.unmodifiableList(list);
		System.out.println(result);
		assertThatCode(() -> result.add("Grape")).isInstanceOf(UnsupportedOperationException.class);

		List<Integer> intList = new ArrayList<>();
		intList.add(1);
		intList.add(2);
		intList.add(3);
		List<Integer> intResult = Collections.unmodifiableList(intList);
		System.out.println(intResult);
		assertThatCode(() -> intResult.add(Integer.MAX_VALUE)).isInstanceOf(UnsupportedOperationException.class);
	}	
[Apple, Banana, Cherry]
[1, 2, 3]

ImmutableList_of

Initializing ArrayList in Java 建立一個 List ，使用 Guava ImmutableList of 初始化三個元素，為不可變的集合，若操作新增、刪除等方法，會拋出例外。

	@Test
	public void ImmutableList_of() {
		int expectedSize = 3;
		List<String> list = ImmutableList.of("Apple", "Banana", "Cherry");
		System.out.println(list);
		assertEquals(expectedSize, list.size());
		assertThatCode(() -> list.add("Grape")).isInstanceOf(UnsupportedOperationException.class);

		List<Integer> intList = ImmutableList.of(1, 2, 3);
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
		assertThatCode(() -> intList.add(Integer.MAX_VALUE)).isInstanceOf(UnsupportedOperationException.class);
	}
[Apple, Banana, Cherry]
[1, 2, 3]

Lists_newArrayList

Initializing ArrayList in Java 建立一個 ArrayList ，使用 Guava Lists newArrayList 初始化三個元素。

	@Test
	public void Lists_newArrayList() {
		int expectedSize = 3;
		List<String> list = Lists.newArrayList("Apple", "Banana", "Cherry");
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = Lists.newArrayList(1, 2, 3);
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
	}
[Apple, Banana, Cherry]
[1, 2, 3]

ArrayListInitializationTest.java

Initializing ArrayList in Java 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.list.arraylist;

import static org.assertj.core.api.Assertions.assertThatCode;
import static org.junit.Assert.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.junit.Test;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;

public class ArrayListInitializationTest {

	@Test
	public void constructor() {
		int expectedSize = 3;
		List<String> list = Arrays.asList("Apple", "Banana", "Cherry");
		List<String> newList = new ArrayList<>(list);
		System.out.println(newList);
		assertEquals(expectedSize, newList.size());

		List<Integer> intList = Arrays.asList(1, 2, 3);
		List<Integer> newIntList = new ArrayList<>(intList);
		System.out.println(newIntList);
		assertEquals(expectedSize, newIntList.size());
	}

	@Test
	public void Stream_of() {
		int expectedSize = 3;
		List<String> list = Stream.of("Apple", "Banana", "Cherry").collect(Collectors.toList());
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = Stream.of(1, 2, 3).collect(Collectors.toList());
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());

		List<Integer> intArrayList = Stream.of(new int[] { 1, 2, 3 }).flatMapToInt(e -> Arrays.stream(e)).boxed()
				.collect(Collectors.toList());
		System.out.println(intArrayList);
		assertEquals(expectedSize, intArrayList.size());
	}

	@Test
	public void Collections_addAll() {
		int expectedSize = 3;
		List<String> list = new ArrayList<>();
		Collections.addAll(list, "Apple", "Banana", "Cherry");
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = new ArrayList<>();
		Collections.addAll(intList, 1, 2, 3);
		System.out.println(intList);
		assertEquals(expectedSize, list.size());
	}

	@Test
	public void Collections_unmodifiableList() {
		List<String> list = new ArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		List<String> result = Collections.unmodifiableList(list);
		System.out.println(result);
		assertThatCode(() -> result.add("Grape")).isInstanceOf(UnsupportedOperationException.class);

		List<Integer> intList = new ArrayList<>();
		intList.add(1);
		intList.add(2);
		intList.add(3);
		List<Integer> intResult = Collections.unmodifiableList(intList);
		System.out.println(intResult);
		assertThatCode(() -> intResult.add(Integer.MAX_VALUE)).isInstanceOf(UnsupportedOperationException.class);
	}

	@Test
	public void ImmutableList_of() {
		int expectedSize = 3;
		List<String> list = ImmutableList.of("Apple", "Banana", "Cherry");
		System.out.println(list);
		assertEquals(expectedSize, list.size());
		assertThatCode(() -> list.add("Grape")).isInstanceOf(UnsupportedOperationException.class);

		List<Integer> intList = ImmutableList.of(1, 2, 3);
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
		assertThatCode(() -> intList.add(Integer.MAX_VALUE)).isInstanceOf(UnsupportedOperationException.class);
	}

	@Test
	public void Lists_newArrayList() {
		int expectedSize = 3;
		List<String> list = Lists.newArrayList("Apple", "Banana", "Cherry");
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = Lists.newArrayList(1, 2, 3);
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
	}
}

心得分享

Java Initializing ArrayList 初始化集合元素，會依新增順序保持排序，以陣列索引為基礎，是一個非同步的操作，允許隨機訪問， Initializing ArrayList in Java 提供了幾種 ArrayList 初始化的操作範例，使用單元測試驗證產出結果。

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