Table of Contents

Initialize ArrayList in Java

初始化 ArrayList 元素不能直接用於基本類型，如 int 、 char 等，必須將基本型別其包裝成類別，才能存入集合，介紹常見的 add 、 addAll 、 doubleBrace 、 asList 等方法，了解初始化集合的不同操作和方法， Initialize ArrayList Java 使用動態陣列來存儲元素，實現可調整大小陣列的功能，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
       |                       +- InitializeArrayListTest.java

單元測試

Java Initialize ArrayList 提供初始化操作列表中的元素。

add

建立一個 ArrayList ，初始化三個元素。

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

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

[Apple, Banana, Cherry]
[1, 2, 3]

doubleBrace

建立一個 ArrayList ，初始化三個元素，代表建立並載入一個新的類別，對效能有不良影響。

	@Test
	public void doubleBrace() {
		int expectedSize = 3;
		List<String> list = new ArrayList<String>() {
			private static final long serialVersionUID = -5948701413773974786L;
			{
				add("Apple");
				add("Banana");
				add("Cherry");
			}
		};
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = new ArrayList<Integer>() {
			private static final long serialVersionUID = -1238701413773974786L;
			{
				add(1);
				add(2);
				add(3);
			}
		};
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
	}

[Apple, Banana, Cherry]
[1, 2, 3]

addAll

Java Initialize ArrayList 建立兩個 ArrayList ，使用給定的 List 物件，初始化三個元素。

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

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

[Apple, Banana, Cherry]
[1, 2, 3]

Arrays_asList

Java Initialize ArrayList 建立一個 List ，使用 Arrats asList 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 Arrays_asList() {
		int expectedSize = 3;
		List<String> list = Arrays.asList("Apple", "Banana", "Cherry");
		System.out.println(list);
		assertEquals(expectedSize, list.size());
		assertThatCode(() -> list.add("Grape")).isInstanceOf(UnsupportedOperationException.class);

		List<Integer> intList = Arrays.asList(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]

Arrays_asList_withMutable

Java Initialize ArrayList 建立一個 ArrayList ，使用 Arrats asList 初始化三個元素，再傳入 ArrayList 建構子，建立集合，則會成為可變集合。

	@Test
	public void Arrays_asList_withMutable() {
		int expectedSize = 3;
		List<String> list = new ArrayList<>(Arrays.asList("Apple", "Banana", "Cherry"));
		System.out.println(list);
		assertEquals(expectedSize, list.size());
		list.add("Grape");
		System.out.println(list);
		assertEquals(4, list.size());

		List<Integer> intList = new ArrayList<>(Arrays.asList(1, 2, 3));
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
		intList.add(Integer.MAX_VALUE);
		System.out.println(intList);
		assertEquals(4, intList.size());
	}

[Apple, Banana, Cherry]
[Apple, Banana, Cherry, Grape]
[1, 2, 3]
[1, 2, 3, 2147483647]

InitializeArrayListTest.java

ArrayList Initialization Java 新增單元測試，驗證 Java ArrayList Initialize 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.assertEquals;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.junit.Test;

public class InitializeArrayListTest {

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

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

	@Test
	public void doubleBrace() {
		int expectedSize = 3;
		List<String> list = new ArrayList<String>() {
			private static final long serialVersionUID = -5948701413773974786L;
			{
				add("Apple");
				add("Banana");
				add("Cherry");
			}
		};
		System.out.println(list);
		assertEquals(expectedSize, list.size());

		List<Integer> intList = new ArrayList<Integer>() {
			private static final long serialVersionUID = -1238701413773974786L;
			{
				add(1);
				add(2);
				add(3);
			}
		};
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
	}

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

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

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

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

	@Test
	public void Arrays_asList_withMutable() {
		int expectedSize = 3;
		List<String> list = new ArrayList<>(Arrays.asList("Apple", "Banana", "Cherry"));
		System.out.println(list);
		assertEquals(expectedSize, list.size());
		list.add("Grape");
		System.out.println(list);
		assertEquals(4, list.size());

		List<Integer> intList = new ArrayList<>(Arrays.asList(1, 2, 3));
		System.out.println(intList);
		assertEquals(expectedSize, intList.size());
		intList.add(Integer.MAX_VALUE);
		System.out.println(intList);
		assertEquals(4, intList.size());
	}
}