Java CopyOnWriteArrayList Class - Java 147

Java CopyOnWriteArrayList Class – Java 147

Java CopyOnWriteArrayList Class

實作了 List 接口,是 ArrayList 的增強版本 ,其中所有修改像是:新增、修改、刪除等,都是透過建立新副本來實現的,位於 java.util.concurrent 套件中,是為在並發環境中使用而建立的資料結構,使用起來成本很高,因為每次更新操作都會建立一個新的副本,因此,如果頻繁操作是讀取操作,那麼是最好的選擇,CopyOnWriteArrayList Class 介紹常見的 contains 、 indexOf 、 sort 、 stream 、 subList 等方法,了解陣列列表的不同操作和方法,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
       |                   +- copyonwritearraylist
       |                       +- CopyOnWriteArrayListClassTest.java   

單元測試

CopyOnWriteArrayList Class 提供檢查包含、搜索指定、排序、串流操作列表中的元素。

contains

建立一個 CopyOnWriteArrayList ,增加三個元素,檢查包含指定的元素。

	@Test
	public void contains() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		boolean contains = list.contains("Apple");
		System.out.println(contains);
		assertTrue(contains);

		contains = list.contains("Grape");
		System.out.println(contains);
		assertFalse(contains);
	}
true
false

containsAll

CopyOnWriteArrayList Class Java 建立一個 CopyOnWriteArrayList ,內有三個元素,,檢查包含所有指定的元素。

	@Test
	public void containsAll() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		List<String> list2 = new CopyOnWriteArrayList<>();
		list2.add("Apple");
		list2.add("Banana");

		boolean contains = list.containsAll(list2);
		System.out.println(contains);
		assertTrue(contains);

		contains = list2.containsAll(list);
		System.out.println(contains);
		assertFalse(contains);
	}
true
false

indexOf

CopyOnWriteArrayList Class Java 建立一個 CopyOnWriteArrayList ,內有三個元素,搜索指定元素,傳回索引。

	@Test
	public void indexOf() {
		int expected = 1;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		int index = list.indexOf("Banana");
		System.out.println(index);
		assertEquals(expected, index);

		index = list.indexOf("Grape");
		System.out.println(index);
		assertEquals(-1, index);
	}
1
-1

lastIndexOf

CopyOnWriteArrayList Class Java 建立一個 CopyOnWriteArrayList ,從最後搜索指定元素,傳回索引。

	@Test
	public void lastIndexOf() {
		int expected = 3;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list.add("Apple");

		int index = list.lastIndexOf("Apple");
		System.out.println(index);
		assertEquals(expected, index);

		index = list.indexOf("Grape");
		System.out.println(index);
		assertEquals(-1, index);
	}
3
-1

sort

CopyOnWriteArrayList Class Java 建立一個 CopyOnWriteArrayList ,內有三個元素,排序元素。

	@Test
	public void sort() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Banana");
		list.add("Apple");
		list.add("Cherry");
		list.sort(Comparator.naturalOrder());
		System.out.println(list);

		list.add("Grape");
		list.sort(Comparator.reverseOrder());
		System.out.println(list);
	}
[Apple, Banana, Cherry]
[Grape, Cherry, Banana, Apple]

stream

CopyOnWriteArrayList Class in Java 建立一個 CopyOnWriteArrayList ,內有三個元素,執行串流,取得長度小於 6 的元素。

	@Test
	public void stream() {
		int expectedSize = 1;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list = list.stream().filter(e -> e.length() < 6).collect(Collectors.toList());
		System.out.println(list);
		assertEquals(expectedSize, list.size());
	}
[Apple]

parallelStream

CopyOnWriteArrayList Class in Java 建立一個 CopyOnWriteArrayList ,內有三個元素,並行執行串流。

	@Test
	public void parallelStream() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list.parallelStream().forEach(System.out::println);
		System.out.println("----------");
		list.parallelStream().forEachOrdered(System.out::println);
	}
Banana
Cherry
Apple
----------
Apple
Banana
Cherry

subList

CopyOnWriteArrayList Class in Java 建立一個 CopyOnWriteArrayList ,內有三個元素,建立指定位置的子列表 。

	@Test
	public void subList() {
		int expectedSize = 2;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		System.out.println(list);
		List<String> list2 = list.subList(0, 2);
		System.out.println(list2);

		assertEquals(expectedSize, list2.size());
	}
[Apple, Banana, Cherry]
[Apple, Banana]

replaceAll

CopyOnWriteArrayList Class in Java 建立一個 CopyOnWriteArrayList ,內有三個元素,替換所有元素 。

	@Test
	public void replaceAll() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list.replaceAll(e -> e.toUpperCase());
		System.out.println(list);
	}
[APPLE, BANANA, CHERRY]

retainAll

Java CopyOnWriteArrayList Class Example 建立兩個 CopyOnWriteArrayList ,內各有三個元素,只保留相同元素 。

	@Test
	public void retainAll() {
		int expectedSize = 1;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		List<String> list2 = new CopyOnWriteArrayList<>();
		list2.add("Apple");
		list2.add("Lemon");
		list2.add("Mango");

		list.retainAll(list2);
		System.out.println(list);
		assertEquals(expectedSize, list.size());
	}
[Apple]

CopyOnWriteArrayListClassTest.java

CopyOnWriteArrayList Class in Java 新增單元測試,驗證 Java CopyOnWriteArrayList Class Example 是否符合預期。

package org.ruoxue.java_147.list.copyonwritearraylist;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import java.util.Comparator;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.stream.Collectors;

import org.junit.Test;

public class CopyOnWriteArrayListClassTest {

	@Test
	public void contains() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		boolean contains = list.contains("Apple");
		System.out.println(contains);
		assertTrue(contains);

		contains = list.contains("Grape");
		System.out.println(contains);
		assertFalse(contains);
	}

	@Test
	public void containsAll() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		List<String> list2 = new CopyOnWriteArrayList<>();
		list2.add("Apple");
		list2.add("Banana");

		boolean contains = list.containsAll(list2);
		System.out.println(contains);
		assertTrue(contains);

		contains = list2.containsAll(list);
		System.out.println(contains);
		assertFalse(contains);
	}

	@Test
	public void indexOf() {
		int expected = 1;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		int index = list.indexOf("Banana");
		System.out.println(index);
		assertEquals(expected, index);

		index = list.indexOf("Grape");
		System.out.println(index);
		assertEquals(-1, index);
	}

	@Test
	public void lastIndexOf() {
		int expected = 3;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list.add("Apple");

		int index = list.lastIndexOf("Apple");
		System.out.println(index);
		assertEquals(expected, index);

		index = list.indexOf("Grape");
		System.out.println(index);
		assertEquals(-1, index);
	}

	@Test
	public void sort() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Banana");
		list.add("Apple");
		list.add("Cherry");
		list.sort(Comparator.naturalOrder());
		System.out.println(list);

		list.add("Grape");
		list.sort(Comparator.reverseOrder());
		System.out.println(list);
	}

	@Test
	public void stream() {
		int expectedSize = 1;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list = list.stream().filter(e -> e.length() < 6).collect(Collectors.toList());
		System.out.println(list);
		assertEquals(expectedSize, list.size());
	}

	@Test
	public void parallelStream() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list.parallelStream().forEach(System.out::println);
		System.out.println("----------");
		list.parallelStream().forEachOrdered(System.out::println);
	}

	@Test
	public void subList() {
		int expectedSize = 2;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		System.out.println(list);
		List<String> list2 = list.subList(0, 2);
		System.out.println(list2);

		assertEquals(expectedSize, list2.size());
	}

	@Test
	public void replaceAll() {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");
		list.replaceAll(e -> e.toUpperCase());
		System.out.println(list);
	}

	@Test
	public void retainAll() {
		int expectedSize = 1;
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("Apple");
		list.add("Banana");
		list.add("Cherry");

		List<String> list2 = new CopyOnWriteArrayList<>();
		list2.add("Apple");
		list2.add("Lemon");
		list2.add("Mango");

		list.retainAll(list2);
		System.out.println(list);
		assertEquals(expectedSize, list.size());
	}
}

心得分享

Java CopyOnWriteArrayList Class Example 使用場景,當讀多寫少,如:白名單,黑名單,商品類目的訪問和更新場景,因為寫的時候會複製新集合,此外當集合不大,因為寫的時候會複製新集合,最後即時性要求不高,因為有可能會讀取到舊的集合資料,提供了幾種 CopyOnWriteArrayList 常見方法的操作範例, CopyOnWriteArrayList Class in Java 在應用上相當廣泛,熟悉這些方法的操作,能夠提高開發效率,節省維護上的成本。

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