Table of Contents

Java ConcurrentHashSet Class

從 JDK 8 開始，可以使用新加入的 keySet (defaultValue) 和 newKeySet 方法，建立一個由 ConcurrentHashMap 支援的 ConcurrentHashSet ，ConcurrentHashSet Class 介紹常見的 contains 、 stream 、 retainAll 等方法，了解 Set 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
       |               +- set
       |                   +- concurrenthashset
       |                       +- ConcurrentHashSetClassTest.java

單元測試

ConcurrentHashSet Class Java 提供檢查包含、串流操作列表中的元素。

contains

使用 ConcurrentHashMap newKeySet ，取得 Set ，增加三個元素，檢查包含指定的元素。

	@Test
	public void contains() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");

		boolean contains = set.contains("Longan");
		System.out.println(contains);
		assertTrue(contains);

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

true
false

containsAll

ConcurrentHashSet Class Java 使用 ConcurrentHashMap newKeySet ，取得 Set ，增加三個元素，檢查是否包含值。

	@Test
	public void containsAll() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");

		Set<String> set2 = ConcurrentHashMap.newKeySet();
		set2.add("Longan");
		set2.add("Tomato");

		boolean contains = set.containsAll(set2);
		System.out.println(contains);
		assertTrue(contains);

		contains = set2.containsAll(set);
		System.out.println(contains);
		assertFalse(contains);
	}

true
false

stream

ConcurrentHashSet Class Java 使用 ConcurrentHashMap newKeySet ，取得 Set ，增加三個元素，執行串流，取得長度小於 6 的元素。

	@Test
	public void stream() {
		int expectedSize = 1;
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");
		set = set.stream().filter(e -> e.length() < 6).collect(Collectors.toSet());
		System.out.println(set);
		assertEquals(expectedSize, set.size());
	}

[Pear]

parallelStream

ConcurrentHashSet Class Java 使用 ConcurrentHashMap newKeySet ，取得 Set ，增加三個元素，並行執行串流。

	@Test
	public void parallelStream() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");
		set.parallelStream().forEach(System.out::println);
		System.out.println("----------");
		set.parallelStream().forEachOrdered(System.out::println);
	}

Pear
Longan
Tomato
----------
Longan
Tomato
Pear

retainAll

使用 ConcurrentHashMap newKeySet ，取得 Set ，增加三個元素，保留指定集合中包含的所有元素。

	@Test
	public void retainAll() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");

		Set<String> set2 = ConcurrentHashMap.newKeySet();
		set2.add("Longan");
		set2.add("Lemon");
		set2.add("Mango");

		boolean result = set.retainAll(set2);
		System.out.println(result);
		System.out.println(set);
		assertTrue(result);

		result = set.retainAll(set2);
		System.out.println(result);
		System.out.println(set);
		assertFalse(result);
	}

true
[Longan]
false
[Longan]

ConcurrentHashSetClassTest.java

Concurrent Hash Set Class in Java 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.set.concurrenthashset;

import static org.junit.Assert.*;

import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.stream.Collectors;

import org.junit.Test;

public class ConcurrentHashSetClassTest {

	@Test
	public void contains() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");

		boolean contains = set.contains("Longan");
		System.out.println(contains);
		assertTrue(contains);

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

	@Test
	public void containsAll() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");

		Set<String> set2 = ConcurrentHashMap.newKeySet();
		set2.add("Longan");
		set2.add("Tomato");

		boolean contains = set.containsAll(set2);
		System.out.println(contains);
		assertTrue(contains);

		contains = set2.containsAll(set);
		System.out.println(contains);
		assertFalse(contains);
	}

	@Test
	public void stream() {
		int expectedSize = 1;
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");
		set = set.stream().filter(e -> e.length() < 6).collect(Collectors.toSet());
		System.out.println(set);
		assertEquals(expectedSize, set.size());
	}

	@Test
	public void parallelStream() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");
		set.parallelStream().forEach(System.out::println);
		System.out.println("----------");
		set.parallelStream().forEachOrdered(System.out::println);
	}

	@Test
	public void retainAll() {
		Set<String> set = ConcurrentHashMap.newKeySet();
		set.add("Longan");
		set.add("Tomato");
		set.add("Pear");

		Set<String> set2 = ConcurrentHashMap.newKeySet();
		set2.add("Longan");
		set2.add("Lemon");
		set2.add("Mango");

		boolean result = set.retainAll(set2);
		System.out.println(result);
		System.out.println(set);
		assertTrue(result);

		result = set.retainAll(set2);
		System.out.println(result);
		System.out.println(set);
		assertFalse(result);
	}
}

心得分享

Java ConcurrentHashSet Class Example 調用 keySet (defaultValue) 和 newKeySet 的方法傳回 Set ，提供了幾種常見方法的操作範例， Concurrent Hash Set 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。