LinkedHashMap in Java with Examples - Java 147

LinkedHashMap in Java with Examples – Java 147

LinkedHashMap in Java with Examples

可以有許多 Null Value 值對應到不同的鍵值,但只允許 1 個 Null Key 鍵值,保證依照新增順序保持排序,是一個非同步的操作,LinkedHashMap in Java 介紹常見的 entrySet 、 forEach 、 toArray 等方法,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
       |               +- map
       |                   +- linkedhashmap
       |                       +- LinkedHashMapWithExamplesTest.java   

單元測試

HashMap Java 提供循環訪問、轉成陣列等操作 Map 中的元素。

Fruit

建立 Fruit 物件,覆寫 equals 、 hashCode ,定義屬性和方法,用來建立一個物件。

	@NoArgsConstructor
	@Getter
	@Setter
	@Builder
	public static class Fruit {
		private String name;
		private double quantity;
		private int type;

		public Fruit(String name, double quantity, int type) {
			this.name = name;
			this.quantity = quantity;
			this.type = type;
		}

		public String toString() {
			ToStringBuilder builder = new ToStringBuilder(this, ToStringStyle.JSON_STYLE);
			builder.appendSuper(super.toString());
			builder.append("name", name);
			builder.append("quantity", quantity);
			builder.append("type", type);
			return builder.toString();
		}

		public boolean equals(Object object) {
			if (!(object instanceof Fruit)) {
				return false;
			}
			if (this == object) {
				return true;
			}
			Fruit other = (Fruit) object;
			return new EqualsBuilder().append(getName(), other.getName()).isEquals();
		}

		public int hashCode() {
			return new HashCodeBuilder().append(getName()).toHashCode();
		}
	}

entrySet

LinkedHashMap Java 建立一個 LinkedHashMap ,內有三個元素,迴圈取得元素。

	@Test
	public void entrySet() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		for (Map.Entry<String, Fruit> e : map.entrySet()) {
			System.out.println(e.getKey() + ", " + e.getValue());
		}
	}
Grape, {"name":"Grape","quantity":1.0,"type":1}
Kiwifruit, {"name":"Kiwifruit","quantity":2.0,"type":1}
Lemon, {"name":"Lemon","quantity":3.0,"type":1}

forEach

LinkedHashMap Java 建立一個 LinkedHashMap ,內有三個元素,迴圈取得元素。

	@Test
	public void forEach() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		map.forEach((k, v) -> System.out.println(k + ", " + v));
	}
Grape, {"name":"Grape","quantity":1.0,"type":1}
Kiwifruit, {"name":"Kiwifruit","quantity":2.0,"type":1}
Lemon, {"name":"Lemon","quantity":3.0,"type":1}

keyForEach

LinkedHashMap Java 建立一個 LinkedHashMap ,內有三個元素,迴圈取得 key 元素。

	@Test
	public void keyForEach() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		map.keySet().forEach(e -> System.out.println(e));
	}
Grape
Kiwifruit
Lemon

keyForEachRemaining

LinkedHashMap Java 建立一個 LinkedHashMap ,內有三個元素,迴圈取得 key 剩餘元素。

	@Test
	public void keyForEachRemaining() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Set<String> set = map.keySet();
		Iterator<String> it = set.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}
Grape
Kiwifruit
----------
Lemon

keyIterator

Java LinkedHashMap 建立一個 LinkedHashMap ,內有三個元素,迴圈取得 key 元素。

	@Test
	public void keyIterator() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Iterator<String> it = map.keySet().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}
Grape
Kiwifruit
Lemon

valueForEach

Java LinkedHashMap 建立一個 LinkedHashMap ,內有三個元素,迴圈取得 value 元素。

	@Test
	public void valueForEach() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		map.values().forEach(System.out::println);
	}
{"name":"Grape","quantity":1.0,"type":1}
{"name":"Kiwifruit","quantity":2.0,"type":1}
{"name":"Lemon","quantity":3.0,"type":1}

valueForEachRemaining

Java LinkedHashMap 建立一個 LinkedHashMap ,內有三個元素,迴圈取得 value 剩餘元素。

	@Test
	public void valueForEachRemaining() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Collection<Fruit> collection = map.values();
		Iterator<Fruit> it = collection.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}
{"name":"Grape","quantity":1.0,"type":1}
{"name":"Kiwifruit","quantity":2.0,"type":1}
----------
{"name":"Lemon","quantity":3.0,"type":1}

valueIterator

Java LinkedHashMap 建立一個 LinkedHashMap ,內有三個元素,迴圈取得 value 元素。

	@Test
	public void valueIterator() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Iterator<Fruit> it = map.values().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}
{"name":"Grape","quantity":1.0,"type":1}
{"name":"Kiwifruit","quantity":2.0,"type":1}
{"name":"Lemon","quantity":3.0,"type":1}

keyToArray

Java LinkedHashMap 建立一個 LinkedHashMap ,內有三個元素, key 轉換成 String 陣列。

	@Test
	public void keyToArray() {
		int expectedSize = 3;
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));

		String[] array = new String[map.size()];
		map.keySet().toArray(array);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}
Grape
Kiwifruit
Lemon

keyStreamToArray

Java LinkedHashMap 建立一個 LinkedHashMap ,內有三個元素,使用 stream , key 轉換成 String 陣列。

	@Test
	public void keyStreamToArray() {
		int expectedSize = 3;
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));

		String[] array = map.keySet().stream().toArray(String[]::new);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}

Grape
Kiwifruit
Lemon

LinkedHashMapWithExamplesTest.java

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

package org.ruoxue.java_147.map.linkedhashmap;

import static org.junit.Assert.*;

import java.util.Collection;
import java.util.LinkedHashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

import org.apache.commons.lang3.builder.EqualsBuilder;
import org.apache.commons.lang3.builder.HashCodeBuilder;
import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;
import org.junit.Test;

import lombok.Builder;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;

public class LinkedHashMapWithExamplesTest {
	
	@NoArgsConstructor
	@Getter
	@Setter
	@Builder
	public static class Fruit {
		private String name;
		private double quantity;
		private int type;

		public Fruit(String name, double quantity, int type) {
			this.name = name;
			this.quantity = quantity;
			this.type = type;
		}

		public String toString() {
			ToStringBuilder builder = new ToStringBuilder(this, ToStringStyle.JSON_STYLE);
			builder.appendSuper(super.toString());
			builder.append("name", name);
			builder.append("quantity", quantity);
			builder.append("type", type);
			return builder.toString();
		}

		public boolean equals(Object object) {
			if (!(object instanceof Fruit)) {
				return false;
			}
			if (this == object) {
				return true;
			}
			Fruit other = (Fruit) object;
			return new EqualsBuilder().append(getName(), other.getName()).isEquals();
		}

		public int hashCode() {
			return new HashCodeBuilder().append(getName()).toHashCode();
		}
	}

	@Test
	public void entrySet() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		for (Map.Entry<String, Fruit> e : map.entrySet()) {
			System.out.println(e.getKey() + ", " + e.getValue());
		}
	}

	@Test
	public void forEach() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		map.forEach((k, v) -> System.out.println(k + ", " + v));
	}

	@Test
	public void keyForEach() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		map.keySet().forEach(e -> System.out.println(e));
	}

	@Test
	public void keyForEachRemaining() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Set<String> set = map.keySet();
		Iterator<String> it = set.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}

	@Test
	public void keyIterator() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Iterator<String> it = map.keySet().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}

	@Test
	public void valueForEach() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		map.values().forEach(System.out::println);
	}

	@Test
	public void valueForEachRemaining() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Collection<Fruit> collection = map.values();
		Iterator<Fruit> it = collection.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}

	@Test
	public void valueIterator() {
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));
		Iterator<Fruit> it = map.values().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}

	@Test
	public void keyToArray() {
		int expectedSize = 3;
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));

		String[] array = new String[map.size()];
		map.keySet().toArray(array);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}

	@Test
	public void keyStreamToArray() {
		int expectedSize = 3;
		Map<String, Fruit> map = new LinkedHashMap<>();
		map.put("Grape", new Fruit("Grape", 1, 1));
		map.put("Kiwifruit", new Fruit("Kiwifruit", 2, 1));
		map.put("Lemon", new Fruit("Lemon", 3, 1));

		String[] array = map.keySet().stream().toArray(String[]::new);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}
}

心得分享

Java LinkedHashMap Example 繼承 HashMap ,實作集合框架的 Map 接口,採用雙向鏈結實作功能,保證新增時的順序,提供了幾種 LinkedHashMap 常見方法的操作範例,在應用上相當廣泛,熟悉 Java LinkedHashMap 這些方法的操作,像是: entrySet 、 forEach 、 toArray 等方法。

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