Table of Contents

Initialize HashMap in Java

繼承了一個 AbstractMap ，將資料儲存在 ( Key, Value ) 對中，是集合框架的一部分，提供無參數建構子，預設初始容量為 16 ，加載因子為 0.75 ， Initialize HashMap Java 初始化集合，會隨著元素增加或移除，大小自動增長或縮小，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，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
       |                   +- hashmap
       |                       +- InitializeHashMapTest.java

單元測試

Java Initialize HashMap 提供初始化操作集合中的元素。

put

建立一個 HashMap ，初始化三個元素。

	@Test
	public void put() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

{Grape=1, Kiwifruit=2, Lemon=3}

doubleBrace

建立一個 HashMap ，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;
		Map<String, Integer> map = new HashMap<String, Integer>() {
			private static final long serialVersionUID = -5487223135233714632L;
			{
				put("Grape", 1);
				put("Kiwifruit", 2);
				put("Lemon", 3);
			}
		};
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

{Grape=1, Kiwifruit=2, Lemon=3}

putAll

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

	@Test
	public void putAll() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Map<String, Integer> newMap = new HashMap<String, Integer>();
		newMap.putAll(map);
		System.out.println(newMap);
		assertEquals(expectedSize, newMap.size());
	}

{Kiwifruit=2, Lemon=3, Grape=1}

constructor

Java Initialize HashMap 建立一個 HashMap ，初始化三個元素。

	@Test
	public void constructor() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Map<String, Integer> newMap = new HashMap<String, Integer>(map);
		System.out.println(newMap);
		assertEquals(expectedSize, newMap.size());
	}

{Kiwifruit=2, Lemon=3, Grape=1}

immutableMap

HashMap Initialization Java 建立一個 ImmutableMap 不可變的 Map ，初始化三個元素。

	@Test
	public void immutableMap() {
		int expectedSize = 3;
		Map<String, Integer> map = ImmutableMap.of("Grape", 1, "Kiwifruit", 2, "Lemon", 3);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

{Grape=1, Kiwifruit=2, Lemon=3}

removeThrowException

HashMap Initialization Java 建立一個 ImmutableMap 不可變的 Map ，初始化三個元素，操作 remove 方法會拋出例外。

	@Test(expected = UnsupportedOperationException.class)
	public void removeThrowException() {
		int expectedSize = 3;
		Map<String, Integer> map = ImmutableMap.of("Grape", 1, "Kiwifruit", 2, "Lemon", 3);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
		map.remove("Grape");
	}

java.lang.UnsupportedOperationException
	at com.google.common.collect.ImmutableMap.remove(ImmutableMap.java:652)
	at org.ruoxue.java_147.map.InitializeHashMapTest.removeThrowException(InitializeHashMapTest.java:78)

InitializeHashMapTest.java

HashMap Initialization Java 新增單元測試，驗證 Java HashMap Initialize 是否符合預期。

package org.ruoxue.java_147.map.hashmap;

import static org.junit.Assert.*;
import java.util.HashMap;
import java.util.Map;
import org.junit.Test;
import com.google.common.collect.ImmutableMap;

public class InitializeHashMapTest {

	@Test
	public void put() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

	@Test
	public void doubleBrace() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>() {
			private static final long serialVersionUID = -5487223135233714632L;
			{
				put("Grape", 1);
				put("Kiwifruit", 2);
				put("Lemon", 3);
			}
		};
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

	@Test
	public void putAll() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Map<String, Integer> newMap = new HashMap<String, Integer>();
		newMap.putAll(map);
		System.out.println(newMap);
		assertEquals(expectedSize, newMap.size());
	}

	@Test
	public void constructor() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Map<String, Integer> newMap = new HashMap<String, Integer>(map);
		System.out.println(newMap);
		assertEquals(expectedSize, newMap.size());
	}

	@Test
	public void immutableMap() {
		int expectedSize = 3;
		Map<String, Integer> map = ImmutableMap.of("Grape", 1, "Kiwifruit", 2, "Lemon", 3);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

	@Test(expected = UnsupportedOperationException.class)
	public void removeThrowException() {
		int expectedSize = 3;
		Map<String, Integer> map = ImmutableMap.of("Grape", 1, "Kiwifruit", 2, "Lemon", 3);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
		map.remove("Grape");
	}
}