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Java HashMap Methods - Java 147

Java HashMap Methods – Java 147

Java HashMap Methods

將資料存儲在鍵、值對中，可以通過另一種類型的索引訪問，插入重複鍵，會替換相應鍵的元素，允許存儲 Null Key，但應該只有一個空鍵物件，不保證新增時的順序，HashMap Java Methods 介紹常見的 put 、 get 、 clear 、 size 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
       |                   +- hashmap
       |                       +- HashMapMethodsTest.java

單元測試

HashMap Java Methods 提供新增、取得、修改、刪除等操作列表中的元素。

put

HashMap Methods Java 建立一個 HashMap ，增加三個元素。

	@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}

putIfAbsent

HashMap Methods Java 建立一個 HashMap ，當元素不存在時，新增元素。

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

get

HashMap Methods Java 建立一個 HashMap ，內有三個元素，取得指定 Key 元素。

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

getOrDefault

HashMap Methods in Java 建立一個 HashMap ，內有三個元素，取得指定 Key 元素，若不存在傳回預設值。

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

update

HashMap Functions in Java 建立一個 HashMap ，內有三個元素，更新指定 Key 元素。

	@Test
	public void update() {
		Integer expected = 10;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);

		Integer put = map.put("Grape", 10);
		System.out.println(put);
		assertEquals(1, put.intValue());
		System.out.println(map);
		assertEquals(expected, map.get("Grape"));
	}
{Grape=1, Kiwifruit=2, Lemon=3}
1
{Grape=10, Kiwifruit=2, Lemon=3}

remove

HashMap Java Methods 建立一個 HashMap ，內有三個元素，刪除指定 Key 元素。

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

{Kiwifruit=2, Lemon=3}

clear

建立一個 HashMap ，內有三個元素，刪除所有元素。

	@Test
	public void clear() {
		int expectedSize = 0;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.clear();
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}
{}

size

建立一個 HashMap ，內有三個元素，取得長度。

	@Test
	public void size() {
		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.size());
		assertEquals(expectedSize, map.size());
	}
3

putAll

建立兩個 HashMap ，內各有三個元素，合併成為一個 Map 。

	@Test
	public void putAll() {
		int expectedSize = 6;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Apple", 1);
		map.put("Banana", 2);
		map.put("Cherry", 3);

		Map<String, Integer> map2 = new HashMap<String, Integer>();
		map.put("Grape", 4);
		map.put("Kiwifruit", 5);
		map.put("Lemon", 6);

		map.putAll(map2);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}
{Apple=1, Cherry=3, Grape=4, Kiwifruit=5, Lemon=6, Banana=2}

isEmpty

建立一個 HashMap ，檢查是否為空 Map 。

	@Test
	public void isEmpty() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		System.out.println(map.isEmpty());
		assertTrue(map.isEmpty());
		map.put("Grape", 4);
		map.put("Kiwifruit", 5);
		map.put("Lemon", 6);
		System.out.println(map.isEmpty());
		assertFalse(map.isEmpty());
	}
true
false

HashMapMethodsTest.java

HashMap Methods Java 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.map.hashmap;

import static org.junit.Assert.*;

import java.util.HashMap;
import java.util.Map;

import org.junit.Test;

public class HashMapMethodsTest {

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

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

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

	@Test
	public void update() {
		Integer expected = 10;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);

		Integer put = map.put("Grape", 10);
		System.out.println(put);
		assertEquals(1, put.intValue());
		System.out.println(map);
		assertEquals(expected, map.get("Grape"));
	}

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

	@Test
	public void clear() {
		int expectedSize = 0;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.clear();
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

	@Test
	public void size() {
		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.size());
		assertEquals(expectedSize, map.size());
	}

	@Test
	public void putAll() {
		int expectedSize = 6;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Apple", 1);
		map.put("Banana", 2);
		map.put("Cherry", 3);

		Map<String, Integer> map2 = new HashMap<String, Integer>();
		map.put("Grape", 4);
		map.put("Kiwifruit", 5);
		map.put("Lemon", 6);

		map.putAll(map2);
		System.out.println(map);
		assertEquals(expectedSize, map.size());
	}

	@Test
	public void isEmpty() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		System.out.println(map.isEmpty());
		assertTrue(map.isEmpty());
		map.put("Grape", 4);
		map.put("Kiwifruit", 5);
		map.put("Lemon", 6);
		System.out.println(map.isEmpty());
		assertFalse(map.isEmpty());
	}
}

心得分享

HashMap Methods in Java 提供了幾種 HashMap 常見方法的操作範例，在應用上相當廣泛，熟悉 HashMap Functions in Java 這些方法的操作，可以快速撰寫程式，降低錯誤率，再輔以單元測試驗證，建置高效穩定的服務或系統。

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