Table of Contents

Java HashMap compute Method

計算一個新值並將它與指定的鍵相關聯，提供該鍵可能存在或不存在於 Map 中的關聯計算， Compute Java HashMap 介紹常見的 compute 、 computeIfAbsent 、 computeIfPresent 等方法，本篇增加了範例，並透過單元測試來驗證產出結果。

default V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
}

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- map
       |                   +- hashmap
       |                       +- HashMapComputeTest.java

單元測試

HashMap Compute Java 提供該鍵存在或已存在於 Map 中的關聯計算。

compute

建立一個 HashMap ，增加三個元素，計算符合條件的新值。

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

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

computeFunction

建立一個 HashMap ，增加三個元素，計算符合條件的新值。

	@Test
	public void computeFunction() {
		Integer expected = 2;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = "Grape";
		map.compute(key, (k, v) -> {
			v += 1;
			return v;
		});
		System.out.println(map);
		assertEquals(expected, map.get(key));
	}

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

computeNullPointerException

HashMap Compute Java 建立一個 HashMap ，內有三個元素，計算不存在的 Key 新值，會拋出 NullPointerException 例外。

	@Test(expected = NullPointerException.class)
	public void computeNullPointerException() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = null;
		map.compute(key, (k, v) -> v + 1);
		System.out.println(map);
	}

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

computeCount

HashMap Compute Java 建立一個 HashMap ，計算 String 個數。

	@Test
	public void computeCount() {
		String value = "Hello World, Java Learn";
		Map<String, Integer> map = new HashMap<String, Integer>();
		for (int i = 0; i < value.length(); i++) {
			String key = String.valueOf(value.charAt(i));
			map.compute(key, (k, v) -> {
				v = (v == null ? 1 : v + 1);
				return v;
			});
		}
		System.out.println(map);
	}

{ =3, a=3, d=1, e=2, H=1, J=1, L=1, ,=1, l=3, n=1, o=2, r=2, v=1, W=1}

computeIfAbsent

HashMap Compute Java 建立一個 HashMap ，內有三個元素，計算不存在的 Key 新值。

	@Test
	public void computeIfAbsent() {
		Integer expected = 4;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = "Mango";
		Integer result = map.computeIfAbsent(key, k -> 4);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = "Grape";
		result = map.computeIfAbsent(key, k -> 2);
		System.out.println(result);
		System.out.println(map);
		assertEquals(1, result.intValue());
	}

{Grape=1, Kiwifruit=2, Lemon=3}
4
{Grape=1, Mango=4, Kiwifruit=2, Lemon=3}
1
{Grape=1, Mango=4, Kiwifruit=2, Lemon=3}

computeIfPresent

HashMap Compute Java 建立一個 HashMap ，內有三個元素，計算已存在的 Key 新值。

	@Test
	public void computeIfPresent() {
		Integer expected = 2;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = "Grape";
		Integer result = map.computeIfPresent(key, (k, v) -> v + 1);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = "Mango";
		result = map.computeIfPresent(key, (k, v) -> 4);
		System.out.println(result);
		System.out.println(map);
		assertNull(result);
	}

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

HashMapComputeTest.java

HashMap Compute Example 新增單元測試，驗證 Java HashMap Compute 是否符合預期。

package org.ruoxue.java_147.map.hashmap;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNull;

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

import org.junit.Test;

public class HashMapComputeTest {

	public HashMapComputeTest() {

	}

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

	@Test
	public void computeFunction() {
		Integer expected = 2;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = "Grape";
		map.compute(key, (k, v) -> {
			v += 1;
			return v;
		});
		System.out.println(map);
		assertEquals(expected, map.get(key));
	}

	@Test(expected = NullPointerException.class)
	public void computeNullPointerException() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = null;
		map.compute(key, (k, v) -> v + 1);
		System.out.println(map);
	}

	@Test
	public void computeCount() {
		String value = "Hello World, Java Learn";
		Map<String, Integer> map = new HashMap<String, Integer>();
		for (int i = 0; i < value.length(); i++) {
			String key = String.valueOf(value.charAt(i));
			map.compute(key, (k, v) -> {
				v = (v == null ? 1 : v + 1);
				return v;
			});
		}
		System.out.println(map);
	}

	@Test
	public void computeIfAbsent() {
		Integer expected = 4;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = "Mango";
		Integer result = map.computeIfAbsent(key, k -> 4);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = "Grape";
		result = map.computeIfAbsent(key, k -> 2);
		System.out.println(result);
		System.out.println(map);
		assertEquals(1, result.intValue());
	}

	@Test
	public void computeIfPresent() {
		Integer expected = 2;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		System.out.println(map);
		String key = "Grape";
		Integer result = map.computeIfPresent(key, (k, v) -> v + 1);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = "Mango";
		result = map.computeIfPresent(key, (k, v) -> 4);
		System.out.println(result);
		System.out.println(map);
		assertNull(result);
	}
}