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Java EnumMap compute Method - Java 147

Java EnumMap compute Method – Java 147

Java EnumMap compute Method

使用所提供的映射方法 BiFunction 為指定鍵計算一個新值，該鍵可能存在或不存在於 Map 中， Compute Java EnumMap 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
       |                   +- enummap
       |                       +- EnumMapComputeTest.java

單元測試

提供該鍵存在或已存在於 Map 中的關聯計算。

Fruit

建立 Fruit enum 類別。

	public enum Fruit {
		GRAPE, KIWIFRUIT, LEMON,

		EMPTY,

		APPLE, BANANA, CHERRY
	}

compute

建立一個 EnumMap ，增加三個元素，計算符合條件的新值。

	@Test
	public void compute() {
		Integer expected = 2;
		Map<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.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

EnumMap Compute Java 建立一個 EnumMap ，增加三個元素，計算符合條件的新值。

	@Test
	public void computeFunction() {
		Integer expected = 2;
		Map<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.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

EnumMap Compute Java 建立一個 EnumMap ，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<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = null;
		map.compute(key, (k, v) -> v + 1);
		System.out.println(map);
	}
java.lang.NullPointerException
	at org.ruoxue.java_147.map.enummap.EnumMapComputeTest.lambda$2(EnumMapComputeTest.java:60)
	at java.util.Map.compute(Map.java:1093)
	at org.ruoxue.java_147.map.enummap.EnumMapComputeTest.computeNullPointerException(EnumMapComputeTest.java:60)

computeIfAbsent

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

	@Test
	public void computeIfAbsent() {
		Integer expected = 4;
		Map<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.APPLE;
		Integer result = map.computeIfAbsent(key, k -> 4);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = Fruit.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, KIWIFRUIT=2, LEMON=3, APPLE=4}
1
{GRAPE=1, KIWIFRUIT=2, LEMON=3, APPLE=4}

computeIfPresent

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

	@Test
	public void computeIfPresent() {
		Integer expected = 2;
		Map<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.GRAPE;
		Integer result = map.computeIfPresent(key, (k, v) -> v + 1);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = Fruit.APPLE;
		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}

EnumMapComputeTest.java

EnumMap Compute Example 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.map.enummap;

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

import java.util.EnumMap;
import java.util.Map;

import org.junit.Test;

public class EnumMapComputeTest {

	public enum Fruit {
		GRAPE, KIWIFRUIT, LEMON,

		EMPTY,

		APPLE, BANANA, CHERRY
	}

	@Test
	public void compute() {
		Integer expected = 2;
		Map<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.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<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.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<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = null;
		map.compute(key, (k, v) -> v + 1);
		System.out.println(map);
	}

	@Test
	public void computeIfAbsent() {
		Integer expected = 4;
		Map<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.APPLE;
		Integer result = map.computeIfAbsent(key, k -> 4);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = Fruit.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<Fruit, Integer> map = new EnumMap<>(Fruit.class);
		map.put(Fruit.GRAPE, 1);
		map.put(Fruit.KIWIFRUIT, 2);
		map.put(Fruit.LEMON, 3);
		System.out.println(map);
		Fruit key = Fruit.GRAPE;
		Integer result = map.computeIfPresent(key, (k, v) -> v + 1);
		System.out.println(result);
		System.out.println(map);
		assertEquals(expected, result);
		key = Fruit.APPLE;
		result = map.computeIfPresent(key, (k, v) -> 4);
		System.out.println(result);
		System.out.println(map);
		assertNull(result);
	}
}

心得分享

Java EnumMap Compute 提供了幾種 EnumMap 常見方法的操作範例，在應用上相當廣泛，熟悉 EnumMap Compute Example 這些方法的操作，例如： compute 、 computeIfAbsent 、 computeIfPresent 等方法，可以讓程式碼更加地簡潔及容易維護。

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