Table of Contents

Java Get Keys From Value Map

採用迴圈或 Stream 的方式，操作 entrySet 等方法，從 Value 取得 Keys 的方法，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
       |                   +- GetKeysFromValueMapTest.java

單元測試

Java Get Key from Map by Value 從符合條件的 Value 元素中，取得 Key。

keySet

建立一個 HashMap ，增加五個元素，keySet 遍歷元素，輸出在 console 上。

	@Test
	public void keySet() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 5);
		for (String key : map.keySet()) {
			System.out.println(key);
		}
	}

null
Grape
Mango
Kiwifruit
Lemon

getKeysFromValue

建立一個 HashMap ，增加五個元素，從 value 取得 keys ，輸出在 console 上。

	public Set<String> getKeys(Map<String, Integer> map, Integer value) {
		Set<String> result = new HashSet<>();
		for (Map.Entry<String, Integer> entry : map.entrySet()) {
			if (Objects.equals(entry.getValue(), value)) {
				result.add(entry.getKey());
			}
		}
		return result;
	}

	@Test
	public void getKeysFromValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		for (String key : getKeys(map, 3)) {
			System.out.println(key);
		}
	}

null
Lemon

getKeysFromNullValue

建立一個 HashMap ，增加五個元素，從 value = null 取得 keys ，輸出在 console 上。

	@Test
	public void getKeysFromNullValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		for (String key : getKeys(map, null)) {
			System.out.println(key);
		}
	}

Mango

getKeysUseStreamFromValue

建立一個 HashMap ，增加五個元素，使用 stream 從 value 取得 keys ，輸出在 console 上。

	public Set<String> getKeysUseStream(Map<String, Integer> map, Integer value) {
		return map
				.entrySet()
				.stream()
				.filter(e -> Objects.equals(e.getValue(), value))
				.map(Map.Entry::getKey)
				.collect(Collectors.toSet());

	}

	@Test
	public void getKeysUseStreamFromValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		for (String key : getKeysUseStream(map, 3)) {
			System.out.println(key);
		}
	}

null
Lemon

getKeysOptionalFromValue

建立一個 HashMap ，增加五個元素，使用從 value 取得第一個 key optional ，此時因 key = null ，會拋出 NullPointerException。

	public Optional<String> getKeysOptional(Map<String, Integer> map, Integer value) {
		return map
				.entrySet()
				.stream()
				.filter(e -> Objects.equals(e.getValue(), value))
				.map(Map.Entry::getKey)
				.findFirst();

	}

	@Test(expected = NullPointerException.class)
	public void getKeysOptionalFromValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		Optional<String> optional = getKeysOptional(map, 3);
		if (optional.isPresent()) {
			System.out.println(optional.get());
		}
	}

GetKeysFromValueMapTest.java

Get Key from Value in a Java Map 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.map;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;

import org.junit.Test;

public class GetKeysFromValueMapTest {

	public GetKeysFromValueMapTest() {

	}

	@Test
	public void keySet() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 5);
		for (String key : map.keySet()) {
			System.out.println(key);
		}
	}

	public Set<String> getKeys(Map<String, Integer> map, Integer value) {
		Set<String> result = new HashSet<>();
		for (Map.Entry<String, Integer> entry : map.entrySet()) {
			if (Objects.equals(entry.getValue(), value)) {
				result.add(entry.getKey());
			}
		}
		return result;
	}

	@Test
	public void getKeysFromValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		for (String key : getKeys(map, 3)) {
			System.out.println(key);
		}
	}

	@Test
	public void getKeysFromNullValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		for (String key : getKeys(map, null)) {
			System.out.println(key);
		}
	}

	public Set<String> getKeysUseStream(Map<String, Integer> map, Integer value) {
		return map.entrySet().stream().filter(e -> Objects.equals(e.getValue(), value)).map(Map.Entry::getKey)
				.collect(Collectors.toSet());

	}

	@Test
	public void getKeysUseStreamFromValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		for (String key : getKeysUseStream(map, 3)) {
			System.out.println(key);
		}
	}

	public Optional<String> getKeysOptional(Map<String, Integer> map, Integer value) {
		return map.entrySet().stream().filter(e -> Objects.equals(e.getValue(), value)).map(Map.Entry::getKey)
				.findFirst();

	}

	@Test(expected = NullPointerException.class)
	public void getKeysOptionalFromValue() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.put("Mango", null);
		map.put(null, 3);
		Optional<String> optional = getKeysOptional(map, 3);
		if (optional.isPresent()) {
			System.out.println(optional.get());
		}
	}
}