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[map-randomly-pick] get a random key, value, entry from hashmap #18904

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Nov 3, 2025
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[map-randomly-pick] get a random key, value, entry from hashmap #18904

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137 changes: 137 additions & 0 deletions ...9/src/test/java/com/baeldung/map/randomselection/RandomlyPickDataFromHashMapUnitTest.java
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package com.baeldung.map.randomselection;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Random;

import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class RandomlyPickDataFromHashMapUnitTest {

private static final Logger LOG = LoggerFactory.getLogger(RandomlyPickDataFromHashMapUnitTest.class);
private static final HashMap<String, String> dataMap = new HashMap<>();

@BeforeAll
public static void setupMap() {
dataMap.put("Key-A", "Value: A");
dataMap.put("Key-B", "Value: B");
dataMap.put("Key-C", "Value: C");
dataMap.put("Key-D", "Value: D");
dataMap.put("Key-E", "Value: E");
dataMap.put("Key-F", "Value: F");
dataMap.put("Key-G", "Value: G");
dataMap.put("Key-H", "Value: H");
dataMap.put("Key-I", "Value: I");
}

<K, V> K randomKeyUsingArray(HashMap<K, V> map) {
K[] keys = (K[]) map.keySet().toArray();
Random random = new Random();
return keys[random.nextInt(keys.length)];
}

<K, V> K randomKeyUsingIterator(HashMap<K, V> map) {
Random random = new Random();
int targetIndex = random.nextInt(map.size());

Iterator<K> iterator = map.keySet()
.iterator();
K currentKey = null;

for (int i = 0; i <= targetIndex; i++) {
currentKey = iterator.next();
}

return currentKey;
}

<K, V> K randomKeyUsingStream(HashMap<K, V> map) {
Random random = new Random();
return map.keySet()
.stream()
.skip(random.nextInt(map.size()))
.findFirst()
.orElseThrow();
}

// random value
<K, V> V randomValueUsingStream(HashMap<K, V> map) {
Random random = new Random();
return map.values()
.stream()
.skip(random.nextInt(map.size()))
.findFirst()
.orElseThrow();
}

//random entry
<K, V> HashMap.Entry<K, V> randomEntryUsingStream(HashMap<K, V> map) {
Random random = new Random();
return map.entrySet()
.stream()
.skip(random.nextInt(map.size()))
.findFirst()
.orElseThrow();
}

@Test
void whenGetRandomKeyUsingArray_thenCorrect() {
for (int i = 0; i < 3; i++) {
String key = randomKeyUsingArray(dataMap);
LOG.info("Random Key (Array): {}", key);
}
}

@Test
void whenGetRandomKeyUsingIterator_thenCorrect() {
for (int i = 0; i < 3; i++) {
String key = randomKeyUsingIterator(dataMap);
LOG.info("Random Key (Iterator): {}", key);
}
}

@Test
void whenGetRandomKeyUsingStream_thenCorrect() {
for (int i = 0; i < 3; i++) {
String key = randomKeyUsingStream(dataMap);
LOG.info("Random Key (Stream): {}", key);
}
}

@Test
void whenGetRandomValueByARandomKey_thenCorrect() {
for (int i = 0; i < 3; i++) {
String key = randomKeyUsingStream(dataMap);
String value = dataMap.get(key);
LOG.info("Random Value (by a random key): {}", value);
}
}

@Test
void whenGetRandomValueUsingStream_thenCorrect() {
for (int i = 0; i < 3; i++) {
String value = randomValueUsingStream(dataMap);
LOG.info("Random Value (Stream): {}", value);
}
}

@Test
void whenGetRandomKeyValueByARandomKey_thenCorrect() {
for (int i = 0; i < 3; i++) {
String key = randomKeyUsingStream(dataMap);
String value = dataMap.get(key);
LOG.info("Random Key-Value (by a random key): {} -> {}", key, value);
}
}

@Test
void whenGetRandomEntryUsingStream_thenCorrect() {
for (int i = 0; i < 3; i++) {
HashMap.Entry<String, String> entry = randomEntryUsingStream(dataMap);
LOG.info("Random Entry (Stream): {} -> {}", entry.getKey(), entry.getValue());
}
}
}