Converting Array to HashMap using Java 8 Stream

Question

I am writing a function to convert array to Map using Java 8 Stream.

 public static <K, V> Map<K, V> toMap(Object... entries) {
 // Requirements:
 // entries must be K1, V1, K2, V2, .... ( even length )
 if (entries.length % 2 == 1) {
 throw new IllegalArgumentException("Invalid entries");
 }
 // TODO
 Arrays.stream(entries).????
}

Valid usages

 Map<String, Integer> map1 = toMap("k1", 1, "k2", 2);
 Map<String, String> map2 = toMap("k1", "v1", "k2", "v2", "k3", "v3");

Invalid usages

 Map<String, Integer> map1 = toMap("k1", 1, "k2", 2, "k3");

Any helps?

Thanks!

• chrylis -cautiouslyoptimistic-: What is your purpose for this? Map.of exists.

Answer 1

Did you realize it already exists? It's been around since Java 9.

The following creates an immutable map of the keys and values.

 Map<String, String> map2 = Map.of("k1", "v1", "k2", "v2", "k3", "v3");

For a mutable map do

 Map<String, String> map2 = new HashMap<>(Map.of("k1", "v1", "k2", "v2", "k3", "v3"));

• Ashvin Sharma: The question explicitly mentions Java 8 so Java 9 is out of the scope of this question

Answer 2

You may use

 public static <K, V> Map<K, V> toMap(
 Class<K> keyType, Class<V> valueType, Object... entries) {
 if(entries.length % 2 == 1)
 throw new IllegalArgumentException("Invalid entries");
 return IntStream.range(0, entries.length/2).map(i -> i*2)
 .collect(HashMap::new,
 (m,i)->m.put(keyType.cast(entries[i]), valueType.cast(entries[i+1])),
 Map::putAll);
}

Answer 3

You can use it this way

Map map = ArrayUtils.toMap(arrayName);

And you can import ArrayUtils from :

import org.apache.commons.lang3.ArrayUtils;

Just include Apache JARs