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!

What is your purpose for this? Map.of exists.

chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-

2019-07-29 05:07:57 +00:00
Commented Jul 29, 2019 at 5:07 — chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-, Commented Jul 29, 2019 at 5:07

WJS · Accepted Answer · 2019-07-29 04:19:45Z

4

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"));

answered Jul 29, 2019 at 4:19

WJS

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1 Comment

Ashvin Sharma Over a year ago

The question explicitly mentions Java 8 so Java 9 is out of the scope of this question

Kapil Kumar Shishodia · Accepted Answer · 2019-07-29 04:15:23Z

3

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);
}

answered Jul 29, 2019 at 4:15

Kapil Kumar Shishodia

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Comments

Vaibhav Balyan · Accepted Answer · 2019-07-29 09:29:14Z

0

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

answered Jul 29, 2019 at 9:29

Vaibhav Balyan

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