How to convert Array to HashMap using Java 8 Stream

Question

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

Here is what I wanted

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!

Answer 1

You may use

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

but it will give you a (founded) unchecked warning. Your method can’t hold the promise to return a correctly typed Map<K, V> for an array of arbitrary objects and, even worse, it will not fail with an exception, but silently return an inconsistent map if you pass in objects of the wrong type.

A cleaner, commonly used, solution is

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);
}

This can be compiled without a warning, as the correctness will be checked at runtime. The calling code has to be adapted:

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

Besides the need to specify the actual types as class literals, it has the disadvantage of not supporting generic key or value types (as they can’t be expressed as Class) and still having no compile-time safety, only a runtime check.

---

It’s worth looking at Java 9. There, you will be able to do:

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

This will create an immutable map of an unspecified type, rather than a HashMap, but the interesting point is the API.

There is a method <K,V> Map.Entry<K,V> entry(K k, V v) which can be combined with
<K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries) to create a map of a variable length (varargs are still limited to 255 parameters, though).

You can implement a similar thing:

public static <K,V> Map.Entry<K,V> entry(K k, V v) {
    return new AbstractMap.SimpleImmutableEntry<>(k, v);
}
public static <K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries) {
    return Arrays.stream(entries)
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
}

The convenience method(s) of are implemented the only way, this can be done with type safety: as overloaded methods with different numbers of arguments, like

public static <K,V> Map<K,V> of() {
    return new HashMap<>();// or Collections.emptyMap() to create immutable maps
}
static <K,V> Map<K,V> of(K k1, V v1) {
    return ofEntries(entry(k1, v1));
}
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2) {
    return ofEntries(entry(k1, v1), entry(k2, v2));
}
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
    return ofEntries(entry(k1, v1), entry(k2, v2), entry(k3, v3));
}
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
    return ofEntries(entry(k1, v1), entry(k2, v2), entry(k3, v3), entry(k4, v4));
}   
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
    return ofEntries(entry(k1, v1), entry(k2, v2), entry(k3, v3), entry(k4, v4));
}   

(Java 9 makes the cut at ten mappings, if you have more, you have to use the ofEntries(entry(k1, v1), …) variant).

If you follow this pattern, you should keep your toMap name or use just map, rather than calling at “of”, as you are not writing the Map interface.

These overloads might not look very elegant, but they solve all problems. You can write the code just as in your question, without specifying Class objects, but gain compile-time type safety and even rejection of attempts to call it with an odd number of arguments.

You have to make a cut at a certain number of parameters, but, as already noted, even varargs do not support unlimited parameters. And the ofEntries(entry(…), …) form isn’t so bad for larger maps.

---

The collector Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue) returns an unspecified map type, which might even be immutable (though it’s a HashMapin the current version). If you want to have a guaranty that a HashMap instance is returned, you have to use Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (v1,v2)->{throw new IllegalArgumentException("duplicate key");}, HashMap::new) instead.