java 8 map initialization

Java 8 has a number of features that can be used to create and initialize maps. Take a look at this method:       

    protected static Map<Integer, String> stdJava8() {

        return Collections.unmodifiableMap(Stream.of(
                new SimpleEntry<>(0, "zero"),
                new SimpleEntry<>(1, "one"),
                new SimpleEntry<>(2, "two"),
                new SimpleEntry<>(3, "three"),
                new SimpleEntry<>(4, "four"),
                new SimpleEntry<>(5, "five"),
                new SimpleEntry<>(6, "six"),
                new SimpleEntry<>(7, "seven"),
                new SimpleEntry<>(8, "eight"),
                new SimpleEntry<>(9, "nine"),
                new SimpleEntry<>(10, "ten"),
                new SimpleEntry<>(11, "eleven"),
                new SimpleEntry<>(12, "twelve"))
                .collect(Collectors.toMap((e) -> e.getKey(), (e) -> e.getValue())));
    }

Here we create a Stream of map entries. At least two implementations of Entry already exists in the
standard Java libraries and here I have used SimpleEntry. After the Stream is constructed, we collect all the entries and creates a Map by splitting up each Entry in a key and a value. It is important to include the diamond operator for each SimpleEntry or else the toMap() method will not be able to infer the types of the Entry.

As can be seen, many intermediate objects need to be created before the final Map can be constructed. This can be a disadvantage if many instances are created rapidly. One advantage with this method is that it is using a standard Java 8 stream pattern, so it is very easy to start out with this pattern and then modify it to do something else.

The Java 8 Way Simplified  

We can further simplify the process of creating maps the Java 8 way, by adding a small number of support methods to our Maps class like this:       

    public static <K, V> Map.Entry<K, V> entry(K key, V value) {
        return new AbstractMap.SimpleEntry<>(key, value);
    }

    public static <K, U> Collector<Map.Entry<K, U>, ?, Map<K, U>> entriesToMap() {
        return Collectors.toMap((e) -> e.getKey(), (e) -> e.getValue());
    }

    public static <K, U> Collector<Map.Entry<K, U>, ?, ConcurrentMap<K, U>> entriesToConcurrentMap() {
        return Collectors.toConcurrentMap((e) -> e.getKey(), (e) -> e.getValue());
    }

 Now we can create the desired Map using this method:   

    protected static Map<Integer, String> extJava8() {
        return Collections.unmodifiableMap(Stream.of(
                entry(0, "zero"),
                entry(1, "one"),
                entry(2, "two"),
                entry(3, "three"),
                entry(4, "four"),
                entry(5, "five"),
                entry(6, "six"),
                entry(7, "seven"),
                entry(8, "eight"),
                entry(9, "nine"),
                entry(10, "ten"),
                entry(11, "eleven"),
                entry(12, "twelve")).
                collect(entriesToMap()));
    }

Do not forget to import the support methods statically. Now, this looks much nicer than the previous attempt because is is much shorter and does not contain so much boiler plate code.