builder for HashMap

Question

Guava provides us with great factory methods for Java types, such as Maps.newHashMap().

But are there also builders for java Maps?

HashM         


        

Answer 1

Since Java 9 Map interface contains:

• Map.of(k1,v1, k2,v2, ..)
• Map.ofEntries(Map.entry(k1,v1), Map.entry(k2,v2), ..).

Limitations of those factory methods are that they:

• can't hold nulls as keys and/or values (if you need to store nulls take a look at other answers)
• produce immutable maps

If we need mutable map (like HashMap) we can use its copy-constructor and let it copy content of map created via Map.of(..)

Map<Integer, String> map = new HashMap<>( Map.of(1,"a", 2,"b", 3,"c") );

Answer 2

You can use the fluent API in Eclipse Collections:

Map<String, Integer> map = Maps.mutable.<String, Integer>empty()
        .withKeyValue("a", 1)
        .withKeyValue("b", 2);

Assert.assertEquals(Maps.mutable.with("a", 1, "b", 2), map);

Here's a blog with more detail and examples.

Note: I am a committer for Eclipse Collections.

Answer 3

A simple map builder is trivial to write:

public class Maps {

    public static <Q,W> MapWrapper<Q,W> map(Q q, W w) {
        return new MapWrapper<Q, W>(q, w);
    }

    public static final class MapWrapper<Q,W> {
        private final HashMap<Q,W> map;
        public MapWrapper(Q q, W w) {
            map = new HashMap<Q, W>();
            map.put(q, w);
        }
        public MapWrapper<Q,W> map(Q q, W w) {
            map.put(q, w);
            return this;
        }
        public Map<Q,W> getMap() {
            return map;
        }
    }

    public static void main(String[] args) {
        Map<String, Integer> map = Maps.map("one", 1).map("two", 2).map("three", 3).getMap();
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            System.out.println(entry.getKey() + " = " + entry.getValue());
        }
    }
}

Answer 4

This is something I always wanted, especially while setting up test fixtures. Finally, I decided to write a simple fluent builder of my own that could build any Map implementation - https://gist.github.com/samshu/b471f5a2925fa9d9b718795d8bbdfe42#file-mapbuilder-java

    /**
     * @param mapClass Any {@link Map} implementation type. e.g., HashMap.class
     */
    public static <K, V> MapBuilder<K, V> builder(@SuppressWarnings("rawtypes") Class<? extends Map> mapClass)
            throws InstantiationException,
            IllegalAccessException {
        return new MapBuilder<K, V>(mapClass);
    }

    public MapBuilder<K, V> put(K key, V value) {
        map.put(key, value);
        return this;
    }

    public Map<K, V> build() {
        return map;
    }

Answer 5

There's ImmutableMap.builder() in Guava.

Answer 6

HashMap is mutable; there's no need for a builder.

Map<String, Integer> map = Maps.newHashMap();
map.put("a", 1);
map.put("b", 2);