builder for HashMap

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

But are there also builders for java Maps?

HashMap<String,Integer> m = Maps.BuildHashMap.
    put("a",1).
    put("b",2).
    build();

There is no such thing for HashMaps, but you can create an ImmutableMap with a builder:

final Map<String, Integer> m = ImmutableMap.<String, Integer>builder().
      put("a", 1).
      put("b", 2).
      build();

And if you need a mutable map, you can just feed that to the HashMap constructor.

final Map<String, Integer> m = Maps.newHashMap(
    ImmutableMap.<String, Integer>builder().
        put("a", 1).
        put("b", 2).
        build());

Not quite a builder, but using an initializer:

Map<String, String> map = new HashMap<String, String>() {{
    put("a", "1");
    put("b", "2");
}};

This is similar to the accepted answer, but a little cleaner, in my view:

ImmutableMap.of("key1", val1, "key2", val2, "key3", val3);

There are several variations of the above method, and they are great for making static, unchanging, immutable maps.

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") );

Here is a very simple one ...

public class FluentHashMap<K, V> extends java.util.HashMap<K, V> {
  public FluentHashMap<K, V> with(K key, V value) {
	put(key, value);
	return this;
  }

  public static <K, V> FluentHashMap<K, V> map(K key, V value) {
	return new FluentHashMap<K, V>().with(key, value);
  }
}

then

import static FluentHashMap.map;

HashMap<String, Integer> m = map("a", 1).with("b", 2);

See https://gist.github.com/culmat/a3bcc646fa4401641ac6eb01f3719065

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());
        }
    }
}

You can use:

HashMap<String,Integer> m = Maps.newHashMap(
    ImmutableMap.of("a",1,"b",2)
);

It's not as classy and readable, but does the work.

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

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

Using java 8:

This is a approach of Java-9 Map.ofEntries(Map.entry(k1,v1), Map.entry(k2,v2), ...)

public class MapUtil {
	import static java.util.stream.Collectors.toMap;

	import java.util.AbstractMap.SimpleEntry;
	import java.util.Map;
	import java.util.Map.Entry;
	import java.util.stream.Stream;

	private MapUtil() {}
	
	@SafeVarargs
	public static Map<String, Object> ofEntries(SimpleEntry<String, Object>... values) {
		return Stream.of(values).collect(toMap(Entry::getKey, Entry::getValue));
	}
	
	public static SimpleEntry<String, Object> entry(String key, Object value) {
		return new SimpleEntry<String, Object>(key, value);
	}
}

How to Use:

import static your.package.name.MapUtil.*;

import java.util.Map;

Map<String, Object> map = ofEntries(
		entry("id", 1),
		entry("description", "xyz"),
		entry("value", 1.05),
		entry("enable", true)
	);

There's ImmutableMap.builder() in Guava.

I had a similar requirement a while back. Its nothing to do with Guava but you can do something like this to be able to cleanly construct a Map using a fluent builder.

Create a base class that extends Map.

public class FluentHashMap<K, V> extends LinkedHashMap<K, V> {
    private static final long serialVersionUID = 4857340227048063855L;

    public FluentHashMap() {}

    public FluentHashMap<K, V> delete(Object key) {
        this.remove(key);
        return this;
    }
}

Then create the fluent builder with methods that suit your needs:

public class ValueMap extends FluentHashMap<String, Object> {
    private static final long serialVersionUID = 1L;

    public ValueMap() {}

    public ValueMap withValue(String key, String val) {
        super.put(key, val);
        return this;
    }

... Add withXYZ to suit...

}

You can then implement it like this:

ValueMap map = new ValueMap()
      .withValue("key 1", "value 1")
      .withValue("key 2", "value 2")
      .withValue("key 3", "value 3")

Here's one I wrote

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.function.Supplier;

public class MapBuilder<K, V> {

    private final Map<K, V> map;

    /**
     * Create a HashMap builder
     */
    public MapBuilder() {
        map = new HashMap<>();
    }

    /**
     * Create a HashMap builder
     * @param initialCapacity
     */
    public MapBuilder(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    /**
     * Create a Map builder
     * @param mapFactory
     */
    public MapBuilder(Supplier<Map<K, V>> mapFactory) {
        map = mapFactory.get();
    }

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

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

    /**
     * Returns an unmodifiable Map. Strictly speaking, the Map is not immutable because any code with a reference to
     * the builder could mutate it.
     *
     * @return
     */
    public Map<K, V> buildUnmodifiable() {
        return Collections.unmodifiableMap(map);
    }
}

You use it like this:

Map<String, Object> map = new MapBuilder<String, Object>(LinkedHashMap::new)
    .put("event_type", newEvent.getType())
    .put("app_package_name", newEvent.getPackageName())
    .put("activity", newEvent.getActivity())
    .build();

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.

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;
    }

Underscore-java can build hashmap.

Map<String, Object> value = U.objectBuilder()
        .add("firstName", "John")
        .add("lastName", "Smith")
        .add("age", 25)
        .add("address", U.arrayBuilder()
            .add(U.objectBuilder()
                .add("streetAddress", "21 2nd Street")
                .add("city", "New York")
                .add("state", "NY")
                .add("postalCode", "10021")))
        .add("phoneNumber", U.arrayBuilder()
            .add(U.objectBuilder()
                .add("type", "home")
                .add("number", "212 555-1234"))
            .add(U.objectBuilder()
                .add("type", "fax")
                .add("number", "646 555-4567")))
        .build();
    // {firstName=John, lastName=Smith, age=25, address=[{streetAddress=21 2nd Street,
    // city=New York, state=NY, postalCode=10021}], phoneNumber=[{type=home, number=212 555-1234},
    // {type=fax, number=646 555-4567}]}