Java 8, Initializing Maps in the Smartest Way

by Per Minborg

on December 10, 2014


Create maps
Illustration: Elis Minborg
When I was a kid, I was taught that, in the Swedish language, one should write out numbers using their text representation in the range from zero to twelve, otherwise one should use their number representation. For example, one should write "There are two birds and 21 flowers" (but in Swedish then of course).

In Java, we are likely to use Maps to perform such translations. The question in this post is: How does one initialize Maps in the best way? What are the alternatives and what are their pros and cons?


The objective in this post is to write a method that returns a Map with a translation from an Integer to a String. The Map shall be Unmodifiable, because we might want to reuse the Map in several places in our code and we want to be sure that is has not been tampered with. The Map shall contain the mapping pairs (or Entries as they are called in Java): 0->"zero", 1->"one", 2->"two", ... , 12->"twelve".

The Imperative Way

The straight forward solution is to declare a Map and then just put() entries into the Map. After that is done, we return an Unmodifiable view of the newly created map like this:
protected static Map<Integer, String> imperative() {
        final Map<Integer, String> numMap = new HashMap<>();
        numMap.put(0, "zero");
        numMap.put(1, "one");
        numMap.put(2, "two");
        numMap.put(3, "three");
        numMap.put(4, "four");
        numMap.put(5, "five");
        numMap.put(6, "six");
        numMap.put(7, "seven");
        numMap.put(8, "eight");
        numMap.put(9, "nine");
        numMap.put(10, "ten");
        numMap.put(11, "eleven");
        numMap.put(12, "twelve");
        return Collections.unmodifiableMap(numMap);
In this solution, as opposed to the other solutions that are shown later in this post, we have to declare an intermediate result variable (numMap). We then have to reference this result variable over and over again for each put() operation , which is disturbing. There is a risk that this result variable can "leak", effectively rendering the Unmodifiable map modifiable, because the Collections.unmodifiableMap() method just provides a view of the provided Map. So if we retain a reference to the provided Map, we can still change it.

Double Brace Initialization

The Double Brace Initialization Idiom firsts appears as very appealing. In the beginning, I liked it and started to use it frequently in my code. This is how it works:
    protected static Map<Integer, String> doubleBracket() {

        return Collections.unmodifiableMap(new HashMap<Integer, String>() {
                put(0, "zero");
                put(1, "one");
                put(2, "two");
                put(3, "three");
                put(4, "four");
                put(5, "five");
                put(6, "six");
                put(7, "seven");
                put(8, "eight");
                put(9, "nine");
                put(10, "ten");
                put(11, "eleven");
                put(12, "twelve");
There is actually no magic with it at all. The first pair of braces will create an anonymous class and the second pair of braces will create an instance initializer block that is run when the anonymous inner class is instantiated. In any class, you can have such brackets containing code that will be run when your create instances of your class (or when the class is used the first time, if you precede the brackets with the keyword static). This looks much better because now you do not have to create an intermediate result and you do not have to reference a variable each time you call put().

However, there are a number of snags with this scheme that is not immediately apparent. As previously stated, when you declare a class with double braces, an anonymous class will be created. Because the anonymous class is not static, it will also hold a reference (this$0) to its containing class (if defined within another class, which is the most normal case). This means that the containing instance (this$0) can not be garbage-collected as long as your Map is alive. A big concern according to my view.

Another disadvantage is that you must provide the types of the Map explicitly, because the compiler is not able to infer the types using the diamond (<>) operator. Also, the new anonymous class does not define a serialVersionUID variable, so we must suppress the resulting warning using the @SuppressWarnings("serial") annotation if you want it to compile nicely. You should really think twice before you use the Double Brace Idiom.

The Builder Pattern

This is one of my favorites and you can read more about this pattern in my previous posts "Creating Objects Using The Builder Pattern" and "The Interface Builder Pattern". The idea here is to create a Builder that has methods that returns the Builder itself again and again (like the StringBuilder does). This way the Builder can be called several time until the final build() method is called and the result is returned. I have created builders that can be used to create instance of Map and ConcurrentMap and placed them in a utility class called Maps:
public class Maps {

    private Maps() {

    public static <K, V> MapBuilder<K, V> builder() {
        return builder(HashMap::new);

    public static <K, V> MapBuilder<K, V> builder(Supplier<Map<K, V>> mapSupplier) {
        return new MapBuilder<>(mapSupplier.get());

    public static <K, V> ConcurrentMapBuilder<K, V> concurrentBuilder() {
        return concurrentBuilder(ConcurrentHashMap::new);

    public static <K, V> ConcurrentMapBuilder<K, V> concurrentBuilder(Supplier<ConcurrentMap<K, V>> mapSupplier) {
        return new ConcurrentMapBuilder<>(mapSupplier.get());

    private static class BaseBuilder<M extends Map<K, V>, K, V> {

        protected final M map;

        public BaseBuilder(M map) {
   = map;

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

        public M build() {
            return map;


    public static class MapBuilder<K, V> extends BaseBuilder<Map<K, V>, K, V> {

        private boolean unmodifiable;

        public MapBuilder(Map<K, V> map) {

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

        public MapBuilder<K, V> unmodifiable(boolean unmodifiable) {
            this.unmodifiable = unmodifiable;
            return this;

        public Map<K, V> build() {
            if (unmodifiable) {
                return Collections.unmodifiableMap(;
            } else {


    public static class ConcurrentMapBuilder<K, V> extends BaseBuilder<ConcurrentMap<K, V>, K, V> {

        public ConcurrentMapBuilder(ConcurrentMap<K, V> map) {

        public ConcurrentMapBuilder<K, V> put(K key, V value) {
            super.put(key, value);
            return this;


For the sake of completeness, I have also included methods to create concurrent maps and also two builder()  methods that takes a map Provider, allowing any type of Map to be created using the Builder. Using the support methods, we can now create maps easily like this:

    protected static Map<Integer, String> builderPattern() {
        return Maps.<Integer, String>builder().
                put(0, "zero").
                put(1, "one").
                put(2, "two").
                put(3, "three").
                put(4, "four").
                put(5, "five").
                put(6, "six").
                put(7, "seven").
                put(8, "eight").
                put(9, "nine").
                put(10, "ten").
                put(11, "eleven").
                put(12, "twelve").

Although we must enter the generic types of the Map (i.e. <Integer, String>) it looks much better. We can avoid all the other disadvantages that the Double Brace Idiom had. Note the Builder method unmodifiable() that sets a flag, controlling if the build() method shall return a normal Map or an unmodifiable Map. Very convenient! Another advantage is that this pattern works with older Java versions and that no extra objects are created during map creation.

The Java 8 Way

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")).

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.


There are many ways to create and initialize a Map. The choice is really up to you. Personally, I like the Builder Pattern best because it is very efficient, flexible and straight forward to use. The Java 8 pattern is also nice. A final warning on using the Double Brace Idiom should also be made.

How do we do the actual translation of text using the Map we have created using any of the various methods in this post? One nice way of doing it is to use Java 8's Map method getOrDefault() like this:

     public static String toText(Map<Integer, String> map, int val) {
         return map.getOrDefault(val, Integer.toString(val));
     final Map<Integer, String> map = imperative();
     System.out.println("There are " + toText(map, 2) + " birds and " + toText(map, 21) + " flowers.");

   "There are two birds and 21 flowers."

UPDATE: Learn how you can implement a type safe dynamic Map builder in this newer post.


Per Minborg

Per Minborg is a Palo Alto based developer and architect, currently serving as CTO at Speedment, Inc. He is a regular speaker at various conferences e.g. JavaOne, DevNexus, Jdays, JUGs and Meetups. Per has 15+ US patent applications and invention disclosures. He is a JavaOne alumni and co-author of the publication “Modern Java”.