How Can I Initialize a HashMap in Java Efficiently?

Initializing a HashMap Literall

Initializing a Java HashMap with specific key-value pairs in a concise and literal manner has been an elusive feature for developers. However, advancements in Java have paved the way for simplified map creation.

Java 9 and Later: Factory Methods

From Java version 9 onwards, the Map class introduces factory methods that streamline map creation:

• Map.of: Can create maps with up to 10 elements using a concise syntax:

  Map<String, String> test1 = Map.of("a", "b", "c", "d");

• Map.ofEntries: Creates maps with any number of elements:

  import static java.util.Map.entry;
  
  Map<String, String> test2 = Map.ofEntries(
    entry("a", "b"),
    entry("c", "d")
  );

However, it's important to note that these methods result in immutable maps. For mutable maps, you can create a copy using:

mutableMap = new HashMap<>(Map.of("a", "b"));

Java 8 and Earlier: Anonymous Class or Function

Prior to Java 9, direct literal initialization was not supported. To achieve a similar effect, you can employ the following techniques:

• Anonymous Class with Initializer: Define an anonymous subclass that initializes the map in its constructor:

  Map<String, String> myMap = new HashMap<String, String>() {{
    put("a", "b");
    put("c", "d");
  }};

• Function for Initialization: Create a function to generate a map and use it in the initializer:

  Map<String, String> myMap = createMap();
  
  private static Map<String,String> createMap() {
    Map<String,String> myMap = new HashMap<String,String>();
    myMap.put("a", "b");
    myMap.put("c", "d");
    return myMap;
  }

The latter approach is cleaner and avoids any potential pitfalls associated with anonymous subclasses.

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