Question

Is it possible in Java to make a Dictionary with the items already declared inside it? Just like the below C# code:

   Dictionary<string, int> d = new Dictionary<string, int>()
    {
        {"cat", 2},
        {"dog", 1},
        {"llama", 0},
        {"iguana", -1}
    };

How do I do this and what type do I use? I’ve read that Dictionary is obsolete.

Answer 1

This will do what you want:

Map<String,Integer> map = new HashMap<String, Integer>(){{
    put("cat", 2);
    put("dog", 1);
    put("llama", 0);
    put("iguana", -1);
}};

This statement creates an anonymous subclass of HashMap, where the only difference from the parent class is that the 4 entries are added during instance creation. It’s a fairly common idiom in the Java world (although some find it controversial because it creates a new class definition).

Because of this controversy, as of Java 9 there is a new idiom for conveniently constructing maps: the family of static Map.of methods.

With Java 9 or higher you can create the map you need as follows:

Map<String, Integer> map = Map.of(
    "cat", 2,
    "dog", 1,
    "llama", 0,
    "iguana", -1
);

With larger maps, this alternative syntax may be less error-prone:

Map<String, Integer> map = Map.ofEntries(
    Map.entry("cat", 2),
    Map.entry("dog", 1),
    Map.entry("llama", 0),
    Map.entry("iguana", -1)
);

(This is especially nice if Map.entry is statically imported instead of being referenced explicitly).

Besides only working with Java 9+, these new approaches are not quite equivalent to the previous one:

• They don’t allow you to specify what Map implementation is used
• They only create immutable maps
• They don’t create an anonymous subclass of Map

However, these differences shouldn’t matter for many use cases, making this a good default approach for newer versions of Java.