How to create a static Map of String -> Array

dictionary, java, static

Solution

You don't need to separate declaration and initialization. If you know how, it can all be done in one line!

// assumes your code declaring the constants ONE, FRED_TEXT etc is before this line
private static final Map<String, int[]> myMap = Collections.unmodifiableMap(
    new HashMap<String, int[]>() {{
        put(FRED_TEXT, new int[] {ONE, TWO, FOUR});
        put(DAVE_TEXT, new int[] {TWO, THREE});
    }});

What we have here is an anonymous class with an initialization block, which is a block of code that executes on construction after constructor, which we've used here to load the map.

This syntax/construct is sometimes erroneously called "double brace initialization" - I suppose because there's two adjacent braces - but there's actually no such thing.

The two cool things about this are:

- it marries the declaration with the contents, and

- because the initialization is in-line, you can also make an in-line call to `Collections.unmodifiableMap()`, resulting in a neat one-line declaration, initialization and conversion to unmodifiable.

If you don't need/want the map to be unmodifiable, leave out that call:

private static final Map<String, int[]> myMap = new HashMap<String, int[]>() {{
    put(FRED_TEXT, new int[] {ONE, TWO, FOUR});
    put(DAVE_TEXT, new int[] {TWO, THREE});
}};

Problem

I need to create a static `Map` which maps a given `String` to an array of `int`'s. In other words, I'd like to define something like: ``` "fred" -> {1,2,5,8} "dave" -> {5,6,8,10,11} "bart" -> {7,22,10010} ... etc ``` Is there an easy way to do this in Java? And if possible, I'd like to use `static` constants for both the `String` and the `int` values. EDIT: To clarify what I meant by `static` constants for the values, and to give what I see to be the correct code, here is my first stab at the solution: ``` public final static String FRED_TEXT = "fred"; public final static String DAVE_TEXT = "dave"; public final static int ONE = 1; public final static int TWO = 2; public final static int THREE = 3; public final static int FOUR = 4; public final static HashMap<String, int[]> myMap = new HashMap<String, int[]>(); static { myMap.put(FRED_TEXT, new int[] {ONE, TWO, FOUR}); myMap.put(DAVE_TEXT, new int[] {TWO, THREE}); } ``` Note, these names are not what I'd actually be using. This is just a contrived example.

Original source