apache-commons colectionutils source code

Question

The following code snippet has been taken from Commons framework's CollectionUtils.java file:

private static class CardinalityHelper<O> {
    final Map<O, Integer> cardinalityA, cardinalityB;

    public CardinalityHelper(final Iterable<? extends O> a,
            final Iterable<? extends O> b) {
        cardinalityA = CollectionUtils.<O> getCardinalityMap(a);
        cardinalityB = CollectionUtils.<O> getCardinalityMap(b);
    }

    public final int max(final Object obj) {
        return Math.max(freqA(obj), freqB(obj));
    }

    public final int min(final Object obj) {
        return Math.min(freqA(obj), freqB(obj));
    }

    public int freqA(final Object obj) {
        return getFreq(obj, cardinalityA);
    }

    public int freqB(final Object obj) {
        return getFreq(obj, cardinalityB);
    }

    private final int getFreq(final Object obj,
            final Map<?, Integer> freqMap) {
        final Integer count = freqMap.get(obj);
        if (count != null) {
            return count.intValue();
        }
        return 0;
    }
}

getCardinalityMap returns <O> Map<O, Integer> Can somebody please help me to understand

1. what does cardinalityA = CollectionUtils.<O> getCardinalityMap(a); means? Is this some kind of typecasting?
2. What does return type <O> Map<O, Integer> means?
3. What is this trick called?

Thanks

Answer 1

1) what does cardinalityA = CollectionUtils. getCardinalityMap(a); means? Is this some kind of typecasting?

It is not a typecasting. It is explicit setting generic parameter for generic method. CardinalityHelper has generic parameter <O> as you can see from the declaration:

private static class CardinalityHelper<O> {

It is typesafe way for the clients of the CardinalityHelper to use this class.

2) What does return type <O> Map<O, Integer> means?

- generic parameter declaration. Map - return type.

3) What is this trick called?

It is using Java Generic methods.

You also can find simular usage in standart JDK library. For example: java.util.Collections.sort(java.util.List)

Java implementation of Metaprogramming principles is very limited, so you are able to use only Generic Type parameters to create typesafe containers, which required by strongly typed nature of Java.