在Java中转换通用Collection的类型

Question

In several spots in my code I have ArrayLists and TreeSets whose generic type I wish to convert. So for example I have an ArrayList<Integer> which I wish to convert to an ArrayList<Long> . Or I have a TreeSet<BigInteger> which I wish to convert to a TreeSet<String> .

All of these conversions can be made, but then I have to create for each type conversion a different function. Therefore I want to create a generic function whose signature looks something like this:

public static <Q,T> Collection<Q> convert(Collection<T> col, Class<Q> Q)

What I want is to get the class from col (eg ArrayList ), create a new collection of that class and type Q (called newCol ), and then iterate through col and convert each element which is of type T to type Q and add it to newCol and lastly return newCol .

How can I do this?

Answer 1

There's no special mechanism like casting of incompatible classes in Java. You need to specify an explicit function which will perform a conversion. Using Java 8 it's really easy:

public static <Q,T,C extends Collection<Q>> C convert(Collection<T> col, Function<T, Q> fn, 
                   Supplier<C> supplier) {
    return col.stream().map(fn).collect(Collectors.toCollection(supplier));
}

Use it like this:

TreeSet<BigInteger> values = // fill them somehow
TreeSet<String> converted = convert(values, BigInteger::toString, TreeSet::new);

Answer 2

@Tagir Valeev is right. You can do it easily in Java 8. But if you use Java 7, you can try to do something like this:

    public static <F, T> Collection<T> transform(Collection<F> fromCollection, Function<? super F, T> function) {
        return new TransformedCollection<F, T>(fromCollection, function);
    }

    static class TransformedCollection<F, T> extends AbstractCollection<T> {
        final Collection<F> fromCollection;
        final Function<? super F, ? extends T> function;

        TransformedCollection(Collection<F> fromCollection, Function<? super F, ? extends T> function) {
            this.fromCollection = checkNotNull(fromCollection);
            this.function = checkNotNull(function);
        }

        @Override public void clear() {
            fromCollection.clear();
        }

        @Override public boolean isEmpty() {
            return fromCollection.isEmpty();
        }

        @Override public Iterator<T> iterator() {
            return Iterators.transform(fromCollection.iterator(), function);
        }

        @Override public int size() {
            return fromCollection.size();
        }
    }

It's code from Guava library.