Java April 6, 2016

Java > Filter a Generic Collection

Basically, I’d like to use a Predicate to filter a generic (which extends Collection), and then return an instance of the same generic Collection implementation (preferably a new instance) e.g. implement the method signature F removeNulls(F inputs).

I have the following examples, but there are caveats to each (removeNulls4 is probably the closest to what I’m trying to achieve):

Caveats

removeNulls1:

the returned list may(/will) not be an instance of F (requires casting)

removeNulls2:

the implementation of F may not have an empty constructor
use of the forReturn object is not thread safe (if run in parallel)

removeNulls3:

modifies the original list
is reinventing the wheel/not parallelisable (would prefer to use the filter method)
Iterator.remove() can throw an UnsupportedOperationException

removeNulls4:

modifies the original list

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class QuickTest<I, F extends Collection<I>> {

    Predicate<I> removeNullsPredicate = x -> x != null;

    @SuppressWarnings("unchecked")
    public F removeNulls1(F inputs) throws Exception {
        return (F) inputs.stream().filter(removeNullsPredicate)
                .collect(Collectors.toList());
    }

    @SuppressWarnings("unchecked")
    public F removeNulls2(F inputs) throws Exception {
        F forReturn = (F) inputs.getClass().newInstance();
        inputs.stream().filter(removeNullsPredicate)
                .collect(Collectors.toCollection(() -> forReturn));
        return forReturn;
    }

    public F removeNulls3(F inputs) throws Exception {
        Iterator<I> iter = inputs.iterator();
        while (iter.hasNext()){
            I next = iter.next();
            boolean test = removeNullsPredicate.test(next);
            if (!test){
                iter.remove();
            }
        }
        return inputs;
    }

    public F removeNulls4(F inputs) throws Exception {
        List<I> forRemoval = inputs.stream().filter(removeNullsPredicate.negate())
                .collect(Collectors.toList());
        inputs.removeAll(forRemoval);
        return inputs;
    }
}

Answer

You could provide a Supplier<F> as an argument:

public F removeNulls(F inputs, Supplier<F> factory) {
    return inputs.stream().filter(removeNullsPredicate)
            .collect(Collectors.toCollection(factory));
}

Then simply call:

List<I> nonNulls = removeNulls(inputs, ArrayList::new);

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