Java: iterating through list of lists?

Question

The question but in C#. So does Java have C#'s command? I need it for Matches-SearchTerm-Files-relationship.

foreach(var i in BunchOfItems.SelectMany(k => k.Items)) {}

[Why not for-loops?] I have done such structures in nested for loops but they soon become bloated. So I prefer something more succint like the above.

public static Stack<Integer[]> getPrintPoss(String s,File f,Integer maxViewPerF)
{
    Stack<File> possPrint = new Stack<File>();
    Integer[] poss = new Integer[4]();
    int u,size;
    for(File f:files)
    { 
        size = f2S(f).length();
        u = Math.min(maxViewsPerF,size);
        for(int i=0; i<u;i++)
        {
           // Do something --- bloated, and soon out of control
           // wants more succintly

        }
    }
    return possPrint;
}

This library has a selectMany() implementation that would look like this for your requirement: for(val i : BunchOfItems.selectMany(bunchItemSelector()) { }. See https://github.com/nicholas22/jpropel-light — NT_, Oct 08 '11 at 10:33

score 10 · Answer 1 · answered May 06 '10 at 17:05

10

for (List<Object> lo : list) {
    for (Object o : lo) {
        // etc etc
    }
}

I don't think there's a simpler solution.

answered May 06 '10 at 17:05

someguy

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1

The OP is asking for a Linq-type of solution that C# provides. He already knows how to iterate through Lists. He wants an elegant solution. Unfortunately, Java does not have something equivalent to Linq. – Kevin Crowell May 06 '10 at 17:16

score 7 · Answer 2 · answered May 06 '10 at 18:21

If you can get the data into an Iterable<Iterable<T>>, then you can get from that to a flattened Iterable<T> using Guava's Iterables.concat method. If what you have is really an Iterable<S>, with some way to get from an S to an Iterable<T>, well, then you have to first use Iterables.transform to view that as the Iterable<Iterable<T>> needed by concat.

All this will look a lot nicer if and when Java has something resembling closures, but at least today it's possible.

http://guava-libraries.googlecode.com

score 3 · Answer 3 · answered Aug 19 '14 at 19:24

3

With Java 8, you can say

Collection bunchOfItems = ...;
bunchOfItems.stream().flatMap(k::getItems).forEach(i -> /* operate on i */);

or

Item[] bunchOfItems = ...;
Stream.of(bunchOfItems).flatMap(k::getItems).forEach(i -> /* operate on i */);

depending upon whether you have a Collection or an Array.

answered Aug 19 '14 at 19:24

Kyle Krull

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Can you please explain your code here? – Tarnished-Coder Oct 15 '21 at 06:41

score 1 · Answer 4 · answered May 06 '10 at 17:15

I have my own version. Waiting desperately for Closures in Java :

public static <T, E> Iterable<T> transformMany(Iterable<E> iterable, Func<E, Iterable<T>> f) {
    if (null == iterable)
        throw new IllegalArgumentException("null iterable");
    if (null == f)
        throw new IllegalArgumentException("null f");

    return new TransformManyIterable<E, T>(iterable, f);
}

public interface Func<E, T> {
    T execute(E e);
}

public class TransformManyIterable<TOriginal, TResult> implements Iterable<TResult> {
    private Iterable<TOriginal> iterable;
    private Func<TOriginal, Iterable<TResult>> func;

    public TransformManyIterable(Iterable<TOriginal> iterable,
            Func<TOriginal, Iterable<TResult>> func) {
        super();
        this.iterable = iterable;
        this.func = func;
    }

    class TransformIterator implements Iterator<TResult> {
        private Iterator<TOriginal> iterator;
        private Iterator<TResult> currentIterator;

        public TransformIterator() {
            iterator = iterable.iterator();
        }

        @Override
        public boolean hasNext() {
            if (currentIterator != null && currentIterator.hasNext())
                return true;
            else {
                while (iterator.hasNext()) {
                    Iterable<TResult> iterable = func.execute(iterator.next());
                    if (iterable == null)
                        continue;
                    currentIterator = iterable.iterator();
                    if (currentIterator.hasNext())
                        return true;
                }
            }

            return false;
        }

        @Override
        public TResult next() {
            if (currentIterator != null && currentIterator.hasNext())
                return currentIterator.next();
            else {
                while (iterator.hasNext()) {
                    Iterable<TResult> iterable = func.execute(iterator.next());
                    if (iterable == null)
                        continue;
                    currentIterator = iterable.iterator();
                    if (currentIterator.hasNext())
                        return currentIterator.next();
                }
            }
            throw new NoSuchElementException();
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }
    }

    @Override
    public Iterator<TResult> iterator() {
        return new TransformIterator();
    }

}

Usage:

    Iterable<SomeType> result = transformMany(input, new Func<InputType, Iterable<SomeType>>() {
        @Override
        public Iterable<SomeType> execute(InputType e) {
            return new ArrayList<SomeType>();
        }
    });

score 1 · Answer 5 · edited May 23 '17 at 12:07

1

The SelectMany method is part of LINQ which is .Net-specific. This question asks about a LINQ equilvalent for java. Unfortunately, it doesn't look like there is a direct equivalent.

edited May 23 '17 at 12:07

Community

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answered May 06 '10 at 17:17

Robin

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score 1 · Accepted Answer · answered May 07 '10 at 02:40

1

Have about half a year patience until JDK7 is final which will include Closures. This provides simliar syntax and the same possibilities as LINQ which was demonstrated in the answer you're talking about.

answered May 07 '10 at 02:40

BalusC

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Java: iterating through list of lists?

6 Answers6