Java：迭代列表列表？

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;
}

Answer 1

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

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

Answer 2

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

Answer 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 .

Answer 4

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>();
        }
    });

Answer 5

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.

Answer 6

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.