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 github.com/nicholas22/jpropel-light — NT_
– NT_, Commented Oct 8, 2011 at 10:33

someguy · Accepted Answer · 2010-05-06 17:05:02Z

10

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

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

answered May 6, 2010 at 17:05

someguy

7,41412 gold badges46 silver badges58 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Kevin Crowell Over a year ago

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 Bourrillion · Accepted Answer · 2010-05-06 18:21:55Z

7

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

answered May 6, 2010 at 18:21

Kevin Bourrillion

41k11 gold badges77 silver badges87 bronze badges

Comments

Kyle Krull · Accepted Answer · 2014-08-19 19:24:04Z

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, 2014 at 19:24

Kyle Krull

1,7432 gold badges18 silver badges27 bronze badges

1 Comment

Tarnished-Coder Over a year ago

Can you please explain your code here?

BalusC · Accepted Answer · 2010-05-07 02:40:37Z

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 7, 2010 at 2:40

BalusC

1.1m377 gold badges3.7k silver badges3.6k bronze badges

Comments

bruno conde · Accepted Answer · 2010-05-06 17:15:57Z

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>();
        }
    });

Community · Accepted Answer · 2017-05-23 12:07:28Z

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, 2017 at 12:07

CommunityBot

11 silver badge

answered May 6, 2010 at 17:17

Robin

1,0221 gold badge11 silver badges24 bronze badges

Collectives™ on Stack Overflow

Java: iterating through list of lists?

6 Answers 6

1 Comment

Comments

1 Comment

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

1 Comment

Comments

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related