How can I generate a List backed by a Function?

Question

I have a function myFunction of Function<Integer, T>, and I want to construct an object mylist of size size, implementing List<T> (or maybe some kind of immutable list interface), backed by the function, in the sense that mylist.get(i) == myFunction.apply(i).

I can do this manually, but is there some (Guava) code which does the same?

score 5 · Accepted Answer · answered Feb 26 '13 at 21:22

5

Just use java.util.AbstractList:

 new AbstractList<T>() {
   public T get(int i) {
     Preconditions.checkElementIndex(i, size);
     return function.apply(i);
   }
   public int size() {
     return size;
   }
 }

The result would not necessarily be immutable, since the function output could vary. In all likelihood, you could get rid of the Function entirely, and just write the implementation of the Function in your AbstractList implementation.

answered Feb 26 '13 at 21:22

Louis Wasserman

191,574
25
345
413

where is `size` coming from? – Kevin Welker Feb 26 '13 at 21:57
Wherever you want it to come from. There's no such thing as a list without a size, so presumably the OP had a size coming from somewhere else. – Louis Wasserman Feb 26 '13 at 22:03

score 1 · Answer 2 · edited May 23 '17 at 10:24

Perhaps instead of a list you should consider an Iterator<T>.

// Example simple Function that returns each element from the array.
static class Function<T> {
    final T[] t;
    Function(T[] t) {
        this.t = t;
    }
    T apply (Integer i) {
        return t[i];
    }
}

static class FunctionIterator<T> implements Iterator<T> {
    final Function<T> f;
    Integer i;
    Integer to;
    Integer step;

    FunctionIterator(Function<T> f, Integer from, Integer to) {
        this.f = f;
        if ( to > from ) {
            step = 1;
            i = from;
            this.to = to;
        } else {
            step = -1;
            i = to;
            this.to = from;
        }
    }

    @Override
    public boolean hasNext() {
        return i != to + step;
    }

    @Override
    public T next() {
        T next = f.apply(i);
        i += step;
        return next;
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException("Not supported.");
    }
}

This code offers an Iterator. You can make an Iterable from it quite easily. Here is an excellent and neat example of how to do it.

How can I generate a List backed by a Function?

2 Answers2

Linked