Generator functions equivalent in Java

Question

I would like to implement an Iterator in Java that behaves somewhat like the following generator function in Python:

def iterator(array):    for x in array:       if x!= None:         for y in x:           if y!= None:             for z in y:               if z!= None:                 yield z 

x on the java side can be multi-dimensional array or some form of nested collection. I am not sure how this would work. Ideas?

Answer 1

Had the same need so wrote a little class for it. Here are some examples:

Generator<Integer> simpleGenerator = new Generator<Integer>() {     public void run() throws InterruptedException {         yield(1);         // Some logic here...         yield(2);     } }; for (Integer element : simpleGenerator)     System.out.println(element); // Prints "1", then "2". 

Infinite generators are also possible:

Generator<Integer> infiniteGenerator = new Generator<Integer>() {     public void run() throws InterruptedException {         while (true)             yield(1);     } }; 

The Generator class internally works with a Thread to produce the items. By overriding finalize(), it ensures that no Threads stay around if the corresponding Generator is no longer used.

The performance is obviously not great but not too shabby either. On my machine with a dual core i5 CPU @ 2.67 GHz, 1000 items can be produced in < 0.03s.

The code is on GitHub. There, you'll also find instructions on how to include it as a Maven/Gradle dependency.

Answer 2

Indeed Java has no yield, but you can now use Java 8 streams. IMO it's really a complicated iterator since it's backed by an array, not a function. Given it's a loop in a loop in a loop can be expressed as a Stream using filter (to skip the nulls) and flatMap to stream the inner collection. It's also about the size of the Python code. I've converted it to an iterator to use at your leisure and printed to demonstrate, but if all you were doing was printing, you could end the stream sequence with forEach(System.out::println) instead of iterator().

public class ArrayIterate {     public static void main(String args[])     {         Integer[][][] a = new Integer[][][] { { { 1, 2, null, 3 },                                                 null,                                                 { 4 }                                               },                                               null,                                               { { 5 } } };          Iterator<Object> iterator = Arrays.stream(a)                                           .filter(ax -> ax != null)                                           .flatMap(ax -> Arrays.stream(ax)                                                .filter(ay -> ay != null)                                                .flatMap(ay -> Arrays.stream(ay)                                                .filter(az -> az != null)))                                           .iterator();          while (iterator.hasNext())         {             System.out.println(iterator.next());         }     } } 

I'm writing about implementation of generators as part of my blog on Java 8 Functional Programming and Lambda Expressions at http://thecannycoder.wordpress.com/ which might give you some more ideas for converting Python generator functions into Java equivalents.