Converting Iterator types in Java

Question

I have an Iterator<TypeA> that I want to convert to Iterator<TypeB>. TypeA and TypeB cannot be directly cast to each other but I can write a rule how to cast them. How can I accomplish this? Should I extend and override Iterator<TypeA>'s next, hasNext and remove methods?

Answer 1

You don't need to write this yourself. Guava (formerly Google-collections) has you covered in Iterators.transform(...)

You provide your iterator and a function that converts TypeA to TypeB and you're done.

Iterator<TypeA> iterA = ....;
Iterator<TypeB> iterB = Iterators.transform(iterA, new Function<TypeA, TypeB>() {
   @Override
   public TypeB apply(TypeA input) {
     TypeB output = ...;// rules to create TypeB from TypeA
     return output;
   }
});

Answer 2

Write an adapter. Here's an example in Java. OReilly's book 'Head First: Design Patterns' gives the best explanation on the topic.

Answer 3

I guess you should write your own iterator(it can implements java.util.iterator) with your ((converting)) rule in a method and use it.

Answer 4

You can use the following snippet :

public static void main(String... args){
    Iterator<TypeA> iteratorTypeA = methodToGetIteratorTypeA();
    Iterator<TypeB> iteratorTypeB = new IteratorConveter<TypeA, TypeB>(iteratorTypeA,
        new Converter<TypeA, TypeB>(){
            public TypeB convert(TypeA typeA){
                return null; //Something to convert typeA to type B
            }
        });
}

public class IteratorConveter<F, T> implements Iterator<T> {
    private final Converter<? super F, ? extends T> converter;
    private final Iterator<F> iterator;

    public IteratorConveter(Iterator<F> iterator, Converter<? super F, ? extends T> converter) {
        this.converter = converter;
        this.iterator = iterator;
    }

    public boolean hasNext() {
        return iterator.hasNext();
    }

    public T next() {
        return converter.convert(iterator.next());
    }

    public void remove() {
        iterator.remove();
    }
}


interface Converter<F, T> {
    T convert(F object);
}