Java Generics - Implementing the Iterable Interface

Jakob Jenkov
Last update: 2014-06-23

It is possible to use your own collection type classes with the new for-loop. To do so, your class must implement the java.lang.Iterable<E> interface. Here is a very basic example:

public class MyCollection<E> implements Iterable<E>{

    public Iterator<E> iterator() {
        return new MyIterator<E>();
    }
}

And here is the corresponding implementation skeleton of the MyIterator class:

public class MyIterator <T> implements Iterator<T> {

    public boolean hasNext() {
    
        //implement...
    }

    public T next() {
        //implement...;
    }

    public void remove() {
        //implement... if supported.
    }
}

Here is how to use the MyCollection generified, and with the new for-loop:

public static void main(String[] args) {
    MyCollection<String> stringCollection = new MyCollection<String>();

    for(String string : stringCollection){

    }
}

Next: Java Generic's Wildcards

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	Jakob Jenkov