for-each loop can only iterate over an array or an instance of java.lang.Iterable [duplicate]

Question

The following code doesn't work. What's wrong with this code? Compiler complains in the for loop that NumberList isn't a Iterable class.

What kind of class can be used in for-each loop? How to make NumberList iterable? I tried making NumberList implement Iterable, but it doesn't seem to work because I don't know how to define the Iterator properly.

If someone could demonstrate how to make this code work, or link me to a tutorial that'd be great.

public class Test{
    public class NumberList{
        private int numItems;
        private Number[] numbers;

        public NumberList(int size){
            this.numbers = new Number[size];
            this.numItems=0;
        }

        public void add(Number n){
            this.numbers[this.numItems++]=n;
        }
    }

    public void printPairs() {
        ArrayList<Integer> num=new ArrayList<Integer>();

        NumberList numbers = new NumberList(50);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);

        for(Number n1: numbers){
            System.out.println(n1);
        }
    }
}

Answer 1

NumberList does not implement Iterable. As far as the compiler is concerned its just any other class.

You need to do something like this

public class NumberList implements Iterable<Number> {

    private int numItems;
    private Number[] numbers;

    public NumberList(int size) {
        this.numbers = new Number[size];
        this.numItems = 0;
    }

    public void add(Number n) {
        this.numbers[this.numItems++] = n;
    }

    @Override
    public Iterator<Number> iterator() {
        return Arrays.asList(numbers).subList(0, numItems).iterator();
    }
}