Enhanced for loop and iterator in Java

Question

I have a class MyList that implements Iterable interface. And here is a toString() method from one of my classes:

public String toString() {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < 4; i++) {
        // First example using iterator
        for (Iterator<Integer> itr = array[i].iterator(); itr.hasNext();) {
            sb.append(itr.next() + " ");
        }
        // And the same result using enhanced for loop
        for (int val : array[i]) {
            sb.append(val + " ");
        }
    }
    return sb.toString();
}

This loop iterates over the nodes in my list:

for (int val : array[i]) {
    sb.append(val + " ");
}

How does this for loop uses the iterator?

Naman Gala · Accepted Answer · 2015-08-11 11:26:46Z

1

Because your class (MyList) implements Iterable. So internally it will call your iterator() method and then with the use of hasNext() and next() methods of ListIterator it will iterate in for loop.

Refer : Using Enhanced For-Loops with Your Classes

edited Aug 11, 2015 at 11:26

answered Aug 11, 2015 at 11:13

Naman Gala

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Eran · Accepted Answer · 2015-08-11 11:12:23Z

1

The enhanced for loop works for any class that implements Iterable. It internally calls the iterator() method implemented by that class to obtain an Iterator, and uses hasNext() and next() methods of the Iterator to iterate over the elements of the Iterator.

answered Aug 11, 2015 at 11:12

Eran

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Comments

Tushar · Accepted Answer · 2024-06-21 13:37:01Z

1

When you use an enhanced for loop to iterate over elements in your MyList class, which implements the Iterable interface, Java is actually using the iterator under the hood.

for (int val : array[i]) {
sb.append(val + " ");
}

is syntactic sugar that simplifies iteration. The Java compiler translates this loop into something similar to the following code that explicitly uses an iterator:

for (Iterator<Integer> itr = array[i].iterator(); itr.hasNext();) {
int val = itr.next();
sb.append(val + " ");
}

answered Jun 21, 2024 at 13:37

Tushar

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