Iterable Interface in Java Collections

Direct Answer

Iterable<T> has one abstract method, iterator(), plus default methods such as forEach() and spliterator(). Collection extends it so collection objects can be used in an enhanced for loop. A class may implement Iterable without supporting Collection operations. Arrays also work with enhanced for through a separate language rule.

Code Example

import java.util.*;

// Generates IDs on the fly - no storage, not a Collection,
// yet works with for-each because it implements Iterable.
class OrderIdRange implements Iterable<String> {
    private final int start, end;
    OrderIdRange(int start, int end) { this.start = start; this.end = end; }

    @Override public Iterator<String> iterator() {
        return new Iterator<>() {
            private int cur = start;
            @Override public boolean hasNext() { return cur < end; }
            @Override public String next() {
                if (!hasNext()) throw new NoSuchElementException();
                return "ORDER-" + (cur++);
            }
        };
    }

    public static void main(String[] args) {
        for (String id : new OrderIdRange(100, 103)) {
            System.out.println(id);   // ORDER-100, ORDER-101, ORDER-102
        }
    }
}

Common Mistakes & Pitfalls

• Iterable vs Iterator confusion. Iterable is the factory (iterator()); Iterator is the cursor (hasNext/next). You implement Iterable; it hands back an Iterator.
• Thinking for-each is only for arrays/collections. Any class implementing Iterable gets for-each for free.
• Implementing Collection when Iterable is enough. A read-only sequence forced to implement Collection must throw UnsupportedOperationException from add/remove — just implement Iterable instead.
• Forgetting arrays are a special case. Arrays support for-each via a compiler rule, not by being Iterable.