All 15 Collections Framework questions
More in Collections Framework
What is the Java Collections Framework, and what are the core interfaces (List, Set, Map, Queue) at its foundation? What are the key performance differences between ArrayList and LinkedList for common operations (random access, insertion/removal at various positions), and when would you choose one over the other? What is the contract between equals() and hashCode() that a custom class must satisfy to work correctly as a HashMap key or HashSet element? What is the difference between fail-fast and fail-safe iterators in the Collections Framework, and which collections use each? What are the immutable collection factory methods (List.of(), Set.of(), Map.of()) introduced in Java 9, and how do they differ from Collections.unmodifiableList()? What is the purpose of the Deque interface, and how does it unify both stack (LIFO) and queue (FIFO) behavior in a single interface? What is the purpose of the Collections utility class's synchronization wrappers (Collections.synchronizedList(), etc.), and how do they differ from genuinely concurrent collections like ConcurrentHashMap? How does PriorityQueue work internally (as a binary heap), and what ordering does it use by default versus with a custom Comparator? How does HashMap's internal implementation work (hashing, bucket array, treeification), and how did its collision-handling strategy change in Java 8? How do you choose between HashMap, LinkedHashMap, and TreeMap based on their different ordering guarantees and performance characteristics?
Advertisement
Java › Collections Framework

What is the difference between the Iterable and Iterator interfaces, and how does the enhanced for-each loop rely on them?

Beginner

Answer

Iterable is implemented by any class that can produce an Iterator via its single iterator() method, making it eligible for use in a for-each loop; Iterator itself provides the actual traversal mechanism via hasNext() (checks if more elements remain) and next() (returns the next element, advancing position), plus an optional remove() -- the compiler desugars a for-each loop over an Iterable into the equivalent explicit while-loop calling hasNext()/next() internally, meaning any custom class implementing Iterable automatically gains for-each loop support.
public class Range implements Iterable<Integer> {
    private final int start, end;
    public Range(int start, int end) { this.start = start; this.end = end; }

    @Override
    public Iterator<Integer> iterator() {
        return new Iterator<>() {
            int current = start;
            public boolean hasNext() { return current < end; }
            public Integer next() { return current++; }
        };
    }
}

for (int i : new Range(1, 5)) { System.out.println(i); }  // works because Range implements Iterable
Real-world example A custom linked data structure implements Iterable<T>, letting application code iterate over it naturally with a standard for-each loop rather than requiring callers to know about and use a custom, non-standard traversal method.

Common follow-ups: What does the for-each loop actually compile down to under the hood?;Why does Iterator not extend Iterable, and what would go wrong if it did?

Streams & Lambdas;Generics
All 15 Collections Framework questions