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Iterable Interface

The Iterable Interface in Java​

The Iterable interface is the foundation of iteration in Java. It is implemented by all collection types (e.g., List, Set, Queue) and enables the use of enhanced for-loops (for (T t : iterable)) and streams (stream()). Understanding Iterable is essential for working with collections effectively.

Key Characteristics of Iterable​

  • Single Abstract Method: Defines the iterator() method.
  • Enhanced For-Loop: Enables iteration without explicitly creating an Iterator.
  • Stream Support: Required for creating streams (stream() and parallelStream()).

Methods in the Iterable Interface​

MethodDescription
Iterator<T> iterator()Returns an iterator over the elements
default void forEach(Consumer<? super T> action)Performs the given action for each element (Java 8+)
default Spliterator<T> spliterator()Creates a Spliterator for parallel traversal (Java 8+)

Code Examples​

Example 1: Enhanced For-Loop​

import java.util.*;

public class IterableExample {

    public static void main(String[] args) {

        List<String> fruits = Arrays.asList("Apple", "Banana", "Cherry");
        // Enhanced for-loop
        for (String fruit : fruits) {
            System.out.println(fruit);
        }
    }
}

Example 2: Using forEach​

import java.util.*;

public class ForEachExample {

    public static void main(String[] args) {
        List<String> fruits = Arrays.asList("Apple", "Banana", "Cherry");

        // Using forEach with a lambda expression
        fruits.forEach(fruit -> System.out.println(fruit));
    }
}

Example 3: Custom Iterable Implementation​

import java.util.*;

class Countdown implements Iterable<Integer> {

    private final int start;

    public Countdown(int start) {
        this.start = start;
    }

    @Override
    public Iterator<Integer> iterator() {

        return new Iterator<>() {

            private int current = start;

            @Override
            public boolean hasNext() {
                return current > 0;
            }

            @Override
            public Integer next() {

                if (!hasNext()) {
                    throw new NoSuchElementException();
                }

                return current--;
            }
        };
    }
}

// Usage:
public class CustomIterableExample {

    public static void main(String[] args) {

        Countdown countdown = new Countdown(5);

        for (int i : countdown) {
            System.out.println(i);
        }
    }
}

Why Iterable Matters​

  • Uniform Iteration: Enables consistent iteration across all collections.
  • Integration with Modern APIs: Required for streams (stream() and parallelStream()).
  • Custom Types: Allows custom types to integrate seamlessly with the Collection Framework.