Iterable
- Java course
- Intermediate
- Lesson 97/116
- 5 min
Learn for-each support with Iterable.
The Iterable<T> interface
Iterable<T> is the interface that lets a class be used in a for-each loop. Any class that implements Iterator<T> iterator() becomes Iterable. The List, Set and Queue interfaces all inherit Iterable.
Once your class implements Iterable, a for (String s : myObject) loop works. This makes a non-collection structure traversable; for example a tree, a list of lines or a number range can be made Iterable.
A custom Iterable example
When a Names class implements Iterable<String>, it works like this: it holds a List<String> items field; the iterator() method returns items.iterator(). Outside, when you write for (String n : names) Java calls this method.
This is a clean way to hide inner data and expose only read access. Users cannot see the list directly but can traverse it through the Iterator. Writing an Iterator from scratch is not needed; the inner list is already Iterable.
Practice Write a custom Iterable class
Define class Names implements Iterable<String>. Hold a List<String> items = new ArrayList<>(); field; add void add(String n) that calls items.add(n);. Implement @Override public Iterator<String> iterator() that returns items.iterator(). In main, create Names names = new Names();, add two names, then print with for (String n : names) System.out.println(n);.
Starter code
import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;
// Define the Names class: Iterable<String>, items list, add, iterator
public class Main {
public static void main(String[] args) {
// Create Names, add two names, print with for-each
}
}Sample solution
import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;
class Names implements Iterable<String> {
List<String> items = new ArrayList<>();
void add(String n) { items.add(n); }
public Iterator<String> iterator() { return items.iterator(); }
}
public class Main {
public static void main(String[] args) {
Names names = new Names();
names.add("Ayşe");
names.add("Alex");
for (String n : names) {
System.out.println(n);
}
}
}Expected output
Ayşe Alex
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What do you gain when a class is Iterable?
- Comparable ability
- Sorting ability
- for-each loop support Correct answer
- Multiple inheritance
Explanation: An Iterable class can be traversed with for-each.
2 Complete the class declaration
class Names implements Iterable<String> {
public Iterator<String> ____() {
return items.iterator();
}
}class Names implements Iterable<String> {
public Iterator<String> iterator() {
return items.iterator();
}
}Explanation: The iterator method is the only required method of Iterable.
3 Match the Iterable concepts
| Term | Meaning |
|---|---|
Iterable<T> |
Interface enabling for-each |
iterator() |
Single required method |
List |
Inherits Iterable |
for-each |
Short loop syntax |