Arrays, lists, sets and maps are iterable. What else?

Question

Even if only List and Set implement the interface Iterable, I believe that an array, a list, a set and a map are all iterable objects, in that we can use them all through a foreach loop:

for(String s : new String[0]);
for(String s : new ArrayList<String>());
for(String s : new HashSet<String>());
for(Entry<Integer, String> entry : new HashMap<Integer, String>().entrySet());

The case of Map is maybe a bit different, but let consider it as a key-value list (what it actually is).

Starting with that iterable understanding, am I missing a type in the following method?

public boolean isIterable(Object o) {
    return o instanceof Object[] || o instanceof Iterable || o instanceof Map;
}

In other words, are there any other types that can be iterated through a foreach loop?

Side but resulting question: is that list of types exhaustive?

Solution 1

Anything that implements the Iterable<T> interface is iterable. The Iterable API lists many of the core Java classes that do this. And note that this also includes class that you create that implement the interface. I've done this at times if my class contains an ArrayList or other iterable object and I want to conveniently iterate through the contents of this. I simply pass the list's Iterator object as the return result for the iterator() method.

For example:

Person.java

class Person {
   private String lastName;
   private String firstName;
   public Person(String lastName, String firstName) {
      this.lastName = lastName;
      this.firstName = firstName;
   }
   @Override
   public String toString() {
      return "Person [lastName=" + lastName + ", firstName=" + firstName + "]";
   }


}

MyPeople.java

class MyPeople implements Iterable<Person> {
   List<Person> personList = new ArrayList<Person>();

   // ... other methods and constructor

   @Override
   public Iterator<Person> iterator() {
      return personList.iterator();
   }
}

Solution 2

Only classes implementing the Iterable interface and arrays are. Maps do not implement Iterable, instead they provide three different iterable views (key set, value collection and entry set), which you can iterate over. A more complete check would thus be:

return obj instanceof Iterable || obj.getClass().isArray();

Solution 3

The JDK 1.7 Documentation in the Iterable interface lists everything within the JDK API that implements iterable, but nothing precludes a third-party library from implementing Iterable as well in any class.

Solution 4

Apart from Iterables also arrays (which don't implement the Iterable interface) can be used in the enhanced for loop.

View more solutions

Comments

[assylias]

Note that in your map example, you don't iterate over the map, you iterate over a Set<Entry>, i.e. a set.

[sp00m]

Thanks. In fact, arrays aren't implementing the Iterable interface, maps neither (even if we iterate on its entry set, not on itself directly, but that isn't the point). The question was more: are there any type that are not implementing Iterable but that are iterable with that understanding of iterable? I mean, looking through them, from a beginning to an end. When thinking about algorithms, it's common to say "go through a matrix, through a list, through a stack", etc. A matrix is a kind of Map<Map, Object>, although Map isn't implementing Iterable. Hum... you said philosophy?

[Hovercraft Full Of Eels]

@sp00m: map doesn't implement Iterable, and you can't use forEach on Map type, just on the value or key sets that are obtainable.