Contains method for Iterable and Iterator?

Question

Is there a simple method to check if an element is contained in an iterable or iterator, analogous to the Collection.contains(Object o) method?

I.e. instead of having to write:

Iterable<String> data = getData();
for (final String name : data) {
    if (name.equals(myName)) {
        return true;
    }
}

I would like to write:

Iterable<String> data = getData(); 
if (Collections.contains(data, myName)) {
    return true;
}

I'm really surprised there is no such thing.

Answer 1

In Java 8, you can turn the Iterable into a Stream and use anyMatch on it:

String myName = ... ;
Iterable<String> data = getData();

return StreamSupport.stream(data.spliterator(), false)
                    .anyMatch(name -> myName.equals(name));

or using a method reference,

return StreamSupport.stream(data.spliterator(), false)
                    .anyMatch(myName::equals);

Answer 2

Guava has the functions Iterables.contains and Iterators.contains which do what you want. You can look at the source to see how to implement them yourself.

Answer 3

An iterator is a sort of cursor which can be moved over the elements of any collection of elements. So its inner state is mainly the pointer to the current element. If you would try to find out whether it "contains" a certain element you would have to move the cursor and therefore modify the inner state. Modifying the state by simply asking a question is surely a bad thing to do.

That is even the problem with the mentioned Guava. It will modify the iterator object by simply calling the contains method.

An iterable on the other hand is simply an interface telling the compiler that there is something to iterate over. In most cases the iterable object will be the collection itself. If you would add methods like "contains" to the interface Iterable you would get a (simplified) version of the Collection interface - which already exists. So there is no need for that.

If you are stuck in your code at some place where you have a reference to an iterable but need functionality of a collection you should think about refactoring your code. Either you should use the interface collection consistently or ask yourself why you should better not call collection methods at this point. So your problem is most probably a result of a suboptimal code design.

On the other hand I would consider it to be a strange thing to use Iterable as type for parameters or variables anyway. Technically you can do this but I think it is meant to be used in loops only.