How can I use the java for each loop with custom classes?

I think most coders have used code like the following :

ArrayList<String> myStringList = getStringList();
for(String str : myStringList)
{
doSomethingWith(str);
}

How can I take advantage of the for each loop with my own classes? Is there an interface I should be implementing?

You can implement Iterable.

Here's an example. It's not the best, as the object is its own iterator. However it should give you an idea as to what's going on.

The short version of for loop (T stands for my custom type):

for (T var : coll) {
    //body of the loop
}

is translated into:

for (Iterator<T> iter = coll.iterator(); iter.hasNext(); ) {
    T var = iter.next();
    //body of the loop
}

and the Iterator for my collection might look like this:

class MyCollection<T> implements Iterable<T> {

    public int size() { /*... */ }

    public T get(int i) { /*... */ }
    
    public Iterator<T> iterator() {
        return new MyIterator();
    }
    
    class MyIterator implements Iterator<T> {
    
        private int index = 0;
    
        public boolean hasNext() {
            return index < size();
        }
    
        public type next() {
            return get(index++);
        }
    
        public void remove() {
            throw new UnsupportedOperationException("not supported yet");
    
        }
   }
}

You have to implement the Iterable interface, that is to say, you have to implement the method

class MyClass implements Iterable<YourType>
{
Iterator<YourType> iterator()
  {
  return ...;//an iterator over your data
  }
}