Nested Classes and Interfaces - The Java Programming Language - page 210

5.4. Anonymous Inner Classes
When a local inner class seems too much for your needs, you can declare
anonymous classes that extend a class or implement an interface. These
classes are defined at the same time they are instantiated with new . For
example, consider the walkThrough method. The class Iter is fairly light-
weight and is not needed outside the method. The name Iter doesn't add
much value to the codewhat is important is that it is an Iterator object.
The walkThrough method could use an anonymous inner class instead:
public static Iterator<Object>
walkThrough(final Object[] objs) {
return new Iterator<Object>() {
private int pos = 0;
public boolean hasNext() {
return (pos < objs.length);
}
public Object next() throws NoSuchElementException {
if (pos >= objs.length)
throw new NoSuchElementException();
return objs[pos++];
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
Anonymous classes are defined in the new expression itself, as part of
a statement. The type specified to new is the supertype of the anonym-
ous class. Because Iterator is an interface, the anonymous class in
walkThrough implicitly extends Object and implements Iterator . An an-
onymous class cannot have an explicit extends or implements clause, nor
can it have any modifiers, including annotations.