Higher-Kinded Types

Monad.java

interface Monad<T> {
    <U> Monad<U> flatMap(Function<? super T, ? extends Monad<U>> mapper);
}

interface Tree<T> extends Monad<T> {
    @Override
    <U> Tree<U> flatMap(Function<? super T, ? extends Tree<U>> mapper);
}

interface BinaryTree<T> extends Tree<T> {
    @Override
    <U> BinaryTree<U> flatMap(Function<? super T, ? extends BinaryTree<U>> mapper);
}