Very simple Function object with composition in Java.

Function.java

public abstract class Function<A, B> {

    public abstract B apply(A arg);
    
    // Given f :: a -> b and g :: b -> c, we can define h :: a -> c as the composition of f and g
    public static <A, B, C> Function<A, C> compose(final Function<A, B> x, final Function<B, C> y) {
        return new Function<A, C>() {
            @Override
            public C apply(A arg) {
                return y.apply(x.apply(a));
            }
        }
    }

}