monoid.java

public interface Monoid<T> {
  public T apply(T a, T b);
}

public class StringMonoid implements Monoid<String> {
  public static final String IDENTITY = StringUtils.EMPTY_STRING;

  public String apply(String a, String b) {
    return a + b;
  }
}

public class Driver {
  public static void main(String[] args) {
    final StringMonoid m = new StringMonoid();
    String foo = "foo";
    String bar = "bar";
    assert( m.apply(foo, StringMonoid.IDENTITY) == foo );
    assert( m.apply(foo, bar) == (foo + bar) );
  }
}