Standard Functional Interfaces

There are other useful interfaces in the java.util.function package.

• Use the BiPredicate interface when you need to pass in two arguments and get a boolean result.

@FunctionalInterface
public interface BiPredicate<T, U> {
  public boolean test(T t, U u);
}
// Pass test a T and U, get back a boolean.

• Use the Function interface when you want to specify the result type as a generic as well.

@FunctionalInterface
public interface Function<T,R> {
  public R apply(T t);
}
// Pass apply a T, get back a R.

• Avoid the extra overhead of boxing to a wrapper class by using the primitive specific version of each interface.

@FunctionalInterface
public interface IntFunction<R> {
  public R apply(int value);
}
// Pass apply an int, get back a R.

• See the documentation for the java.util.function package for more.

Functional Interfaces and default Methods

A functional interface contains only one abstract method, but can define other default methods with implementations.

Predicate has useful default methods:

• negate() - returns a Predicate that represents the logical negation of this Predicate.

  // ...
  if (ps.negate().test(planet)) {
    tempList.add(planet);
  }
  // ...
  

• and(Predicate p2) - returns a composed Predicate representing the short-circuiting logical AND of this Predicate and p2.

• or(Predicate p2) - returns a composed Predicate representing the short-circuiting logical OR of this Predicate and p2.

The method below combines two Predicates using and().

public static List<Planet> filterPlanetsAND(List<Planet> list, Predicate<Planet> ps1, Predicate<Planet> ps2 ) {
  List<Planet> tempList = new ArrayList<>();
  for (Planet planet : list) {
    // Combine the two Predicates
    if (ps1.and(ps2).test(planet)) {
      tempList.add(planet);
    }
  }
  return tempList;
}

Collection Method - boolean removeIf(Predicate)

Java 8 integrated the Predicate interface into Collection, allowing us to remove items that match a Predicate. * It returns true if any items were removed.

List<String> strings = new ArrayList<>();
strings.add("// This is a comment");
strings.add("public class PlanetUtilities {");
strings.add("\t\t// A comment with tabs...");
strings.add("}");

// Trim the input String for whitespace and see if it starts with an inline comment
Predicate<String> startsWithComment = p -> p.trim().startsWith("//");
boolean removed = strings.removeIf(startsWithComment);

System.out.println(removed);
for (String s : strings) {
  System.out.println(s);
}
// true
// public class PlanetUtilities {
// }

* Be careful, as this modifies the Collection.

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