11.6. More Examples

In the code presented below, we create three more objects using lambda expressions (four total). Using the usual named class approach to implementing the interface would have required four .java files, one for each named class. Using the lambda expression approach, all four (unnamed) classes are created and instantiated using a single .java file, and in this case, all in one method!

// Driver.java (assume proper package and import statements)
public class Driver {

    public static void forEach(String[] strings, Consumer<String> consumer) {
        for (int i = 0; i < strings.length; i++) {
            String str = strings[i];
            consumer.accept(str);
        } // for
    } // forEach

    public static void main(String[] args) {

        Consumer<String> println = t -> System.out.println(t);
        Driver.forEach(args, println);
        System.out.println();

        Consumer<String> shout = t -> System.out.println(t.toUpperCase());
        Driver.forEach(args, shout);
        System.out.println();

        Consumer<String> whisper = t -> System.out.println(t.toLowerCase());
        Driver.forEach(args, whisper);
        System.out.println();

        Consumer<String> repeat2 = t -> System.out.println((t + " ").repeat(2));
        Driver.forEach(args, repeat2);
        System.out.println();

    } // main

} // Driver

Compile ONE file, then run:

java Driver hello WORLD

hello
WORLD

HELLO
WORLD

hello
world

hello hello
WORLD WORLD

Test Yourself

• In the API documentation for the preexisting Predicate interface, you will see that it contains an abstract method called test that takes a single parameter and returns a boolean value. You should also note that the interface is generic.

  Write a lambda expression to implement the test method so that it returns true if a provided String is longer than 8 characters.

Test Yourself Solutions (open after attempting the questions above)

• One possible solution:

  Predicate<String> longString = (String s) -> {
      return s.length() > 8;
  };
  

• Another valid solution:

  Predicate<String> longString = (String s) -> {
      if (s.length() > 8) {
          return true;
      } else {
          return false;
      } // if
  };
  

• Other solutions are possible.