11.5. 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

• Write a method called all that returns true if all the elements of an array pass a test supplied using a Predicate.

  public static <T> boolean all(T[] elements, Predicate<T> predicate) {
     ...
  } // all
  

• Write a method called any that returns true if at least one of the elements of an array passes a test supplied using a Predicate.

  public static <T> boolean any(T[] elements, Predicate<T> predicate) {
     ...
  } // any
  

Test Yourself Solutions (open after attempting the questions above)

• One possible solution:

  public static <T> boolean all(T[] elements, Predicate<T> predicate) {
      for (T t: elements) {
          if (!predicate.test(t)) {
              return false; // return false if an element doesn't pass the test
          } // if
      } // for
  
      // If we make it out of the loop, all elements passed the test
      return true;
  } // all
  

• We won’t provide a solution to the second example above. Try it on your own, test it, and ask questions on Piazza if you get stuck. Feel free to share your code on Piazza since “Test Yourself” questions are not graded assignments.