CS 151

Cay S. Horstmann

Day 7 Clicker Question 1

Which of the following statements are true?

1. The Icon.paintIcon method is abstract.
2. The Icon interface is a functional interface.
3. The IconTester class is coupled with the MarsIcon class.
4. The JOptionPane class is coupled with the MarsIcon class.

Day 7 Clicker Question 2

What does this lambda expression do?

s -> s.equals(s.toUpperCase())

1. It turns a string into uppercase.
2. It tests whether a string is all uppercase.
3. It tests whether two strings are the same when turned into uppercase.
4. It always returns false.

Day 7 Clicker Question 3

Consider this method:

public static <T> T largest(T[] values, Comparator<T> comp)
{
   T result = null;
   for (int i = 0; i < values.length; i++)
      if (i == 0 || comp.compare(result, values[i]) < 0)
         result = values[i];
   return result;
}

What is the result of this call?

String[] words = { "Mary", "had", "a", "little", "zebra" };
String largestWord = largest(words, (s, t) -> Math.max(s.length(), t.length()));

1. "Mary"
2. "little"
3. "zebra"
4. There is a compile- or runtime error.

Mystified By Lambdas?

If you are totally mystified by lambdas, do this exercise. If you understand the basic idea, skip this and move on to the next slide.

• Start JShell. Paste in the method from the preceding slide. Paste in the two lines below it. What happens?
• Now call largest without a lambda expression. Make a class MyFriendlyComparator that implements an interface. Which interface?
• Which method do you need to define in MyFriendlyComparator?
• What is the body of that method?
• What do you now pass to largestWord?

Put it all into JShell—the class that you defined, the instance of that class, and the call to largestWord.

Lambda Practice

• Start JShell. Run

  import java.time.*;
  var ids = ZoneId.getAvailableZoneIds();
  

• There are some unsightly IDs that start with Etc/. Remove them by calling List.removeIf. How did you do that?
• These days, we prefer to return results rather than having side effects. Complete a generic method

<T> Collection<T> removeIf(Collection<T> original, Predicate<T> pred) {
   ArrayList<T> result = new ArrayList<>();
   . . .
   return result;
}

and paste the code into JShell. Verify that you can call

removeIf(ZoneId.getAvailableZoneIds(), id -> id.startsWith("A"))

What is the code of your removeIf method?

Sequence Diagram Practice 1

Consider this call:

JOptionPane.showMessageDialog(. . ., new MarsIcon(50));

Draw a sequence diagram.

Sequence Diagram Practice 2

Consider this method:

public static Comparable largest(Comparable[] values)
{
   Comparable result = null;
   for (int i = 0; i < values.length - 1; i++)
      if (i == 0 || result.compareTo(values[i]) < 0)
         result = values[i];
   return result;
}

and this call:

largest(countries);

where countries is defined as

Country[] countries = new Country[] {
   new Country("Uruguay", 176220),
   new Country("Thailand", 514000),
   new Country("Belgium", 30510) };

Sequence Diagram Practice 3

Repeat with this method

public static T <T> largest(T[] values, Comparator<T> comp)
{
   T result = null;
   for (int i = 0; i < values.length - 1; i++)
      if (i == 0 || comp.compare(result, values[i]) < 0)
         result = values[i];
   return result;
}

and this call:

largest(countries, new CountryComparatorByName());

Discussion