Lab: Anonymous functions

Summary

We explore the use of anonymous functions in Java.

Preparation

a. Review the documentation for the Predicate interface, available at https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/function/Predicate.html.

b. Review the documentation for the Function interface, available at https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/function/Function.html.

c. Review the documentation for the Comparator interface, available at https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Comparator.html.

d. Open the Java Tutorials page on lambda expressions.

e. Create a new class, ListUtils, that you will use for some of the exercises.

f. Create a new class, LambdaExperiments, that you will use for some of the exercises. Your class should include a main method that begins as follows.

    String[] tmp = 
        new String[] { "alpha", "bravo", "charlie", "delta", "echo",
                       "foxtrot", "golf", "hotel", "india",
                       "juliett", "kilo", "lima", "mike", 
                       "november", "oscar", "papa", "quebec",
                       "romeo", "sierra", "tango", "uniform",
                       "victor", "whiskey", "xray", "yankee", "zulu" };
    ArrayList<String> strings = new ArrayList<String>(Arrays.asList(tmp));

Exercises

Exercise 1: Reviewing the reading

In the section entitled “Ideal Use Case for Lambda Expressions” in the reading, the authors present nine different approaches to filtering a list. Write a program that experimentally verifies that each of the nine approaches works as described.

You will likely need to

• Make a copy of the Person class associated with the reading, which is available at https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Person.java.
• Create a new class, JTLambdaExperiment, in which you will write the various experiments.
• Add a main method to JTLambdaExpermiment.
• In the main method of JTLambdaExperiment, create a list of Person objects. (We’d recommend an ArrayList.) You can see the createRoster method in Person for an example.
• For each static method in the reading, add the method to JTLambdaExperiment and then add a call to that method to main to see that the method behaves as expected.

Note: If you are adding interfaces and classes, you should do so outside the body of JTExperiment.

Exercise 2: Selecting values

a. In ListUtils, write a static generic method, select(List<T> vals, Predicate<T> pred), that creates a new ArrayList that contains only the elements of vals for which the predicate holds.

b. In the main method of LambdaExperiments, select and print the values in strings (from the preparation) whose length is at least five. Make sure to define your predicate anonymously.

c. In the main method of LambdaExperiments, select the values in strings which contain two vowels in a row. Once again, make sure to define your predicate anonymously.

Exercise 3: Removing values

Rather than building a new list by selecting values, as in the previous problem, we could also write a method that mutates an existing list by removing values.

a. In ListUtils, write a static generic method, remove(List vals, Predicate)`, that removes from vals any values for which the predicate holds. You may not use any existing methods in the `List` class other than `iterator`. You can assume that the `remove` method is implemented in the given iterator.

b. Extend your program to remove the values in strings whose length is at least five.

c. Extend your program to instead remove the values in strings which contain two vowels in a row.

Exercise 4: Extrema

Another common generic task is to find the largest (or smallest) value in a list.

a. Write a static generic method, largest(List<T> vals, Comparator<T> compare), that finds the largest element in the list, using compare to compare values.

b. Extend your program to find the shortest string in strings. (If there are multiple shortest strings, you can return any of them.)

c. Extend your program to find the longest string in strings. (If there are multiple longest strings, you can return any of them.)

d. Extend your program to find the string in strings whose sum of ASCII values is the smallest.

e. Extend your program to find the string in strings whose sum of ASCII values is the largest.

Exercise 5: Mapping

a. Implement a static generic method, map(Function<T,R> fun, List<T> list) that builds a new ArrayList<R> by applying fun to each element of list.

b. Using map, build a new version of strings in which every string is converted to uppercase.

c. Using map, build a new version of strings in which the first letter in each string is converted to uppercase.

d. Using map, build a new list that contains the lengths of all of the strings in strings.

For those with extra time

In the unexpected situation that you find that you have extra time, you might attempt one or more of the following exercises.

Extra 1: Updates to lambda

The Java Tutorials page for lambdas notes that

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don’t take advantage of improvements introduced in later releases.

Determine what improvements have been added in later releases.

Extra 2: Other functional interfaces

Find at least two other functional interfaces and describe use cases for those interfaces.

Extra 3: Iterators

Lambda provide a nice alternative to anonymous inner classes, with a bit less syntax. Can we use lambda to implement iterators? Why or why not?

Acknowledgements

Some problems on this lab were inspired by a lab on anonymous procedures from the Fall 2014 iteration of CSC 207.