Lab: Practicing with Collections

IDE Setup

Before starting this lab, make sure you have completed the setup instructions in the CS240 Setup instructions.

Starter Files

Download the following starter files for this lab:

• CollectionExercises.java - starter code
• CollectionExercisesTest.java - contains the Gradescope test cases

Important: Package Declaration

The files have the package declaration package labs.collections;. This means that the files should be placed in a directory named collections inside a directory named labs inside your source/project directory.

For example, if your source/project directory is src, you should have a directory structure like this:

src/
├── labs/
│   └── collections/
│       ├── CollectionExercises.java
│       └── CollectionExercisesTest.java

If you don't have a labs directory, you should create it. If you don't have a collections directory, you should create it.

Instructions

Your goal in this lab is to complete the unfinished methods in CollectionExercises.java.

Your solutions for these exercises must satisfy the following requirements:

• You may not use indexed for loops. Use only for-each loops or iterators, as appropriate.
• None of your methods may include nested loops. Nested loops generally take a long time to iterate over large collections.
• You must follow the style guidelines for this lab! Typically, labs will not require style, but we want to make sure you have set up your IDE correctly.

Read the hints below carefully before starting the exercises!

Hints

• For some methods, you should create a new collection object to help you solve the problem. For instance, containsDuplicates can be solved by using a HashSet.

  • Here are the links to the Java documentation for the classes you may need:
    • Iterator
    • Collection
    • List
    • ArrayList
    • HashSet
    • HashMap

• You can create a collection that contains all elements from another collection:

  • Using a constructor: HashSet<Integer> = new HashSet<>(other);
  • Using the addAll method.

Solutions

If you're stuck, learn from the first two solutions:

removeSmallInts

public static void removeSmallInts(List<Integer> list, int minVal) {
    Iterator<Integer> iter = list.iterator();
    while (iter.hasNext()) {
        int val = iter.next();
        if (val < minVal) {
            iter.remove(); // Remember, only the iterator can remove elements while looping.
        }
    }
}

containsDuplicates

public static boolean containsDuplicates(Collection<Integer> ints) {
    // Sets are unique, so if it contains fewer elements, there must have been duplicates.
    HashSet<Integer> set = new HashSet<>(ints);
    return set.size() < ints.size();
}

Submitting

You will submit your completed file through Gradescope. You may submit as many times as you like.