CS173: Intro to Computer Science - The ArrayList

Activity Goals

The goals of this activity are:
  1. To be able to create, access, and iterate an ArrayList
  2. To be able to explain that an ArrayList is a dynamic array that allows for expansion at runtime
  3. To be able to dynamically grow and shrink an ArrayList
  4. To use objects such as the ArrayList that accept templated/generic data types

Supplemental Reading

Feel free to visit these resources for supplemental background reading material.

The Activity

Directions

Consider the activity models and answer the questions provided. First reflect on these questions on your own briefly, before discussing and comparing your thoughts with your group. Appoint one member of your group to discuss your findings with the class, and the rest of the group should help that member prepare their response. Answer each question individually from the activity, and compare with your group to prepare for our whole-class discussion. After class, think about the questions in the reflective prompt and respond to those individually in your notebook. Report out on areas of disagreement or items for which you and your group identified alternative approaches. Write down and report out questions you encountered along the way for group discussion.

Model 1: The ArrayList

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import java.util.ArrayList;
 
public class Main {
   public static void main(String[] args) {
      /* This is a class that encapsulates the behavior of an array,
         but it grows as you add items on the fly. */
      ArrayList<String> arr = new ArrayList<String>(10);
       
      for(int i = 0; i < 10; i++) {
        String x = new String("Test #" + i);
         
        arr.add(x);
      }
       
      for(int i = 0; i < arr.size(); i++) {
        String x = arr.get(i);
         
        System.out.println(x);
      }
       
      arr.remove(arr.size() / 2);
       
      // This is called an Enhanced For Loop - a nice shortcut!
      for(String item: arr) {
        System.out.println(item);
      }
   }
}

Questions

  1. What is the purpose of the <String> notation when creating the ArrayList?
  2. What would be the effect of creating an ArrayList that stored another ArrayList?
  3. What is the size of the ArrayList prior to calling remove()? What is the size after the call?
  4. Suppose a friend needed to remove every other element from an ArrayList (say, the ones with an even numbered index). Looping for i = 0 to arr.size(), they remove each element if (i % 2 == 0) but the wrong elements seem to be removed. What happened, and what can we do instead?
  5. Rewrite this line of code as an enhanced for loop: for(int i = 0; i < arr.size(); i++) {
  6. How might you create an ArrayList of integers or floating point values?

Submission

I encourage you to submit your answers to the questions (and ask your own questions!) using the Class Activity Questions discussion board. You may also respond to questions or comments made by others, or ask follow-up questions there. Answer any reflective prompt questions in the Reflective Journal section of your OneNote Classroom personal section. You can find the link to the class notebook on the syllabus.

For Additional Practice

Feel free to visit these resources for additional practice exercises.