5: Objects & References

We will look more closely at the object-oriented programming and the implications of instantiated objects.

Learning Targets

  • I can describe what it means to be an object-oriented programmer.

  • I can describe the difference between values and references.

  • I can build unit tests to check every method in my class definition.

Class Vocab

Object-Oriented Programming

Why don't we list all of our code in one long file? How do we start to organize big projects? We start with objects. Already we've been using a class with a main method as its starting point. All of our methods have been static. What if we wanted to make a game with a lot of monsters? We can define our own Monster class and create as many instances of that object as we need in the game. We can't use static methods anymore as a result.

Let's try out a simple game in class using while-true loop and a Monster.


Keep your data private, accessible only to methods that allow for more careful controls.

Primitives and Refs

import java.util.ArrayList;

public class Practice {

	public static void main(String[] args) {
		// create a primitive
		int myPrimitive = 5;
		// pass it by value to a method that adds one to the item.

		// print the unchanged value of the primitive
		System.out.println("Primitive: " + myPrimitive);
		// instantiate a list and store the reference to it in myRef
		ArrayList<Integer> myRef = new ArrayList<>();
		// add the value 5 to a new Integer 
		// pass it by reference to a method that adds one to the first element in the list
		// print what's now in the first spot of the object that was passed by reference
		System.out.println("Reference: " + myRef.get(0));
	} // closes main
	public static void addOne(ArrayList x) {
		// get the current value of the first item in the list, convert to int
		int current = (int)x.get(0);
		// set the first item's value to one more than its current
		x.set(0, ++current);
	public static void addOne(int x) {

} // closes the class

Intro to ArrayLists

ArrayLists are introduced in this section because they're objects themselves. They illustrate some of the differences we see when working with objects. Instead of arrays when we can access an element just by printing someCollection[x], we now need to use someOtherCollection.get(x). Let's get into these differences.

Different Kind of Loop

Because ArrayLists are only accessible through methods, you can use the same type of access[index] you can with an array. Here's what a simple traversal looks like with a good ol' array:

int[] myArray = {5, 10, 15, 20};
for(int x = 0; x < myArray.length; x++){

Notice how I used length as a property not as an accessor method() and I accessed an element from the collection using [brackets]. Now let's take a look at an ArrayList:

List myList = new ArrayList<Integer>(); 
myList.add(5);  // I could loop and add all these
for(int x = 0; x < myList.size(); x++){

There's a couple important things happening above you should look closely at:

  • Notice how I declared a List and then turned it into an ArrayList? It wouldn't have worked if I tried to do new List because a List is abstract. It's like mammal. You can tell the computer that you'd like to make a new mammal named x and it should run the new Dog() constructor. That's a polymorphic declaration. Use a general ancestor as a type and then be more specific during the constructor.

  • If you look inside the ArrayList class programmed inside our Java library, you'll notice it has an array at its core. It's a wrapper for an array. So there is a .length property that's relevant to its internal, private programming. Every time you .add(something) to an ArrayList it destroys its old array and copies all the content to a new, longer array. So .length is serious business. Since that's occupied, we use the .size() accessor method to retrieve the length of an ArrayList.

SecureList: An Example

What's the point of creating our own objects? How does an ArrayList differ from an array in practical implementation? Let's go through an example project to help iIllustrate these concepts.

What is a SecureList?

SecureLists are made up. They're a silly object that provides an extra layer of "security" by storing your list of 12 names in two different objects, an array and an ArrayList. Does that actually provide additional security? No, not really. But let's pretend it does so you have an opportunity to work with an array and an ArrayList simultaneously.

Let's set up your project with the following files:

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 * and open the template in the editor.

import java.util.Scanner;

public class App {
    // from a static method, we'll create instances of our made-up object
    public static void main(String[] args) throws Exception {
        System.out.println("Let's build a list of names to test.");
        Scanner s = new Scanner(System.in);
        // our example container
        String[] testList = new String[12];
        // build an example list to work with
        System.out.print("Would you like to enter names? (y/n): ");
            // populate the array with names from user
            for(int x = 0; x < 12; x++){
                System.out.print("\nInsert name at index " + x + ": ");
                testList[x] = s.nextLine();
        } else {
            // populate the array with just numbers
            for(int x = 0; x < 12; x++){
                testList[x] = "Student #" + x;
        System.out.println("\n----- SecureList Unit Tests ------\n");
        // test the constructor
        SecureList test1 = new SecureList(testList);
        // test title mutator and accessor
        System.out.println("\nTITLE TEST: \n");
        test1.title("Demo"); // mutator 
        System.out.println(test1.title());  // accessor

        // implicitly call the .toString() method
        System.out.println("\nPRINT TEST: \n");
        // draw a random name from the list
        System.out.println("\nGET RANDOM: \n");
        // check that each element in both collections match
        System.out.println("\nIS SECURE: \n");
        System.out.println(test1.isSecure()); // should print "true"
        // get the first name in the list
        System.out.println("\nGET STUDENT 0: \n");
        // get the first name in the list using the seat number not the index
        System.out.println("\nGET SEAT 1: \n");
        // randomize the order of the names
        System.out.println("\nSHUFFLE: \n");

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