Lab 2: Inheritance & ExpandableArray

Objectives:

The main goals for this lab are:

1. Review inheritance
2. Implement your first data structure - a self expanding array

This lab will be submitted and autograded on Gradescope.

Notes: in this lab you are not allowed to include any import statements other than import java.lang.IndexOutOfBoundsException

Paired Programming rules

This lab is a paired programming assignment. What exactly does that mean? You will be working in pairs on the CS lab computers. Each pair will be working on one computer. One person will be the driver and the other person will be the navigator. Here is the rule: the driver controlls the lab computer, but the driver can only type what the navigator tells them to type. For this to work well, each pair should be constantly talking among themselves. After each problem, you will switch roles, the navigator will become the driver and the driver will become the navigator.

Exercise 1: Inheritance

You will design all the necessary classes in order to make the following driver program work properly and produce the given expected output (we have broken down the step below).

public static void main(String[] args){
    Mammal[] mammals = new Mammal[4];
    mammals[0] = new Dolphin();
    mammals[1] = new Platypus();
    mammals[2] = new Human();
    mammals[3] = new CSStudent();

    for (int i=0; i< mammals.length; i++){
        System.out.print("Generally, a " + mammals[i].getName());
        System.out.print(" can be found ");
        if(mammals[i].livesInWater() == false){
            System.out.print("on land, ");
        }
        else {
            System.out.print("in water, ");
        }

        System.out.print("it can ");
        if(mammals[i].laysEggs() == false) {
            System.out.print("not ");
        }
        System.out.print("lay eggs, and is often overheard saying'");
        mammals[i].speak();
        System.out.println("'");
    }
}

Will produce the following output:

Generally, a Dolphin can be found in water, it can not lay eggs, and is often overheard saying 'ak, ak, ak, ak'
Generally, a Platypus can be found in water, it can lay eggs, and is often overheard saying 'errrr'
Generally, a Human can be found on land, it can not lay eggs, and is often overheard saying 'I'll take a grande latte with a double-shot of espresso'
Generally, a CSStudent can be found on land, it can not lay eggs, and is often overheard saying 'I love programming!'

Make sure that each class is declared public and stored in their own files.

1.1 Mammal class

Design a Mammal class with the following:

• two private String variables named name and sound
• a value constructor that initializes the two variables
• getters of the two instance variables
• a void method named speak() that prints the object’s sound
• a boolean method named laysEggs()
• a boolean method named livesInWater()

1.2 Dolphin class

Design a Dolphin class that inherits from the Mammal class. As seen in the Driver code, it should include a constructor that takes no arguments. Override methods as appropriate

1.3 Platypus class

Design a Platypus class that inherits from the Mammal class. As seen in the Driver code, it should include a constructor that takes no arguments.

1.4 Human class

Design a Human class that inherits from the Mammal class. As seen in the Driver code, it should include a constructor that takes no arguments. Override methods as appropriate

1.5 CSStudent class

Design a CSStudent class that inherits from the Human class. Override methods as appropriate

Before moving on in this lab, run the Driver program and make sure the output is correct.

======= As seen in the Driver code, it should include a constructor that takes no arguments. Override methods as appropriate

Before moving on in this lab, submit your code to gradescope to make sure the output is correct.

Exercise 2: ExpandableArray

Working with arrays are great, we can easily access data from an array and insert data into an array. Just give me the index, and accessing and inserting data is easy!

But, what happens we run out of space in our array - Oh no! We need to create a new bigger array, then copy over all the data from the older smaller array to the new bigger array - that doesnt sound like fun at all!

Wouldn’t it be nice if we have a data structure that took care of all this building and expanding for us. Thats what we are going to build now. We will create a class called ExpandableArray that takes care of that. So, if a programmer wants to just store data in an array, they dont have to worry about running out of space, your data structure will do that automatically.

You will be using and adding more functionality to your ExpandableArray in Homework02.

Testing

We have provided a file called

ExpandableArrayTests.java that you can use to test your ExpandableArray. You can download it by running:

wget https://raw.githubusercontent.com/BMC-CS-151/BMC-CS-151.github.io/main/labs/lab02/ExpandableArrayTests.java 

The tests will not compile until you’ve written completed the lab.

Below we list some of the instance methods you will need to implement.

2.1 Class definition

Lets start by creating a file called ExpandableArray.java. We can define our class with the following line:

public class ExpandableArray<E>

Question: In our class definition what is E and what does it denote?

2.2 Instance variables

Our ExpandableArray class will need to keep track of some data.

Question: What data do you think we will need to store?

Now add those instance variables

2.3 Constructors

Create two constructors:

1. The first should take an argument to specify the initial capacity of our data structure. We call these kinds of constructors “value constructors.”
2. In the empty constructor, you as the designer get to specify a default value. In order to make sure that the remaining methods work correctly, I would recommend you limit the default to at most 10.

2.4 Inserting

We will create two insert methods. The first will take in an item and insert the item at the begining of the ExpandableArray. The second will take in an item and a location and insert the item in the specified location. When a user inserts an item in the array, make sure to move shift elements up or down to maintain our rule of always storing an element whose list index is i at index i of the array.

For the insert method which takes an index to insert at, ensure that the index is valid. If the user specifies an invalid index, throw an IndexOutOfBoundsException error. An index is invalid if does not correspond to an element in the array. If our underlying array is of size 20, but only 4 of them are filled, any index > 4 is invalid.

Your error should include an informative message. Similar to how the follow code

int[] test = new int[2];
int a = test[3];

would throw the following error with the following message:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 2

Both insert methods should be named insert and return void type.

2.5 Accessing

Write a method named get that will extract an item from the ExpandableArray. The user should be able to specify an index and the ExpandableArray will return the item at that index.

Again, if the user specifies an index that is greater than the number of elements in the ExpandableArray, throw an IndexOutOfBoundsException.

2.6 Remove

Write a remove method that enables a user to specify the index and the ExpandableArray will remove the item at that index. Since we do not want any empty spaces in the middle of our ExpandableArray, make sure to move over every item that you need to when removing the item in the index specified by the user. Your remove method should return the removed element.

2.7 toString()

Write a toString() method that will print out each item in the ExpandableArray. Each item should be seperated by a comma and space.

2.8 set()

Write a set method which takes an element and an index, and sets the value accordingly.

Again, if the user specifies an index that is greater than the number of elements in the ExpandableArray, throw an IndexOutOfBoundsException.

2.9 size()

Write a size method which returns the number of elements in the array.

Test it!

After compiling the test class, make sure to include the ea flag when running the program:

java -ea ExpandableArrayTests

This will ensure that all tests actually run. We are using assert to test the ExpandableArray.

Once your tests pass, submit your code to Gradescope.

Exercise 3: Mammals again

After getting ExpandableArrayTests.java to pass all the test, change the data structure in exercise 1 to use your ExpandableArray. Make sure the program works the same.

Wrap up

In todays lab we covered Inheritance and creating our first data structure.

Signing out

Before leaving, make sure you submit to Gradescope.