Object-Oriented Programming with Java, part I + II

Abstract classes

In Exercise overwriting-methods-2 there’s a problem though. It is possible to execute the following code

Vector2D vector = new Vector3D(10,10,10);

This would not make any sense though, as a Vector3D would be the same as a Vector2D.

In the shape example of last chapter, there’s also a big problem. It is possible in java to make a new Shape

Shape shape = new Shape(Color.green);

This would of course be nonsense, as a ‘shape’ does not really have a surface area or circumference. The methods in this class now return 0, but we can also remove the code for these methods, by turning this class into an abstract class

class Shape {
    private Color color;

    public Shape(Color color) {
        this.color = color;
    }
    public abstract double getArea();
    public abstract double getCircumference();
}

An abstract class can contain abstract methods, which are methods that do not have an implementation. There is no code for these methods because it makes no sense, like in our Shape example. This also makes sure that there can be no new Shape objects, because it has abstract methods.

Shape shape = new Shape(Color.green); // won't compile, as Shape is an abstract class

In order to make an object, just extend the Shape class, and implement the abstract methods, as done before. This new class is not abstract anymore.

Exercise abstract-classes-1: Bank accounts

A bank has 2 kinds of accounts, a deposit account and a savings account. Deposit account costs 5 euro per year, and has no interest. A savings account costs 50 euro per year but has 5% interest. We are going to model this using an abstract class

abstract-classes-1.1 Account class

An account stores the name of the type of the account (Savings or Deposit), and have an amount of money on them. We will model these as attributes. Make an abstract class Account with the following properties

• attribute String name
• attribute double amount
• Constructor with parameters String name and double initialAmount. The name attribute gets set to name + " (initial " + amount + ")"
• getter and setter for amount. Make sure the setter rounds the amount to 2 decimals
• toString that prints name - amount
• abstract void calculateNextYear()

abstract-classes-1.2 Savings and Deposit account

Make a class DepositAccount that extends Account. Implement a constructor with 1 parameter, initialAmount. The name is set to ‘Deposit’. The class DepositAccount overwrites the calculateNextYear, which sets the amount to the amount - 5.

Make a class SavingsAccount that extends Account. Implement a constructor with 1 parameter, initialAmount. The name is set to ‘Savings’. The class SavingsAccount overwrites the calculateNextYear, which sets the amount to the (amount - 50) * 1.05.

abstract-classes-1-3 Using the accounts

Write a main method that tests the accounts. Make a program that outputs the following output (use the toString for the account to print the accounts)

Year 1
Savings (initial 500.0) - 472.5
Savings (initial 1500.0) - 1522.5
Deposit (initial 500.0) - 495.0
Deposit (initial 1500.0) - 1495.0

Year 2
Savings (initial 500.0) - 443.63
Savings (initial 1500.0) - 1546.13
Deposit (initial 500.0) - 490.0
Deposit (initial 1500.0) - 1490.0

Year 3
Savings (initial 500.0) - 413.31
Savings (initial 1500.0) - 1570.94
Deposit (initial 500.0) - 485.0
Deposit (initial 1500.0) - 1485.0

Exercise abstract-classes-2: Walking monsters

Given is a simulation with a Hero and Enemys. All these Characters are simulated, and can walk around, and attack other characters.

In this diagram we can see see it is quite easy to add new enemies, just extend the Enemy class. The Character class provides the following methods:

• getX() and getY() to get the position of the character
• moveTo(int x, int y) to move the character to another position
• distanceTo(int x, int y) and distanceTo(Character other) to calculate the distance to another character
• ArrayList<Character> getCharactersNear(int range, boolean isEnemy) returns a list of all the enemies around this character
• spawn(Character character) adds a new character
• suicide() kills this character

Provided is a simple enemy, the RandomEnemy, that just moves around randomly.

Exercise abstract-classes-2.1 : Running away

Create a new enemy called RunningEnemy that will run away from the hero characters near. Create the class, and override the update() method to run away from any players nearby. Give this enemy a ‘R’ as display character, and a health of 1

Exercise abstract-classes-2.2 : Breeding enemies

Create a new enemy, called BreedingEnemy. The behavious of this enemy should be

• If a player is nearby, it should go towards the player to attack it
• If no player is nearby, it should move towards other BreedingEnemy nearby.
• If the enemy is next to another BreedingEnemy, it should perform suicide on both BreedingEnemies, and make 1 new BreedingEnemy (and spawn it), with the health of both enemies, multiplied by 2

Note Due to time constraints, this exercise does not test the behavious completely