COMP2511

23T2 Week 3

Wednesday 1PM - 4PM (W13A)

Thursday 3PM - 6PM (H15B)

Friday 2PM - 5PM (F14B)

Slides by Alvin Cherk (z5311001)

Today

Design by contract
Exceptions
JUnit testing
Generics & Collections
Design Principles
Polymorphism/Inheritance
UML

Important Notice

Assignment-i is out, please start early
There is an assignment viva (discussion with me) after its due in week 7/8. This is to make it easier for me to mark your design decision & to prevent plagiarism
Go to help sessions if you need help (they get busier towards the due date)
No auto generated UMLs of any sort. You will get 0. If you use programs that generate it based on text, please commit this text as a file in your repo.
Assignment-ii pair preferences are out. See your teams channel for the link and more information

Polymorphism

An object's ability to decide what method to apply to itself, depending on where it is in the inheritance hierarchy, is usually called polymorphism.

What is it?

Group Task

Code Review - 5 minutes

Code Review

Can you override a static method?
What is the output of A.f()?

public class A {
    public static void f() {
        C c = new C();
        c.speak();
        B b = c;
        b.speak();
        b = new B();
        b.speak();
        c.speak();
    }
}


public class B {
    public void speak() {
        System.out.println("moo");
    }
}


public class C extends B {
    public void speak() {
        System.out.println("quack");
    }
}

public class D {
    public static void f() {
        F f1 = new F();
        F f2 = new F();
        f1.incX();
        f2.incY();
        System.out.println(f1.getX() + " " + f1.getY());
        System.out.println(f2.getX() + " " + f2.getY());
    }
}

public class F {
    private int x;
    private static int y;

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public void incX() {
        x++;
    }

    public void incY() {
        y++;
    }
}

3. What is the output of D.f()

Code Review

Can you override a static method?
- No, since the static method is attached to the class, it cannot be overridden
What is the output of A.f()?
- quack
- quack
- moo
- quack

public class A {
    public static void f() {
        C c = new C();
        c.speak();
        B b = c;
        b.speak();
        b = new B();
        b.speak();
        c.speak();
    }
}


public class B {
    public void speak() {
        System.out.println("moo");
    }
}


public class C extends B {
    public void speak() {
        System.out.println("quack");
    }
}

Code Review

What is the output of D.f()

public class D {
    public static void f() {
        F f1 = new F();
        F f2 = new F();
        f1.incX();
        f2.incY();
        System.out.println(f1.getX() + " " + f1.getY());
        System.out.println(f2.getX() + " " + f2.getY());
    }
}

public class F {
    private int x;
    private static int y;

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public void incX() {
        x++;
    }

    public void incY() {
        y++;
    }
}

1 1
0 1

Domain Modelling

UML

Composition is a specialized form of aggregation. In composition, if the parent object is destroyed, then the child objects also cease to exist. Composition is actually a strong type of aggregation and is sometimes referred to as a "death" relationship.

UML

Domain Modelling

Create an OO domain model for a system with the following requirements.

A Car has one or more engines and a producer. The producer is a manufacturing company who has a brand name. Engines are produced by a manufacturer and have a speed. There are only two types of engines within UNSW's cars:

Thermal Engines, which have a default max speed of 114, although they can be produced with a different max speed, and the max speed can change to any value between 100 and 250.
Electrical Engines, which have a default max speed of 180. This is the speed at which they are produced, and the max speed can change to any value that is divisible by 6.

Cars are able to drive to a particular location x, y.

Since UNSW is a world-leader in technology innovation, they want you to be able to model the behaviour of Time Travelling for any vehicle, and to model a time travelling car. A vehicle that travels in time stays in the same location but travels to a LocalDateTime.

Domain Modelling

Create an OO domain model for a system with the following requirements.

A Car has one or more engines and a producer. The producer is a manufacturing company who has a brand name. Engines are produced by a manufacturer and have a speed. There are only two types of engines within UNSW's cars:

Thermal Engines: which have a default max speed of 114, although they can be produced with a different max speed, and the max speed can change to any value between 100 and 250.
Electrical Engines: which have a default max speed of 180. This is the speed at which they are produced, and the max speed can change to any value that is divisible by 6.

Cars are able to drive to a particular location x, y.

LucidCharts

A generally good diagram maker software that is free*

*if you sign up with an .edu account

Tips for UML Diagrams

Make it readable (for me please:) )
Use colours to distinguish parts
- E.g, All satellites are shaded blue...
- This design pattern is shaded green
Try making a draft one before you start coding your assignments

Code Demo

Wonderous.java

package wondrous;

import java.util.ArrayList;
import java.util.List;

public class Wondrous {
    private final int MY_MAGIC_NUMBER = 42;

    public List<Integer> wondrous(int start) {
        int current = start;
        List<Integer> sequence = new ArrayList<Integer>();

        while (current != 1) {
            sequence.add(current);
            if (current % 2 == 0) {
                current /= 2;
            } else {
                current = (current * 3) + 1;
            }
        }

        return sequence;
    }
}

The Wondrous Sequence is generated by the simple rule:

If the current term is even, the next term is half the current term.
If the current term is odd, the next term is three times the current term, plus 1

JUnit Testing

Make sure your VSCode is open in the correct folder, otherwise, these buttons wont appear

JUnit Testing

Make sure your VSCode is open in the correct folder, otherwise, these buttons won't appear

JUnit Testing

Failed test output
See past history

Debug Mode

Set a breakpoint, run the test in debug mode
Stops at a breakpoint it encounters
See variable values, call stack

Debug Mode

Now lets fix it

package wondrous;

import java.util.ArrayList;
import java.util.List;

// If the current term is even, the next term is half the current term.
// If the current term is odd, the next term is three times the current term, plus 1.
public class Wondrous {
    public List<Integer> wondrous(int start) {
        int current = start;
        List<Integer> sequence = new ArrayList<Integer>();

        while (true) {
            sequence.add(current);
            if (current == 1) {
                break;
            } else if (current % 2 == 0) {
                // Even
                current /= 2;
            } else {
                // odd
                current = (current * 3) + 1;
            }
        }
        return sequence;
    }
}

Exceptions

What are they?

An exception is an event, which occurs during the execution of a problem, that disrupt the normal flow of the program's instructions
When error occurs, an exception object is created and given to the runtime system. This is called throwing an exception
The runtime system searches the call stack for a method that contains a block of code that can handle the exception
The exception handler chosen is said to catch the exception

Exceptions

Checked vs Unchecked

Checked: Must be checked, will result in compilation error if not handled
- Any class that inherits from `Exception` is a checked exception.
- E.g., IOException, SQLException
Unchecked: Genuine errors that occur at run time
- Any class that inherits from `RuntimeException` is unchecked
- E.g., ArrayIndexOutOfBoundsExceptions, ArithmeticException

Exceptions - Wonderous

How can we use it in Wonderous to make the code better?
Checked or Unchecked?

Exceptions - Wonderous

How can we use it in Wonderous to make the code better?

package wondrous;

import java.util.ArrayList;
import java.util.List;

// If the current term is even, the next term is half the current term.
// If the current term is odd, the next term is three times the current term, plus 1.
public class Wondrous {
    public List<Integer> wondrous(int start) {
        int current = start;
        List<Integer> sequence = new ArrayList<Integer>();

        if (start < 1) {
            throw new IllegalArgumentException("wondrous start must be >= 1");
        }

        while (true) {
            sequence.add(current);
            if (current == 1) {
                break;
            } else if (current % 2 == 0) {
                // Even
                current /= 2;
            } else {
                // odd
                current = (current * 3) + 1;
            }
        }
        return sequence;
    }
}

JUnit Testing

Lets write some more tests

package wondrous.test;

import static org.junit.Assert.assertThrows;
import static org.junit.jupiter.api.Assertions.assertEquals;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.junit.jupiter.api.Test;

import wondrous.Wondrous;

public class WondrousTest {
    @Test
    public void testBasic() {
        Wondrous w = new Wondrous();
        List<Integer> expected = new ArrayList<Integer>(Arrays.asList(3, 10, 5, 16, 8, 4, 2, 1));

        assertEquals(expected, w.wondrous(3));
    }

    @Test
    public void testOne() {
        Wondrous w = new Wondrous();
        List<Integer> expected = new ArrayList<Integer>(Arrays.asList(1));
        assertEquals(expected, w.wondrous(1));
    }

    @Test
    public void testInvalid() {
        Wondrous w = new Wondrous();

        assertThrows(IllegalArgumentException.class, () -> {
            w.wondrous(0);
        });
    }
}

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