COMP2511

25T2 Week 3

Friday 1pm-4pm (F13A)

Start 1:05pm

By: Sam Zheng (z5418112)

Original Slides by: Alvin Cherk

Today

Design by Contract
Domain Modelling & UML
JUnit Testing
Exceptions

Important Notices

Assignment 1 is out, please start early!
Go to help sessions if you need help (they get busier towards the due date). Find locations/times on the course website
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 2 pair preferences are out. Check Teams for the link and more information!

Design By Contract

At the design time, responsibilities are clearly assigned to different software elements, clearly documented and enforced during the development and using unit testing and/or language support.

Clear demarcation of responsibilities helps prevent redundant checks, resulting in simpler code and easier maintenance

Design By Contract

Every software element should define a specification (or a contract) that govern its transaction with the rest of the software components.

A contract should address the following 3 conditions:

Pre-condition - what does the contract expect?
Post-condition - what does that contract guarantee?
Invariant - What does the contract maintain?

Design By Contract

public class Bird {
	private int height;
    
    public void fly(int height) {
        if (height <= 5) {
            throw new IllegalArgumentException("Height must be greater than 5 meters to fly.");
        }
        this.height = height;
    }
}

public class Penguin extends Bird {
    @Override
    public void fly(int height) {
        if (height != 0) {
            throw new IllegalArgumentException("Penguins cannot fly! Height must be 0 meters.");
        }
		// does nothing
    }
}

Pre-condition has strengthened - inputs for birds don't apply for penguins
Post-condition has weakened - additional states/outputs expected

Precondition Weaking

An implementation or redefinition (method overriding) of an inherited method must comply with the inherited contract for the method
Preconditions may be weakened (relaxed) in a subclass, but it must comply with the inherited contract
An implementation or redefinition may lesson the obligation of the client, but not increase it

from 0 <= theta <= 90 to 0 <= theta <= 180 is weakening
[0, 90] => [0, 180]

Why?

Liskov Substitution Principle (more next week). I should be able to use the subclass's implementation in place of my super class.

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.

On the other hand, aggregation still involves one object being a part of another, but if the parent object is destroyed, the child object can still exist.

Examples:

Aggregation: A classroom has students. If the classroom is removed, the students don't get removed.
Composition: An online class has breakout rooms. If the class is removed, then the breakout rooms are also removed.

UML

- private attribute

+ static method

# protected attribute

- FINAL_VARIABLE

+ public method

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.

Text

- time LocalDateTime

LucidCharts

A generally good diagram maker software that is free for UNSW students:

https://cgi.cse.unsw.edu.au/~cs2511/25T2/setup/lucidchart-setup

Tips for UML Diagrams

Make it readable (for me please :) )
Use colours if you wish (but be consistent!)
- E.g. use colours for all trains
- Use purple for a specific design etc.
Try making a rough one before you start coding your assignments
Don't worry if it's not perfect to start with, refine over time as you work through large projects!

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 let's 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;
    }
}

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);
        });
    }
}

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;
    }
}