COMP2511

24T1 Week 2

Tuesday 6PM - 9PM (T18A)

 

Slides by Alvin Cherk (z5311001)

Today

  • Some VSCode, GitLab, Git tips
  • Classes
  • Commenting & Documentation
  • Basic Inheritance
  • Access Modifiers

 

Assignment I is out, please have a read of the spec and start early!

VSCode Config Tips

Ensure that you open the correct folder

example: cs2511-project

Good

Bad

Name of folder open

The folder I want to actually open

VSCode Config Tips

Ensure you have the correct Java extension installed

VSCode Config Tips

Ensure that you are running code using the "Run" button

Always use this to run code in this course

GitLab Tips

How to view pipeline output

GitLab Tips

How to view pipeline output

View linting output

View tests output

GitLab Tips

How to view pipeline output

Inheritance

Inheritance

What is it?

In Java, a class can inherit attributes and methods from another class. The class that inherits the properties is known as the sub-class or the child class. The class from which the properties are inherited is known as the superclass or the parent class.

Known as a "is-a" relationship

Code Review

Code Review

package shapes;

public class Shape {
    public String color;

    public Shape(String color) {
        System.out.println("Inside Shape constructor");
        this.color = color;
    }
}

public class Rectangle extends Shape {
    public int height;
    public int width;

    public Rectangle(String color) {
        super(color);
        System.out.println("Inside Rectangle constructor with one argument");
    }

    public Rectangle(String name, int width, int height) {
        this(name);
        this.width = width;
        this.height = height;
        System.out.println("Inside Rectangle constructor with three arguments");
    }

    public static void main(String[] args) {
        Rectangle r = new Rectangle("red", 10, 20);
        System.out.println(r.color);
        System.out.println(r.width);
        System.out.println(r.height);
    }
}
  • What is the difference between super and this?
    • super refers to the immediate parent class whereas this refers to the current class
  • What about super(...) and this(...)?
    • super() acts as a parent class constructor and should be the first line in a child class constructor
    • this() acts as a current class constructor (can be used for method overloading)

Code Review

package shapes;

public class Shape {
    public String color;

    public Shape(String color) {
        System.out.println("Inside Shape constructor");
        this.color = color;
    }
}

public class Rectangle extends Shape {
    public int height;
    public int width;

    public Rectangle(String color) {
        super(color); // => Calling constructor of parent `Shape(String color)`
        System.out.println("Inside Rectangle constructor with one argument");
    }

    public Rectangle(String name, int width, int height) {
        this(name); // => Calling constructor `Rectangle(String color)`
        this.width = width;
        this.height = height;
        System.out.println("Inside Rectangle constructor with three arguments");
    }

    public static void main(String[] args) {
        // Rectangle(3 arguments) => Rectangle(1 argument) => Shape(1 argument)
        Rectangle r = new Rectangle("red", 10, 20);
        System.out.println(r.color);
        System.out.println(r.width);
        System.out.println(r.height);
    }
}
  • What is the difference between super and this?
    • super refers to the immediate parent class whereas this refers to the current class
  • What about super(...) and this(...)?
    • super() acts as a parent class constructor and should be the first line in a child class constructor
    • this() acts as a current class constructor (can be used for method overloading)

Code Review

package circle;

public class Circle extends Object {
    // Every class extends Object, it is not needed though
    private static final double pi = 3.14159;
    private int x, y;
    private int r;
	
    // Only 1 variable for all Circle objects
    static int no_circles = 0; 

    public Circle() {
        super(); // not needed
        no_circles++;
    }
    
     public double circumference() {
        return 2 * pi * r;
    }
}

What are static fields and methods?

Static fields are variables that are common and available to all instances of a Class. They belong to the Class, rather than an instance.

Methods are a block of code that perform a task. You can think of them as functions of a class.

Documentation

JavaDoc

Documentation

  • Why is documentation important? When should you use it
  • What does the term "self-documenting" code mean?
    • Code that documents itself. It is readable inherently. Usually accomplished through variable name and function names
  • When can comments be bad (code smell)?
    • Comments become stale & does not get updated with new changes
    • Possibly hinting that your design/code is too complex

https://www.youtube.com/watch?v=Bf7vDBBOBUA

Documentation

Single Line

// Single line comment

Multi-line comment

/**
 * This is multi-line
 * documentation
 */

JavaDoc Documentation

/**
 * Constructor used to create a file
 * @param fileName the name of the file
 * @param content contents of the file
 */

JavaDoc

  • JavaDoc is one way of documenting in Java.
  • JavaDoc is a way of writing your comments
  • It mainly targets class definitions and method/function definitions.
  • In COMP2511, you will not have to use JavaDoc documentation unless asked. Though, it is a good idea to do it anyway in assignments.

JavaDoc

/**
 * File class that stores content under a file name
 */
public class File {
    /**
     * Constructor used to create a file
     * @param fileName the name of the file
     * @param content contents of the file
     */
    public File(String fileName, String content) {}

    /**
     * Constructor used to make a partial file when receiving a new file
     * I.e., content.length() != fileSize with no compression
     * @param fileName
     * @param fileSize
     */
    protected File(String fileName, int fileSize) {}
    
    /**
     * Checks if transfer has been completed
     * @return true if it has been completed
     */
    public boolean hasTransferBeenCompleted() {}
}

Code Demo

Employee.java & Manager.java

Code Demo

  1. Create a Employee class with a name and salary
  2. Create setters & getters with JavaDoc
  3. Create a Manager class that inherits Employee with a hireDate
  4. Override toString() method
  5. Write equals() method

Code Demo

The java extension packs come with some features you can use to generate boilerplate code

Please do not generate the equals method. Write it yourself. This applies for Lab02 marking.

Code Demo

How many constructors does a class need?

Technically none. If a class is defined without a constructor, Java adds a default constructor

However, if a class needs attributes to be assigned (e.g., has a salary), then a constructor must be assigned.

If your class has attributes with no default values, then the constructor must set these attributes. This is because variables with no values are dangerous (null), and is also the  constructor responsibility.

Each class's constructor is also only responsible for setting its own attributes. Do not set the superclass's attributes within the subclasses without using a super(...) constructor call

Code Demo

How many constructors does a class need?

import java.time.LocalDate;

public class BadConstructor {
    public String name;
    public double salary;
    public LocalDate hireDate;

    public BadConstructor() {
        this.hireDate = LocalDate.now();
        // salary and hireDate aren't assigned a value
        // Technically, they're defaulted to null
    }

    public static void main(String[] args) {
        BadConstructor e = new BadConstructor();
        System.out.println(e.name);
        System.out.println(e.salary);
        System.out.println(e.hireDate);
    }
}

Code Demo

How do you write a good equals method?

Since we are overriding an existing method (in the super most class called Object), we must follow the conditions described.

The conditions can be found in the Java Docs

The semantics of this was explored in a past exam

Access Modifiers

Access Modifiers

Private

It is accessible only to the same class (not including main). The most restrictive modifier.

Public

It is accessible to everything. The least restrictive modifier.

Protected

Can be accessed in the same package and in inheritance.

Default

The default access modifier is also called package-private, which means that all members are visible within the same package but aren't accessible from other packages