Java Refresher

Repo: java-refresher
Repository for first session of COM673, focusing on Java refresher for the upcoming Java on Azure content

Introduction

In this module the focus will be on designing, developing, debugging and deploying Java apps and bots on the Azure cloud.
Let's start with a Java refresher. We'll be using Java 10, so some of the latest language features may be new to you.

Basic language features

Many programming languages have variables, input and output, selection, and loops as basic features for writing programs.

Variables

Simple variables have a type, name and value:

int x = 2;
double y = 7.5;
boolean z = false;

Complex variables are objects (see below) that encapsulate data and methods to work with them:

String sentence = "The quick red fox";
int sentenceLength = words.length();

In this example, a String contains the data for the sentence, and .length() is a method that return the number of characters in the sentence.
Java 10 adds type inference, if you use var as the type, Java will attempt to infer the type for you. This is particularly useful for verbose types.

var sentence = "This is a String type";

Operators

Operators allow you to perform operations on variables, including mathematical operations and Boolean logic operations. Remember BODMAS for evaluating complex statements with brackets:

int a = 2;
int b = 3;
int c = a + b;
int d = c - a;
int e = d * d;
int f = ((a * 2) * ((b + 1) / a)) / 2; // BOMDAS

We'll also look at comparison operators when we get to selections below.

Input and Output

Although we'll only be using print statements at the start of the course, we can print with:

 System.out.println("print this and go onto a new line");
 System.out.print("print this and stay on the current line");

Input used to be quite knotty but luckily the Scanner class makes it much more straightforward:

Scanner input = new Scanner(System.in);
System.out.println("What is your name?");
String name = input.nextLine();
System.out.println("What age are you " + name + "?");
int age = input.nextInt();
System.out.println("Do you like marmite? (true or false)");
boolean likesMarmite = input.nextBoolean();
System.out.printf("%s is %d years old and likes marmite: %b\n", name, age, likesMarmite);       
input.close();   

Selection

if, if-else, and if-else if-else statements are common when deciding what to do in a program. Each execution path should be considered when writing a selection, just because a particular path is unlikely, does not mean it will never happen.

boolean a = true;
boolean b = false;
// == simple variable comparison
// && Boolean AND
if(a == true && b == true) {
    System.out.println("a is true, b is true");
}
else if(a == false && b == false) {
    System.out.println("a is false, b is false");
}
else if(a == true && b == false) {
    System.out.println("a is true, b is false");
}
else if(a == false && b == true) {
    System.out.println("a is false, b is true");
}
String first = "Cat";
String second = "Dog";
// .equals() compares the contents of the objects
if(first.equals(second) == true) {
    System.out.println(first + " is the same as " + second);
}
else {
    System.out.println(first + " is not the same as " + second);
}

Loops

A loop performs a certain action a number of times until a stop condition is met.
We have do-while loops which run through the loop at least once, while loops that may not run through the loop at all, for loops which execute a set number of times and for-each loops which are useful for iterating over lists.

int stopVal = 100;
int val = 200;
do
{
    System.out.println(val);
    val += val;
}
while(val < stopVal);
val = 2;
while(val < stopVal)
{
    System.out.println(val);
    val *= val; // What does this operator do?
}
for(int i = 0; i <= 30; i+=3)
{
    System.out.print(i + ", ");
}
System.out.println();
String[] animals = { "cat", "dog", "mouse" };
for(var animal : animals)
    System.out.println(animal);
}

Object oriented programming language features

Java is an OOP language and that we need to be comfortable working with classes which define the structure and capabilities of something we want to use and objects, which are an instance of a class.

Dog roscoe = new Dog();

In this example, the class is Dog and the object is roscoe, a particular instance of a dog.

Constructors & Methods

A method is a piece of code that can be called to perform some tasks. It may take parameters and may return values. A constructor is a special method that is called at object creation time.
Example, a Gaelic Football Team class might look like this:

private class GAAFootballTeam
{
    // Memeber variables
    private String name;
    private int goals;
    private int points;
    // Add Constructor
    GAAFootballTeam(String name) {
        this.name = name;
    }
    // Getter method for name
    public String getName() {
        return name;
    }
    // Setter method for goals
    public void scoreGoals(int goals) {
        this.goals = goals;
    }
    // Setter method for points
    public void scorePoints(int points) {
        this.points = points;
    }
    // Helper method to calculate total points and return value
    public int calculateTotalPoints() {
        return (goals * 3) + points;
    }
}

Some sample code to test the class:

// This test has 2 county teams, Louth and Meath
GAAFootballTeam louth = new GAAFootballTeam("Louth County Team");
GAAFootballTeam meath = new GAAFootballTeam("Meath County Team");
// The teams play each other, and Louth scores 4 goals and 7 points
louth.scoreGoals(4);
louth.scorePoints(7);
// meath scores 3 goals and 8 points
meath.scoreGoals(3);
meath.scorePoints(8);
// A goal is worth 3 points, which team won?
int meathTotal = meath.calculateTotalPoints();
int louthTotal = louth.calculateTotalPoints();
if(louthTotal == meathTotal) {
    System.out.println("The match was a draw!");
}
else if(louthTotal < meathTotal) {
    System.out.println(meath.getName() + " won!");
}
else {
    System.out.println(louth.getName() + " won!");
}