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Java Starter

So you don't know anything about Java, but you've heard so much about it you've decided it might not be that bad to learn. Or you're just a freelance programmer itching at the chance to flex your coding muscles and tackle the hottest new language in the market. Or you can't care less about Java and the only reason why you reading this is that you're forced the use it at school. Whatever reason you may have, if you ever wanted to learn Java, here's your chance.

In this tutorial and the many that will follow afterwards, I will guide you through the basics of Java and give you a slew of examples to aid in your understanding and also to help familiarize yourself with the Java syntax. I also recommend grabbing a Java IDE such as Java Workshop from Sun or Visual Age from IBM so that you can try out some of the code yourself. If you prefer the command line Unix programming or just using a text editor, make sure you grab JDK v1.2 to compile your code. After all, the fastest way to learn a language is to do some programming yourself.

classes

objects

methods

variables

class

Methods

Variables

public class Car {
  public double kilometres;
  public int speed;

  public Car() {
    kilometres = 0.0;
    speed = 150;
  }

  public void tuneUp() {
    speed += 25;
  }

  public int maxSpeed() {
    return speed;
  }

  public void drive( double distance ) {
    kilometres += distance;
  }

  public void showSpecs() {
    System.out.println("Max Speed = " + speed + "km/h");
    System.out.println("Distance Travelled = " + kilometres + "km");
  }
}

In this case, the class Car defines the Car object. Inside the class you have variables such as kilometres and speed (don't worry about the other stuff in front for now) and you also have methods like tuneUp() and maxSpeed() that gives you information about Car.

Now to create a Car object, you type: Car lexus = new Car();

The general syntax is: <object name> name = new <object name>(); where "name" can be anything you want. In the above example, lexus is the same as name. When the above line of code is executed, you have just created a new Car object with the name lexus. What it actually does is declare lexus as a Car object (left side of equal sign) and run the Car() method to create a new Car object (right side of equal sign). This method is known as the Constructor since it creates Car objects. Now lexus has all of Car's methods (ie. tuneUp(), drive()) and variables (ie. kilometres and speed). To access these methods and variables with lexus, use the following dot notation:

lexus.speed; (for variables)
lexus.tuneUp(); (for methods)

Type in these lines of code will access the speed variables and run the tuneUp() method. For a better understanding on how these things work here's another class that will perform the various functions of the Car object. This type of class is referred to as the main class or driver class since all it does is run the program. I will be using "//" to precede comments about the code. The "//" tells Java that they are comments and not code.

public class Main {

  public static void main( String[] args ) {

    //Create a Car Object with the name lexus.
    //It has the attributes speed=150 and
    //kilometres=0.0.
    Car lexus = new Car();

    //Increase speed by 25.
    lexus.tuneUp();

    //Increase kilometres by 74.7.
    lexus.drive(74.7);

    //Sets the local variable max to the value
    //returned by maxSpeed(). In this
    //case max=175.
    int max = lexus.maxSpeed();

    //Print to screen.
    lexus.showSpecs();
  }
}

After compiling and running this code you should see the output:

Max Speed = 175 km/h
Distance Travelled = 74.7 km

As you can see only the lexus.showSpecs() line produced any output. That's because the rest of the program happens in the background. The user only sees the lines sent to output and these are defined by the "System.out.print" command. Now you're ready to write a few of your own methods. Try writing one that returns kilometres or one that finds the total time it takes to travel a certain distance at a certain speed. (*Hint* time = distance/speed). Then run those methods by adding the appropriate lines in the Main class.

So you're still wondering what the heck that int and double stuff in front of the variables meant, aren't ya? Well, read on to the next section to find out...

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This article first appeared May 31, 1999.