Implementing a Demo/Tester Program

Question

I have a project for my java programming course.

The instructions are that we have to create a simple class and a tester class, and the class must include a Default constructor; Parameterized constructor with three parameters (make, model and price); Accessor method called getMake( ) to return the make; Accessor method called getModel( ) to return the model; Accessor method called getPrice( ) to return the price; Mutator method setMake( String newMake) to set the make; Mutator method setModel( String newModel) to set the model; and a Mutator method setPrice( double newPrice ) to set the price..

I have created my class and tester program, and my class compiles perfectly. When I try to run it, though get the error that there is no main method. Now, I followed my professor's example for the tester program and I get several errors on that. If anyone could give me the a pointer in the right direction, I would appreciate it.

My question is this: How do I implement my tester program? Do I need to create a zip file? I've tried doing so and didn't seem to help much...

The following is my code for the class:

public class Automobile
{ 
    private String make;    
    private String model;
    private double price;   

    public Automobile()
    {
        make = "Lexus2017";
        model = "RX";
    }

    public Automobile(String initMake, String initModel, double initPrice)
    {
        make = initMake;
        model = initModel;
        price = initPrice; 
    }

    public String getMake()
    {
        return make;
    }

    public String getModel()
    {
        return model;
    }

    public double getPrice()
    {
        return price;
    }   

    public void setMake(String newMake)
    {
        make = newMake;
    }

    public void setModel(String newModel)
    {
        model = newModel;
    }

Also, the following is my tester class(the one that has a lot of errors):

public class AutomobileTester
{
    public static void main(String[] args)
    {
        Automobile make = new Automobile("Lexus 2017");
        System.out.println("The car is " + make.getMake());

        Automobile model = new Automobile("RX");
        System.out.println("The car is " + Automobile.getModel());

        Automobile price = new Automobile("43020"); 
        System.out.println("The car is " + Automobile.getPrice());  

        // Use the mutator to change the make variable
        Automobile.setMake("Lexus 2017");
        System.out.println("The car is " + backDoor.getState());

        // Use the mutator to change the model variable
        Automobile.setModel("RX");
        System.out.println("The car is called " + backDoor.getName());

        Automobile.setPrice("43020");
        System.out.println("The car is " + price.getPrice());
    }
}

This is my first time working with constructors, and I'm very new to Java, so I'm sorry for any obvious errors. Thank you ahead of time for your time and help.

Answer 1

One of the first problems is that you do not use the proper number of parameters for your calls to the constructor, in Java (and most programming languages) you have to supply all of the required parameters to a method/function/constructor in one call. The fix for your code would be to use:

Automobile car = new Automobile("Lexus 2017", "RX", 43020.0D);

Also when you print out the cars information you first use an instance call then you use a static call, I won't go to much into the difference between the two but basically an instance call requires you to instantiate an object while a static does not. The fix for this problem would be to do:

System.out.println("The car is a " + car.getMake() + ", the brand is " + car.getModel() + ", the price is $" + car.getPrice());

As for changing the variables you should be using:

car.setMake("My New Car Make");

instead of:

Automobile.setMake("My New Car Make");

For the difference between static and instance you can look here, here, and here.

Answer 2

You did this correctly. You accessed the method by using the make instance variable of an Automobile class.

(side note: make is a bad name for an automobile instance, rather call it car1, or something)

Automobile make = new Automobile("Lexus 2017");
System.out.println("The car is " + make.getMake());

Now, everywhere else that you use Automobile.someMethod(), that's not right, because you need to set or get the data on one instance of the class, not the entire class.

Then, finally, you need to test the constructor with three parameters that you have in that class.

Answer 3

You have an error in the constructor call. Your constructor takes three parameters (make, model and price) but when you call the method only send one. That is an error. By default, the Java class constructor takes no parameters (in your case, this would be "new Automobile ()"). To implement the tester you have two options. First, create the car using the constructor without parameters and then set the parameters:

Automobile auto = new Automobile();
auto.setMake("Lexus 2017");
auto.setModel("RX");
auto.setPrice(43020);

Automobile Automobile make = new Automobile ();

Another option is to use your own builder and pass parameters:

Automobile auto2 = new Automobile("Lexus 2017", "RX", 43020);

Automobile.java:

public class Automobile {
private String make;
private String model;
private double price;

public Automobile() {
}

public Automobile(String make, String model, double price) {
    this.make = make;
    this.model = model;
    this.price = price;
}

public String getMake() {
    return make;
}

public void setMake(String make) {
    this.make = make;
}

public String getModel() {
    return model;
}

public void setModel(String model) {
    this.model = model;
}

public double getPrice() {
    return price;
}

public void setPrice(double price) {
    this.price = price;
}
}

AutomobileTester.java:

public class AutomobileTester {

public static void main(String[] args) {

    Automobile auto = new Automobile();
    auto.setMake("Lexus 2017");
    auto.setModel("RX");
    auto.setPrice(43020);
    System.out.println("The car1 is " + auto.getMake() + " " + auto.getModel() + " " + auto.getPrice());

    Automobile auto2 = new Automobile("Lexus 2017", "RX", 43020);
    System.out.println("The car2 is " + auto2.getMake() + " " + auto2.getModel() + " " + auto2.getPrice());

}

}