Thread: Help with Abstract Extended Classes

Hey Everyone,
I am taking my first java class right now(although I am not new to coding, I've taken many c and C++ classes). I am working on an assignment and I have run into some trouble. The assignment is to create an abstract class that has a bunch of extended classes(main class shape with extended classes such as circle, rectangle, etc...). I have created the classes I believe and I was creating a demo program to test/debug it and i have run into a error i can't seem to figure out.
Below is my main Shape class, my extended Circle class, and my ShapeDemo program.

Main Shape class
~~~~~~~~~~~~~~

package CS249Mazur;
 
public abstract class Shape
{
	public double Area;
	public double Perimeter;
 
	public Shape()
	{
		Area = this.getArea();
		Perimeter = this.getPerimeter();
	}
 
	public Shape(Shape original)
	{
		Area = original.Area;
		Perimeter = original.Perimeter;
	}
 
	public String toString()
	{
		return ("Area = "+this.Area+"\nPerimeter = "+this.Perimeter);
	}
 
	public boolean equals(Shape otherShape)
	{
		return (this.Area == otherShape.Area);
	}
 
	public int compareTo(Shape otherShape)
	{
		if (this.Area < otherShape.Area)
			return -1;
		else if(this.Area > otherShape.Area)
			return 1;
		else
			return 0;
	}
 
	public abstract double getArea();
	public abstract double getPerimeter();
}

Extended Circle class
~~~~~~~~~~~~~~~~~~

package CS249Mazur;
 
public abstract class Circle extends Shape
{
	public double radius;
 
	public Circle(double theRadius)
	{
		super();
		if(theRadius > 0)
			radius = theRadius;
		else
		{
			System.out.println("Fatal Error: Illegal Radius amount entered.");
			System.exit(0);
		}
	}
 
	public Circle(Circle original)
	{
		super(original);
		radius = original.radius;
	}
 
	public double getArea()
	{
		return 3.141592*radius*radius;
	}
 
	public double getPerimeter()
	{
		return 2-3.141592*radius;
	}
 
	public double getRadius()
	{
		return this.radius;
	}
 
	public void setRadius(double r)
	{
		this.radius = r;
	}
 
	public String toString()
	{
		return (super.toString()+"\nRadius = "+this.radius);
	}
 
}

Shape Demo program
~~~~~~~~~~~~~~~~~

 package CS249Mazur;
 
public abstract class ShapeDemo
{
	Shape array[];
	double rad=0;
 
	//ask user for shape
	//figure out what shape they entered is
	//Based on what they entered ask for the parts needed
 
	rad=5;
	Circle o1;
	array[0]=o1;
	o1.setRadius(rad);
 
	//move to next spot in the array
	//and get the next shape to enter into the array
}

The demo program is not what I will be using for the actual program just something simple to test if my classes worked. I am getting an error on the line where i create the double rad. some kind of syntax error. saying: Syntax error on token ";", { expected after this token.

Any and all help would be appreciated!
Thanks!

Please review the Announcement topic at the topic of each sub-forum to learn how to post your code in code or highlighting tags. What you've posted is hard to read.

However, it was readable in the email version, where I saw that the ShapeDemo class has no methods. It has statements, but no methods, like a main() method from which to begin the program's execution.

Originally Posted by GregBrannon

Please review the Announcement topic at the topic of each sub-forum to learn how to post your code in code or highlighting tags. What you've posted is hard to read.

However, it was readable in the email version, where I saw that the ShapeDemo class has no methods. It has statements, but no methods, like a main() method from which to begin the program's execution.

Doesn't the ShapeDemo Program gets its Methods from the Classes? Do i need some type of Include statement?

If it helps, i am using Eclipse IDE to do my coding.

The ShapeDemo class has statements - lines that are expected to execute, like:

array[0]=o1;

that must be inside a method.

I would expect there to be a main() method somewhere, and I don't see one. Post your exact error messages, copied and pasted into a post, rather than your interpretation of them.

Oh wow, can't believe i forgot a main method. lol.
So i Added that in and this is the new ShapeDemo class:

package CS249Mazur;

public abstract class ShapeDemo
{
	Shape array[];
	double rad=0;
 
	//ask user for shape
	//figure out what shape they entered is
	//Based on what they entered ask for the parts needed
	 public void Main(String [ ] args)
	 {
		 rad=5;
		 Circle o1;
		 o1 = new Circle(rad);
		 array[0]=o1;
	 }
	//move to next spot in array
	//and get the next shape to enter into the array
}

The error i am getting now is "Cannot instantiate the type Circle", on the line "o1 = new Circle(rad);"
I use eclipse IDE to run the program so that is how i get the error messages.

Abstract classes cannot be instantiated. Does Circle need to be abstract? ShapeDemo shouldn't be abstract.

Originally Posted by GregBrannon

Abstract classes cannot be instantiated. Does Circle need to be abstract? ShapeDemo shouldn't be abstract.

I am new to Java so I am still learning. I am not sure what has to be labeled abstract or not. My assignment was to make an abstract class Shape. Does that me the Main shape classes that uses the getArea() method that is part of it's extended class is the only class that has to be labeled abstract?

You should find reading this helpful. Most of the classes you'll write will be concrete (non-Abstract). When you're writing an Abstract class or an interface, you'll have specific reasons for doing so.

This is where I am at with the program. Thanks to all your help i got it farther(my silly mistakes).

package CS249Mazur;
 
public class ShapeDemo
{
 
 
 
	//ask user for shape
	//figure out what shape they entered is
	//Based on what they entered ask for the parts needed
	 public static void main(String [ ] args)
	 {
		 double rad=0;
		 Shape array[] = null;
		 rad=5;
		 Circle o1;
		 o1 = new Circle(rad);
		 array[1]=o1;
 
		 System.out.println(o1.toString());
	 }
	//move to next spot in array
	//and get the next shape to enter into the array
}

Here is the error I am getting:
Exception in thread "main" java.lang.NullPointerException
at CS249Mazur.ShapeDemo.main(ShapeDemo.java:19)

I am trying to make an array that can hold a bunch of Shapes that will be created. But I am having an issue with the Array part.

Any Suggestions?

I would do it something like:

Shape[] shapes = new Shape[NUMBER_OF_SHAPES];

where NUMBER_OF_SHAPES is a previously defined constant. This:

Shape shapes[], and this:

Shape[] shapes

are both allowed, but the second is preferred. I also believe it's easier to read as "an array of Shape objects, "shapes".

That page I pointed you two is just one of thousands that cover topics like this one. You should practice finding and using those pages and gain confidence in using them to answer these kind of questions.

Your error is coming from the line before that.

Shape array[];

The two brackets should come after the type, not the identifier and I do believe you have to instantiate arrays if I remember correctly.

The two brackets should come after the type, not the identifier . . .

I discussed that and noted that it's a preferred construction but either is acceptable.

. . and I do believe you have to instantiate arrays if I remember correctly.

Ya . . . I think I covered that too, if I understand your meaning.