Java error - “invalid method declaration; return type required”

Question

We are learning how to use multiple classes in Java now, and there is a project asking about creating a class Circle which will contain a rad

Answer 1

As you can see, the code public Circle(double r).... how is that different from what I did in mine with public CircleR(double r)? For whatever reason, no error is given in the code from the book, however mine says there is an error there.

When defining constructors of a class, they should have the same name as its class. Thus the following code

public class Circle
{ 
    //This part is called the constructor and lets us specify the radius of a  
    //particular circle. 
  public Circle(double r) 
  { 
   radius = r; 
  }
 ....
} 

is correct while your code

public class Circle
{
    private double radius;
    public CircleR(double r)
    {
        radius = r;
    }
    public diameter()
    {
       double d = radius * 2;
       return d;
    }
}

is wrong because your constructor has different name from its class. You could either follow the same code from the book and change your constructor from

public CircleR(double r) 

to

public Circle(double r)

or (if you really wanted to name your constructor as CircleR) rename your class to CircleR.

So your new class should be

public class CircleR
{
    private double radius;
    public CircleR(double r)
    {
        radius = r;
    }
    public double diameter()
    {
       double d = radius * 2;
       return d;
    }
}

I also added the return type double in your method as pointed out by Froyo and John B.

Refer to this article about constructors.

HTH.