Is there any way to manipulate a constructor in Java?

Question

I'm currently working on my assignment and I'm stuck rn. The task is to create a program based on this diagram. In that diagram, as you can see, there is no behavior for creating constructors, but in the given Main Class there is. Is there a way to create a program without changing the given Main Class and not creating new methods in addition to the given diagram. The program is to calculate density. Help : )

This is my Box class code:

class Box {
    private double width;
    private double height;
    private double depth;
    private double mass;
    private double density;


    public void setWidth(double width){
        this.width = width;
    }

    public void setHeight(double height){
        this.height = height;
    }

    public void setDepth(double depth){
        this.depth = depth;
    }

    public void setMass(double mass){
        this.mass = mass;
    }

    public double getDensity(){
        density = mass / (width * height * depth);
        return density;
    }
}