public int computeArea()
public int computePerimeter()
public double computeDiagonal()

public class Screen {
	private Rectangle display;
	private Rectangle resolution;

	//etc (more code follows)

}

	public double computePPI()  {
		//Pixels per inch (PPI) = pixels / inches
		double pixels, inches;
		pixels = this.resolution.computeDiagonal();
		inches = this.display.computeDiagonal();
		return pixels/inches;
	}