在Java中，如何防止图形小程序多次运行？

Question

I'm trying to write a simple program that draws fractals, and wrote the following code. I ran it to test whether it worked and found that it runs twice every time. I looked for answers and found that it likely has something to do with the graphics window redrawing and re-running the entire code as a result. In other questions I've seen, no one seemed concerned with making the code not run multiple times, but rather why it was doing so, and no remedies were provided as a result. However, Because of the fact that I'm using Scanners, it's extremely inconvenient to have the program run repeatedly and my primary concern is making it run only once.

import java.awt.*;
import java.applet.*;
import java.util.Scanner;
public class FractalCreator extends Applet
{
     public void paint(Graphics g)
     {
          Scanner s=new Scanner(System.in);
          System.out.println("How many branches do you want each node to have?");
          int branches=s.nextInt();
          System.out.println("How many times do you want the fractal to repeat?");
          int repetitions=s.nextInt();
     }
}

What I want to know is how I can stop the graphics window from redrawing and re-running everything, or, if that isn't possible, what alternatives I have to the Graphics class to draw things in.

I put the code into non-Applet code with a main method and it ran once, as it was intended to.

If it is relevant, I'm using DrJava as an IDE.

Answer 1

The answer to your question is, you can't.

So you screwing a bunch of things up. An applet won't (normally) have a console for use interaction - you especially shouldn't be doing this from within the paint method as this could adversely effect the user interaction with your UI.

GUI's are event driven environments, that is, something happens and you respond to it. You are still thinking in a linear/procedural/console based way

My first piece of advice is, don't use applets, at all, they aren't worth your time to learn right now. If you want a graphical output, I would start by having Creating a GUI With JFC/Swing for a better understanding of how to interact with users through an event driven environment.

I'd then suggest you have a look at Painting in AWT and Swing and Performing Custom Painting for a better understanding of how painting works in Swing.

Answer 2

Solution 1: Use a member variable to indicate whether the Applet has been painted already. Might be prone to errors.

Solution 2: In the init() method, draw your picture to a BufferedImage member variable, and in the paint() method, simply draw that picture.

As the answer before me stated, you are acting inside the wrong event method for handling the painting, because as you noticed, it will get drawn over and over again.