在Java小程序中绘制矩形

Question

I am attempting to create a simple applet that can draw a rectangle, I have the following code:

import java.util.Scanner;
import java.awt.Graphics;
import javax.swing.JApplet;
public class DrawShapes extends JApplet{

    public void paint(Graphics canvas) {
        Scanner reader  = new Scanner(System.in); 
        System.out.println("How many sides do you want your shape to have: ");
        int sides = reader.nextInt();
        reader.close();
        super.paint(canvas);



        if(sides ==4){
        canvas.drawRect(100, 50, 200, 200);
        }

        System.out.println("Done!");

    }

}

I the run this code and when prompted to enter how many sides i want i enter "4"

A new applet window is then created in the upper left corner without any errors. But, the second I attempt to expand the applet the following errors are printed to the console:

Exception in thread "AWT-EventQueue-1" java.util.NoSuchElementException
    at java.util.Scanner.throwFor(Scanner.java:862)
    at java.util.Scanner.next(Scanner.java:1485)
    at java.util.Scanner.nextInt(Scanner.java:2117)
    at java.util.Scanner.nextInt(Scanner.java:2076)
    at DrawShapes.paint(DrawShapes.java:9)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:842)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:814)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
    at javax.swing.RepaintManager.access$1200(RepaintManager.java:64)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1732)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Also, "Applet Started" is repeated in the applet window as such: "Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started Applet Started" What is going on with my code?

Answer 1

This is because whenever you resize the window, the paint() method will be called again.

The first time paint() method is called, the thread will wait for the user input from the console.

After the first time, the thread will not wait for the console input any more because you have closed the input stream:

reader.close();

So either you create only one instance of new Scanner(System.in) and do not close it:

public class DrawShapes extends JApplet{
    Scanner reader  = new Scanner(System.in); //  a single instance for the Applet
    public void paint(Graphics canvas) {
        super.paint(canvas); // this should be the first statement of the method
        System.out.println("How many sides do you want your shape to have: ");
        int sides = reader.nextInt();

        if(sides ==4){
            canvas.drawRect(100, 50, 200, 200);
        }

        System.out.println("Done!");
    }
}

Or you replace the console input with a JTextField since you are using GUI.

Answer 2

Painting methods are for painting only! Don't do I/O in a painting method.

Applets are not designed to do file I/O. Applets execute inside your web browser. There is no command line associated with the browwer.

If you need user input then you should be using a JOptionPane.showInputDialog(...) to prompt the user for input in the init(...) method of the applet. You can also use a JOptionPane.showMessageDialog(...) to display a message.

Read the section from the Swing tutorial on How to Make Dialogs for more information.

Also, custom painting is done by overriding the paintCompnent() method of a JPanel and then you add the panel to the applet. The tutorial also has a section on Custom Painting . I suggest you read the tutorial as it covers the basics better than your class appears to be doing.