Question

package demo;
import java.awt.*;
import java.awt.image.*;
import java.util.*;
import javax.swing.*;

public class ScreenCapturingThread extends Thread{
    public ScreenCapturingThread(Vector<BufferedImage> screenShots,
            int frameRate,
            Icon cursor,
            Rectangle recordingArea){
        this.screenShots = screenShots;
        this.frameRate = frameRate;
        this.cursor = cursor;
        this.recordingArea = recordingArea;

        try{
            bot = new Robot();
        }catch(Exception e){
            System.out.println(e);
        }
        calculateSleepTime();
    }
    @Override
    public void run(){
        while(keepCapturing == true){
            try{
                screenShots.add(takeScreenShot());
                sleep(sleepTime);

                keepCapturing = false; //take only one shot

                System.out.println("here");
                JFrame frame = new JFrame();
                frame.setSize(recordingArea.width,recordingArea.height);
                frame.getGraphics().drawImage(screenShots.firstElement(), 0, 0,frame);
                frame.repaint();
                frame.setVisible(true);

            }catch(InterruptedException e){
                e.printStackTrace();
            }
        }
    }
    public BufferedImage takeScreenShot(){
        p = m.getPointerInfo();
        Point location = p.getLocation();
        image = bot.createScreenCapture(recordingArea);
        if(cursor!=null){
            Graphics g = image.getGraphics();
            g.drawImage(((ImageIcon)cursor).getImage(), location.x,location.y,null);
        }

        return image;
    }
    public void stopIt(){
        keepCapturing = false;
    }
    public void calculateSleepTime(){
        sleepTime = 1/frameRate;
    }

    public static void main(String[] args) {
        Vector<BufferedImage> bufferedImages = new Vector<>(100);
        int frameRate = 10;
        Icon cursor = (Icon) new ImageIcon("src/images/blackCursor.png");
        Rectangle r = new Rectangle(1280,800);
        ScreenCapturingThread sc = new ScreenCapturingThread(bufferedImages,frameRate,cursor,r);

        sc.start();
    }
    Vector<BufferedImage> screenShots;
    int frameRate;
    long sleepTime;
    boolean keepCapturing = true;
    Icon cursor;
    Rectangle recordingArea;
    Robot bot;
    MouseInfo m;
    PointerInfo p;
    BufferedImage image;
}

Explanation

I have designed thread to go along with my screen recorder but I decided to test it first. This is what it is supposed to do:

create a new thread object by passing the appropriate parameters

take only one screen shot (during this test only), store it in the vector and before the run() ends, draw this on the JFrame so I can see what has been captured.

Problem

I keep getting a NullPointerException at

frame.getGraphics().drawImage(screenShots.firstElement(), 0, 0,frame);

I don’t know what is going wrong.
If you could please find out the bug?

Update:

Now although the NullPointerException is gone, the frame is blank while it is not supposed to be

Why?

Answer 1

JFrame will not provide you with any Graphics until it is shown.

If you want to draw before JFrame is shown you never should do like this:

frame.pack();
frame.setSize(recordingArea.width, recordingArea.height);
Graphics g = frame.getContentPane().getGraphics();
g.drawImage(screenShots.firstElement(), 0, 0, frame);

because as Andrew Thompson has correctly written:

Do not use Component.getGraphics(). Instead, subclass and override the paint() (AWT), or paintComponent() (Swing) method.

Component.getGraphics() simply can’t work. Java uses a callback mechanism for drawing graphics. You are not supposed to "push" graphics information into a component using getGraphics(). Instead you are supposed to wait until Java calls your paint()/paintComponent() method. At that moment you are supposed to provide the Component with the drawings you would like to do.

instead you’ll be better off doing something like below:

BufferedImage img = new BufferedImage(recordingArea.width, recordingArea.height,
                    BufferedImage.TYPE_3BYTE_BGR);
            Graphics g = img.createGraphics();
            g.drawImage(screenShots.firstElement(), 0, 0, frame);
            JLabel l = new JLabel(new ImageIcon(img));
            frame.getContentPane().add(l);