Jframe not Disposing

Question

So i'm working on a text based game in Ready for Java. I was able to find a way to put an image on screen, but when i try to use frame.dispose(); to remove image it gives me a NullPointerException .

I have tried looking for an answer but I have not found anything. Can anyone lend a hand?

// The "Pics" class.
import java.awt.*;
import hsa.Console;
import javax.swing.*;

public class Pics extends JPanel
{
static JFrame frame;
static String Continue;


    public static void Screens (String jail_time)
    {
        JFrame frame = new JFrame ("JFrame"); //the pictures

        frame.setDefaultCloseOperation (JFrame.DISPOSE_ON_CLOSE);
        frame.setSize (773, 480);
        JLabel jl = new JLabel (new ImageIcon (jail_time));
        frame.getContentPane ().add (jl);
        frame.setVisible (true);
        frame.pack ();
    }

    static Console c;           // The output console

    public static void main (String[] args)
    {
        c = new Console (30, 60);
        Screens ("guard2.jpg");// the first picture
        Continue = c.readString ();
        frame.dispose (); 
        Screens ("Jail copy.jpg");//the second picture

        // Place your program here.  'c' is the output console
    } // main method
} // Pics class

Answer 1

Don't shadow the "frame" variable.

You have the variable defined as an instance variable, which is null and then you define it again as a local variable. Get rid of the local variable:

//JFrame frame = new JFrame ("JFrame"); //the pictures
frame = new JFrame ("JFrame"); //the pictures

Also, the whole structure of you class is wrong. You should NOT be using static variables and methods. This is the sign of a poorly designed class.

Answer 2

You have a shadowing problem...

First, you declare a static reference to frame ...

static JFrame frame;

Then in your static Screens method, you create a local instance of frame and build your UI...

public static void Screens (String jail_time)
{
    // Local reference here...
    JFrame frame = new JFrame ("JFrame"); //the pictures

Which means when you try and reference it in main , it's still null ...

public static void main (String[] args)
{
    Screens ("guard2.jpg");// the first picture
    // I'm still null...
    frame.dispose ();

Try removing the local declaration in Screens

public static void Screens (String jail_time)
{
    //JFrame frame = new JFrame ("JFrame"); //the pictures
    frame = new JFrame ("JFrame"); //the pictures

Also, take a look at Initial Threads and make sure you're creating your UI's within the context of the Event Dispatching Thread.

Answer 3

You're shadowing your class member JFrame frame , which leaves it uninitialized, causing a NullPointerException anytime you try and call the class member frame. Instead of

JFrame frame = new JFrame ("JFrame"); 

Do

frame = new JFrame ("JFrame"); 

Answer 4

In the main method's first line, change

JFrame frame = new JFrame ("JFrame"); //the pictures

to

frame = new JFrame ("JFrame");