[hardwired]

Code:

C:\jexp>javac gn.java
gn.java:40: non-static variable this cannot be referenced from a static context
        okButton.addActionListener(this);
                                   ^
gn.java:58: cannot find symbol
symbol  : variable okButton
location: class GN
        if(e.getSource() == okButton){
                            ^
2 errors

1 — You cannot use the "this" keyword in static context, ie, inside a method whose signature includes the static modifier or with a field/variable that is declared static. You can use a reference variable:

Code:

public class GN implements ActionListener {
    static JButton okButton;  // member variable has class scope

    public static void main(String[] args) {
        GN gn = new GN();    // reference to the enclosing class

//        JButton okButton;  // local variable
        ...
        okButton.addActionListener(gn);

2 — the JButton "okButton" is declared inside a method (as a local variable) and thus cannot be seen inside the actionPerformed method. To be seen inside actionPerformed "okButton" must be in class scope, ie, declared as a member variable.

When making guis it is more elegant to put them together outside the main method.

Code:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;
import javax.swing.*;
import java.awt.*;

public class GN implements ActionListener{
    JButton okButton;

    private JPanel getContent() {        
        String askForNum = "Please enter what you think the random number is below.";
        
        // generates randomNumber between 1 and 100
        int randomNumber;
        int max = 100;
        Random random = new Random();
        randomNumber = random.nextInt(max +1);
        
        // output of random number for testing purposes remove prior to submission
        System.out.println(randomNumber);
        
        // creates JLabel that asks user to guess what the random
        // number is and enter it below
        JLabel msgLabel = new JLabel(askForNum, JLabel.CENTER);
        msgLabel.setSize(400, 150);
        msgLabel.setFont(new Font("Times New Roman", Font.BOLD, 14));

        JTextField answerTextField = new JTextField();
        // The designers recommend that we specify a columns argument
        // to help establish the size of a JTextField, eg, new JTextField(16)
        // The setSize method is of little value until after realization.
        // Swing components are made to work well with preferredSize.
        // So another option is:
        Dimension d = answerTextField.getPreferredSize();
        d.width = 75;
        answerTextField.setPreferredSize(d);
        // Or answerTextField.setPreferredSize(new Dimension(75, 30));
        // Choice of parent layout manager has a lot to do with how
        // this will be shown/handled.
        answerTextField.setFont(new Font("Times New Roman", Font.PLAIN, 14));
//        answerTextField.setSize(75, 30);
//        answerTextField.setVisible(true);    
        
        okButton = new JButton("Ok");
        okButton.setSize(30, 30);
        okButton.setFont(new Font("Times New Roman", Font.PLAIN, 14));
        okButton.addActionListener(this);  // "this" is okay now
        
        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
        panel.add(msgLabel);
        panel.add(answerTextField);
        panel.add(okButton);
        return panel;
    }
    
    public void actionPerformed(ActionEvent e){
        if(e.getSource() == okButton){
            
        }
    }

    public static void main(String[] args) {
        GN gn = new GN();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(gn.getContent());
        f.setSize(400, 200);
        f.setVisible(true);
    }
}