New to Java: Adding Functionality to my Buttons

Code Java:

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import java.awt.FlowLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import java.awt.*;
 
 
 
 
class test extends JFrame{
//Create Button Variables
	private Button add;
 
	public test(){
		super("Calculator");
		setLayout(new FlowLayout());
 
		//Addition Button
		try{
				add = new Button("+");
				add(add, BorderLayout.SOUTH);
				String addfn = JOptionPane.showInputDialog("Enter First Number: ");
				String addsn = JOptionPane.showInputDialog("Enter Second Number: ");
				double addnum1 = Double.parseDouble(addfn);
				double addnum2 = Double.parseDouble(addsn);
				double addsum;
				addsum = addnum1 + addnum2;
				JOptionPane.showMessageDialog(null, "The Sum of " + addnum1 + " and " + addnum2 + " is " + addsum, "Addition Operation", JOptionPane.PLAIN_MESSAGE );
		}catch(Exception e){
		}
		//Handler Class
		HandlerClass handler = new HandlerClass();
		add.addActionListener(handler);
		}
		private class HandlerClass implements ActionListener{
 
		//actionPerformed is the only method to override
		public void actionPerformed(ActionEvent event){
			JOptionPane.showMessageDialog(null, String.format("%s", event.getActionCommand()));
 
		}
	}
	public static void main(String[] args){
		//Created Object for Window
		test wo = new test();
 
		//Allow Program to Close on Exit
		wo.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
		//Program Window Settings
		wo.setSize(400,300);
		wo.setVisible(true);
 
	}
}

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[This is only a portion of my program for the sake of space]

In the process of teaching myself Java, I'm trying to make a Simple Calculator that opens up a window with several buttons.. Each button sends the user to its corresponding operation.. "+" button sends you to an operation that allows you to add two numbers.. etc..

However, (Sorry if my terminology is off, I will try to explain my issue as best as possible) when I run my program, it goes straight to the content I want my "Addition Button" to contain instead of going to the window with the clickable "+" button.

Basically, the program runs backwards.. Once I type in the two numbers I want to add, it goes to the window with the "+" button.. So I'm pretty much asking these questions.. How can I start at the window containing the buttons? How can I add functionality to each button so the user can choose which operation he/she would like to use? Also, how can I return to my main window containing the buttons once the user has finished their first operation..

I've looked through several Youtube videos and I have also researched this. Either I cannot find anything that is relevant to my issue, or the information I am provided with is too complex for a beginner. Any help will be greatly appreciated.