another calculator problem

[flex567]

I don't know why this code doesn't work as it should?
I am a newbie in programming so any extra explanation would be very helpful.

public class Kalkulator {

	public static void main(String[] args) {
		SwingUtilities.invokeLater(new Runnable(){
			public void run()
			{
				JPanel plosca = new Plosca();
				GraficnoOgrodje.ustvariOkno("Kalkulator",plosca,false);
			}
		});

	}
	private static class Plosca extends JPanel implements ActionListener
	{
		private JTextField vpSt1, vpSt2, vpRez;
		private JButton gmPlus,gmMinus,gmDeljeno,gmKrat;
	
		public Plosca()
		{
			setLayout(new FlowLayout());
			
			vpSt1 = new JTextField(7);
			vpSt2 = new JTextField(7);
			vpRez = new JTextField(7);
			JButton gmPlus = new JButton(" + ");
			JButton gmMinus = new JButton(" - ");
			JButton gmKrat = new JButton(" * ");
			JButton gmDeljeno = new JButton(" / ");
			
			add(new JLabel("Stevilka 1"));
			add(vpSt1);
			add(new JLabel("Stevilka 2"));
			add(vpSt2);
			add(new JLabel("Rezultat"));
			add(vpRez);
			add(gmPlus);
			add(gmMinus);
			add(gmKrat);
			add(gmDeljeno);
			
			vpSt1.setHorizontalAlignment(JTextField.RIGHT);
			vpSt2.setHorizontalAlignment(JTextField.RIGHT);
			vpRez.setHorizontalAlignment(JTextField.RIGHT);
			vpRez.setEditable(false);
			
			gmPlus.addActionListener(this);
			gmMinus.addActionListener(this);
			gmKrat.addActionListener(this);
			gmDeljeno.addActionListener(this);
		}
		
			public void actionPerformed(ActionEvent e)
			{
				
				double stev1 = Double.parseDouble(vpSt1.getText());
				double stev2 = Double.parseDouble(vpSt2.getText());
				double rez = 7; 
				
				Object izvor = e.getSource();
				if( izvor == gmPlus )
				{
					rez = stev1 + stev2;
					System.out.println("Hi");
				}
				else if(izvor == gmMinus)
				{
					rez = stev1 - stev2;
				}
				else if(izvor == gmKrat)
				{
					rez = stev1 * stev2;
				}
				else if(izvor == gmDeljeno)
				{
					rez = stev1 / stev2;
				}
				vpRez.setText(Double.toString(rez)); 	
				
			}
	}
	
}

[keang]

I don't know why this code doesn't work as it should?

Given that you haven't said what it is doing and what it should do, I don't know either

It also doesn't help that your variable names are in a language I don't understand, so please can you provide some English comments in the code.

[flex567]

This is a calculator, I didnt make any notes becase i thought it is an easy program.

import javax.swing.*;

import java.awt.*;
import java.awt.Event.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;


public class Kalkulator {          //Calculator

	public static void main(String[] args) {
		SwingUtilities.invokeLater(new Runnable(){
			public void run()
			{
				JPanel plosca = new Plosca();
				GraficnoOgrodje.ustvariOkno("Kalkulator",plosca,false); //Creates a window
			}
		});

	}
	private static class Plosca extends JPanel implements ActionListener
	{
		private JTextField vpSt1, vpSt2, vpRez;  // Textfields for inputting numbers Rez = result
		private JButton gmPlus,gmMinus,gmDeljeno,gmKrat; // Buttons- minus, plus ...
	
		public Plosca()
		{
			setLayout(new FlowLayout());
			
			vpSt1 = new JTextField(7);
			vpSt2 = new JTextField(7);
			vpRez = new JTextField(7);
			JButton gmPlus = new JButton(" + ");
			JButton gmMinus = new JButton(" - ");
			JButton gmKrat = new JButton(" * ");
			JButton gmDeljeno = new JButton(" / ");
			
			add(new JLabel("Stevilka 1"));  // Stevilka = number
			add(vpSt1);
			add(new JLabel("Stevilka 2"));
			add(vpSt2);
			add(new JLabel("Rezultat")); // just a label for a result
			add(vpRez);
			add(gmPlus);
			add(gmMinus);
			add(gmKrat);
			add(gmDeljeno);
			
			vpSt1.setHorizontalAlignment(JTextField.RIGHT);
			vpSt2.setHorizontalAlignment(JTextField.RIGHT);
			vpRez.setHorizontalAlignment(JTextField.RIGHT);
			vpRez.setEditable(false);
			
			gmPlus.addActionListener(this);
			gmMinus.addActionListener(this);
			gmKrat.addActionListener(this);
			gmDeljeno.addActionListener(this);
		}
		
			public void actionPerformed(ActionEvent e)
			{
				
				double stev1 = Double.parseDouble(vpSt1.getText());
				double stev2 = Double.parseDouble(vpSt2.getText());
				double rez = 7; 
				
				Object izvor = e.getSource(); // izvor = source
				if( izvor == gmPlus )
				{
					rez = stev1 + stev2;
				
				}
				else if(izvor == gmMinus)
				{
					rez = stev1 - stev2;
				}
				else if(izvor == gmKrat)
				{
					rez = stev1 * stev2;
				}
				else if(izvor == gmDeljeno)
				{
					rez = stev1 / stev2;
				}
				vpRez.setText(Double.toString(rez)); 	
				
			}
	}
	
}

[keang]

You still haven't said what bit doesn't work.

[flex567]

I dont know why are all conditions in all IF statesmant always negative(false)?

[keang]

You are assigning the JButtons to local variables with exactly the same name as your instance variables so your instance variables will all be null and hence will never match the event source.

Note: it's nearly always a bad idea to have variables with the same name but different scopes. It leads to all sorts of confusion as you have just found out.

[flex567]

Can you explain this a little bit more. What should i fix in this code in order for this program to work?

[keang]

Your code declares an instance variable gmPlus of type JButton but doesn't ever assign anything to it.

Look at the line where you assign the JButton instance to your variable gmPlus. You will see you are not assigning it to the instance variable gmPlus but to a local variable called gmPlus.

To solve the problem remove the declaration of type JButton from the beginning of the line. ie

Change

JButton gmPlus = new JButton(" + ");

to

gmPlus = new JButton(" + ");

This applies to all your operator buttons.

[flex567]

oh, i understand. I created 2 variables with the same name(JButton gmPlus) - inside the function and
- one inside of class

function Action listener was called from the different object.

[flex567]

How is it possible that we never call actionPerformed and Run functions? We just declare them.

[keang]

How is it possible that we never call actionPerformed and Run functions? We just declare them.

These methods are called by the classes you have registered your object with. For instance looking at ActionPerformed: You declare a class as implementing the ActionListener interface which means you have to implement the ActionPerformed method. You then pass an instance of this class to any/all GUI components that you want to listen to for action events ie by calling their addActionListener(..) method. When the GUI component generates an action event it iterates over it's list of registered listeners and calls each one's actionPerformed(..) method and hence your object's actionPerformed(..) method is called.

[flex567]

so actually the JVM calls these functions?

[keang]

Only in the sense that everything that happens in Java goes through the JVM.

The sequence of events is roughly:

• You click on button on the screen.
• The OS sends a OS mouse click event
• This OS event is translated to a Java mouse click event and is added to the EventQueue.
• The EventQueue dispatches the mouse click event to all listeners. This is done by the EventDispathThread which is the single thread that runs all swing stuff.
• Your button gets sent the event and generates an ActionEvent which it sends to all its ActionListeners.