java - Non Static String cannot be referenced from a static context

Question

When trying to call on a method from another class my method as a error at

private class equalsButton implements ActionListener {

    // makes the equals button listener
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("=");
        InfixToPostfixParens.convert(operator); // Line with error
    }
}

Saying Non Static String cannot be referenced from a static context. The variable isn't static but I still get this problem any ideas?

The Bulk of my main program as it stands now (Sorry for the sloppy coding in places)

/**
 * Graphics of the calculator
 * 
 * @author Collin Blake
 * @version (2-28-11)
 */
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class SimpleCalculator extends JFrame {

    JTextField textField;
    String operator = "";

    public SimpleCalculator() {
        setTitle("Simple Calculator"); // makes the window for the calculator
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new GridLayout(0, 1));
        JPanel topPanel = new JPanel(); // Initilaized for the text window
        topPanel.setLayout(new GridLayout());
        textField = new JTextField(10);
        textField.addActionListener(new NumEntered());
        topPanel.add(textField);
        add(topPanel);
        JPanel centerPanel = new JPanel(); // center panel initilaized
        centerPanel.setLayout(new GridLayout(0, 4));
        JButton oneButton = new JButton("1"); // initilaized values for each
                                                // button in their menus
        JButton twoButton = new JButton("2");
        JButton threeButton = new JButton("3");
        JButton plusButton = new JButton("+");
        JButton fourButton = new JButton("4");
        JButton fiveButton = new JButton("5");
        JButton sixButton = new JButton("6");
        JButton minusButton = new JButton("-");
        JButton sevenButton = new JButton("7");
        JButton eightButton = new JButton("8");
        JButton nineButton = new JButton("9");
        JButton timesButton = new JButton("*");
        JButton zeroButton = new JButton("0");
        JButton decimalButton = new JButton(".");
        JButton powerButton = new JButton("^");
        JButton divideButton = new JButton("/");
        JButton LParButton = new JButton("(");
        JButton RParButton = new JButton(")");
        JButton equalsButton = new JButton("="); // creates equals button
        JButton allClearButton = new JButton("AC");
        oneButton.addActionListener(new oneButton()); // initilaized values for
                                                        // each button's
                                                        // actionlistener
        twoButton.addActionListener(new twoButton());
        threeButton.addActionListener(new threeButton());
        plusButton.addActionListener(new plusButton());
        fourButton.addActionListener(new fourButton());
        fiveButton.addActionListener(new fiveButton());
        sixButton.addActionListener(new sixButton());
        minusButton.addActionListener(new minusButton());
        sevenButton.addActionListener(new sevenButton());
        eightButton.addActionListener(new eightButton());
        nineButton.addActionListener(new nineButton());
        timesButton.addActionListener(new timesButton());
        zeroButton.addActionListener(new zeroButton());
        decimalButton.addActionListener(new decimalButton());
        powerButton.addActionListener(new powerButton());
        divideButton.addActionListener(new divideButton());
        LParButton.addActionListener(new LParButton());
        RParButton.addActionListener(new RParButton());
        equalsButton.addActionListener(new equalsButton()); // creates action
                                                            // listener for
                                                            // equals
        allClearButton.addActionListener(new allClearButton());
        centerPanel.add(oneButton); // Adds the buttons to the sub panel
        centerPanel.add(twoButton);
        centerPanel.add(threeButton);
        centerPanel.add(plusButton);
        centerPanel.add(fourButton);
        centerPanel.add(fiveButton);
        centerPanel.add(sixButton);
        centerPanel.add(minusButton);
        centerPanel.add(sevenButton);
        centerPanel.add(eightButton);
        centerPanel.add(nineButton);
        centerPanel.add(timesButton);
        centerPanel.add(zeroButton);
        centerPanel.add(decimalButton);
        centerPanel.add(powerButton);
        centerPanel.add(divideButton);
        centerPanel.add(LParButton);
        centerPanel.add(RParButton);
        centerPanel.add(equalsButton);
        centerPanel.add(allClearButton);
        add(centerPanel); // Creates the panel with buttons
        pack();
        setLocationRelativeTo(null);
    }

    private class NumEntered implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            // do nothing
        }
    }

    private class oneButton implements ActionListener {

        // makes the one button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("1");
            operator += "1";
            textField.setText(operator);
        }
    }

    private class twoButton implements ActionListener {

        // makes the two button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("2");
            operator += "2";
            textField.setText(operator);
        }
    }

    private class threeButton implements ActionListener {

        // makes the three button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("3");
            operator += "3";
            textField.setText(operator);
        }
    }

    private class fourButton implements ActionListener {

        // makes the four button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("4");
            operator += "4";
            textField.setText(operator);
        }
    }

    private class fiveButton implements ActionListener {

        // makes the five button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("5");
            operator += "5";
            textField.setText(operator);
        }
    }

    private class sixButton implements ActionListener {

        // makes the six button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("6");
            operator += "6";
            textField.setText(operator);
        }
    }

    private class sevenButton implements ActionListener {

        // makes the seven button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("7");
            operator += "7";
            textField.setText(operator);
        }
    }

    private class eightButton implements ActionListener {

        // makes the eight button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("8");
            operator += "8";
            textField.setText(operator);
        }
    }

    private class nineButton implements ActionListener {

        // makes the nine button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("9");
            operator += "9";
            textField.setText(operator);
        }
    }

    private class zeroButton implements ActionListener {

        // makes the zerobutton listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("0");
            operator += "0";
            textField.setText(operator);
        }
    }

    private class plusButton implements ActionListener {

        // makes the plus button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("+");
            operator += "+";
            textField.setText(operator);
        }
    }

    private class minusButton implements ActionListener {

        // makes the minus button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("-");
            operator += "-";
            textField.setText(operator);
        }
    }

    private class timesButton implements ActionListener {

        // makes the times button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("*");
            operator += "*";
            textField.setText(operator);
        }
    }

    private class divideButton implements ActionListener {

        // makes the divide button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("/");
            operator += "/";
            textField.setText(operator);
        }
    }

    private class allClearButton implements ActionListener {

        // makes the all clear button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("AC");
            operator = "";
            textField.setText(operator);
        }
    }

    private class decimalButton implements ActionListener {

        // makes the decimal button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println(".");
            operator += ".";
            textField.setText(operator);
        }
    }

    private class equalsButton implements ActionListener {

        // makes the equals button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("=");
            InfixToPostfixParens.convert(operator);
        }
    }

    private class LParButton implements ActionListener {

        // makes the ( button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("(");
            operator += "(";
            textField.setText(operator);
        }
    }

    private class RParButton implements ActionListener {

        // makes the ) button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println(")");
            operator += ")";
            textField.setText(operator);
        }
    }

    private class powerButton implements ActionListener {

        // makes the ^ button listener
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("^");
            operator += "^";
            textField.setText(operator);
        }
    }
}

The code for the InfixToPostfixParens

import java.util.Stack;
import java.util.EmptyStackException;
import java.util.regex.Pattern;
import java.util.Scanner;

/**
 * Translates an infix expression with parentheses to a postfix expression.
 * 
 * @author Koffman & Wolfgang
 */
public class InfixToPostfixParens {

    // Nested Class
    /** Class to report a syntax error. */
    public static class SyntaxErrorException extends Exception {

        /**
         * Cons

Answer 1

How does your code differ from the following (which compiles)?

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

public class Example1 {
    private String operator;

    private class equalsButton implements ActionListener {
        public void actionPerformed(ActionEvent e) {
           System.out.println("=");
           InfixToPostfixParens.convert(operator);
        }
    }
}

public class InfixToPostfixParens {
    public static void convert(String s) {
      // Do something useful
    }
}