非静态字符串不能从静态上下文中引用

您的代码与以下代码（编译）有何不同？

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    }}