为什么这会抛出一个数字格式异常

Question

它基本上是一个进行乘法运算的按钮事件

  mul.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {

                if(Sans == null){
                    temp = text;
                }else {
                    temp = Sans ;
                }
                text = "";
                equal = "mul" ;
                textArea.setText("*");
            }
        });
           this is the action that happens when pressing equal button
                 ans.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                if(equal == "mul"){
                    double a = Double.parseDouble(text);
                    double b = Double.parseDouble(temp);
                    double ans = b*a;
                    String Sans = String.valueOf(ans);
                    textArea.setText(Sans);
                    text = "" ;
                }
                    }

这段代码给出了第一次乘法的结果，但是对连续的乘法给出了一个numberformatexception

Answer 1

从

更新if条件

if(equal == "mul")

if(equal.equals("mul"))

学习更多关于字符串比较的知识。相关文章https://stackoverflow.com/questions/767372/java-string-equals-versus