为什么这会引发数字格式异常

Question

它基本上是一个做乘法的按钮事件

  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 = "" ;
                }
                    }

此代码给出第一次乘法的结果，但给出了连续乘法的数字格式 相同的代码可以正常进行除法运算。

Answer 1

从

更新if条件

if(equal == "mul")

到

if(equal.equals("mul"))

And learn more about string comparisions. Related post https://stackoverflow.com/questions/767372/java-string-equals-versus