我尝试将jdialogbox中的结果存储在数组中，并调用它在另一个jdialogbox的jtextfield中显示，但它似乎无法显示，并且一直有错误

//create array
String simple [] = new String [20];

//storing into array
for (int i = 0; i < 20; i++) {
    simple [i] = num1 + num2 + "=" + output;
    simple [i].toString();
}

//calling array to display at another dialog box
SimpleCalculator cal = new SimpleCalculator();
String text = "";
for (int i = 0; i < 20; i++) {
    text = text + cal.simple[i] + "\n";
    if(cal.simple[i].equals(null)) {
        cal.simple[i] = "";
        text = text + cal.simple[i] + "\n";
}

源代码如下：https://drive.google.com/drive/folders/16hsnmebuhae8siwzh6s3vthqaly_cn0l?usp=sharing