...
import javax.microedition.lcdui.Alert;
import javax.microedition.lcdui.AlertType;
...
try {
    switch(act[0]) {
        //    
        //   
        case '+':
            res = first + second;
            break;
        case '-':
            res = first - second;
            break;
        case '*':
            res = first * second;
            break;
        case '/':
            res = first / second;
            break;
        default:
            // :   
            resForm.append("Illegal Operation");
            //  
            display.setCurrent(resForm);
            // 
            return;
    }
    //        
    resForm.append((new Integer(res)).toString());
    //  
    display.setCurrent(resForm);

//    
} catch (ArithmeticException aexc) {
    //    
    Alert alert = new Alert("Error","Divide by zero",null,AlertType.ERROR);
    //    
    alert.setTimeout(Alert.FOREVER);
    //      
    display.setCurrent(alert,form);
}
...
