Can't get it to round to 2 decimals, using DecimalFormat, please proof read

Question

Why won't this output #.##? No matter how I change them it still gives me exact answers.
   import javax.swing.*;
   import java.awt.*;
   import java.util.*;
   import java.text.DecimalFormat;
    public class Calculator{
       public static void main (String[] args){
      //Ask for input
         String inputString = JOptionPane.showInputDialog("Input your desired calculation, separating numbers and operation with spaces.(+-/*%)");
      //split input to make switch function
         String[] result = inputString.split("\\s");
      /*I Put both of these in here, but stuck with the Double,
      I think thats the better fit*/
      //float n1 = Float.parseFloat(result[0]);
      //float n2 = Float.parseFloat(result[2]);
         double n1 = Double.parseDouble(result[0]);
         double n2 = Double.parseDouble(result[2]);
         char c = result [1].charAt(0);
         if((c == '+')  || (c == '/') || (c == '*') || (c == '%') || (c == '-'))
         DecimalFormat df = new DecimalFormat("#.##"); 
         //switch function to cover the 5 functions, and divide by zero message
            switch(c)
               case '+':
                  double answer = n1 + n2;           
                  df.format(answer);
                  JOptionPane.showMessageDialog(null, inputString + " = " + (answer));
                  break;
               case '-':
                  //DecimalFormat df = new DecimalFormat("#.#");            
                  double answer2 = n1 + -n2;           
                  df.format(answer2);
                  JOptionPane.showMessageDialog(null, inputString + " = " + (answer2));
                  break;
               case '/':
                  if(n2 == 0)
                     JOptionPane.showMessageDialog(null, "You know that's a no no!");
                  else
                     //DecimalFormat df = new DecimalFormat("#.#");            
                     double answer3 = n1 / n2;           
                     df.format(answer3);             
                     JOptionPane.showMessageDialog(null, inputString + " = " + (answer3));
                  break;
               case '*':
                  //DecimalFormat df = new DecimalFormat("#.#");            
                  double answer4 = n1 * n2;           
                  df.format(answer4);           
                  JOptionPane.showMessageDialog(null, inputString + " = " + (answer4));
                  break;
               case '%':
                  //DecimalFormat df = new DecimalFormat("#.#");            
                  double answer5 = n1 % n2;           
                  df.format(answer5);         
                  JOptionPane.showMessageDialog(null, inputString + " = " + (answer5));
                  break;
         else
            System.out.println("Can't do that operation!");     
   }I commented out some other stuff I was thinking on, so just ignore. Thanks!

Answer 1

cuz you are not using the value returned by the DecimalFormat object.
                  df.format(answer2);
                  JOptionPane.showMessageDialog(null, inputString + " = " + (answer2));DecimalFormat.format() returns a string. By what magic do you think the primitive double answer2 is going to be changed to a String object?
                  String a2 = df.format(answer2);
                  JOptionPane.showMessageDialog(null, inputString + " = " + (a2));