New to Java

Hi i'm new around here, I've been reading about various java programming code and am currently studying it at University, I'm struggling with one part of the program, and that is when a user inputs how many litres of petrol they want it doesn't take that value away permanantly from the tank (which limit is 100). Sometimes it'll seem to be working until I enter the exact amount it says I have left, for example I enter 75 litres, it says I have 25 left, then I enter 25 and it says I have 50 left, I'm not sure why and I can't figure it out. I'm also getting some negative numbers left in the tank but I've put in a clause where if the tank reaches 0 or below it should say there's none left and exit.

Also finally, when case 9 is selected (the exit clause) it says it is exiting but then loops back to the beginning, can anyone see why?

I hope i've sort of explained it, I've tried changing the entire layout from having many if statements to just one, and using a do-while loop over it all. I'm sure it's going to be an amateurish mistake but I suppose I've got to learn somewhere. Here's the code, any help would be appreciated. Thanks

/*
   Petrol Pump Program
*/
import javax.swing.JOptionPane;
class PetrolChooser{
  public static void main (String args[]) {
     String str, strOutput = "";
     int selectedOption;
     int litres;
     int tank = 100;
     Object selectedValue;
     
         if (tank <=0) {
       	JOptionPane.showMessageDialog(null,
       	"There is no petrol remaining, the program will now exit",
       	"RESULTS:",JOptionPane.ERROR_MESSAGE) ;
			System.exit(0);
       	
       }  
		 
		 else {
	  
	  do {
     

     //the following code lists the items from which the user can select
     Object [] menuOptions = {
         "1 - 4 Star Petrol",
         "2 - SuperGrade Petrol",
         "3 - Diesel",
         "4 - SuperGrade Diesel",
         "9 - Exit" };
    
     //this displays the menu
     //selected option is placed in selectedValue
     selectedValue = JOptionPane.showInputDialog(null,
         "Which petrol would you like to use? \n", "Menu",
         JOptionPane.QUESTION_MESSAGE, null,
         menuOptions, menuOptions[0]);
     
     str = (String) selectedValue; //covert selected value to a String

     //next statement converts the first character of str into an int
     selectedOption = Character.getNumericValue(str.charAt (0) ) ;

     //use a switch statement to decide what to do
     switch (selectedOption) {
        case 1: //option 1 selected
           strOutput = "You have selected 4 Star Petrol";
           break;
        case 2: //option 2 selected
           strOutput = "You have selected SuperGrade Petrol";
           break;
        case 3: //option 3 selected
           strOutput = "You have selected Diesel";
           break;
        case 4: //option 4 selected
           strOutput = "You have selected SuperGrade Diesel";
           break;
        case 9: //option 9 selected
           strOutput = "You have selected to Exit";


    }
    JOptionPane.showMessageDialog(null, strOutput, "Output",
                      JOptionPane.INFORMATION_MESSAGE) ;
                      
     str=JOptionPane.showInputDialog("How many litres do you want?");
    			    litres=Integer.parseInt (str);
    			    
    			   
    			    
    JOptionPane.showMessageDialog(null,
       	"You selected " + litres + " There are " + (tank - litres) + " litres left in the tank",
       	"RESULTS:",JOptionPane.INFORMATION_MESSAGE) ;
                   tank=Integer.parseInt (str);   

     
        }
     
     while (tank > 0 || case 9);
     

    

                      
                      
    System.exit (0) ;
  }
}