Autofill a JTextField once another JTextField has been typed into

Hi there

I am trying to get a JTextField to autofill with the result of a calculation made from user input in another JTextField

I am building a virtual ticket machine, where i want the user to select ticket type, type in the quantity of tickets they would like, and then so as soon as they have clicked out of the quantity textfield, I want the payment required textfield to autofil with the correct cost of the users selection, I have wrote the code to do the calculation for the cost, I just dont know how to put the cost in the payment required textfield at this stage.

Here is my code for this bit;

Java Code:

public void actionPerformed(ActionEvent e) {
        
        String TypeName = (String)TicketTypeList.getSelectedItem();
        typeselectedTxt.setText(TypeName);
        int arrayIndex = Arrays.asList(typeStrings).indexOf(TypeName);
        
        String quantityStr = noOfTickets.getText();
        Double quantity = Double.parseDouble(quantityStr);
            
        if (noOfTickets.getText().equals(null) || noOfTickets.getText().equals("")){
            showMessageDialog(null, "Please enter a quantity");
            
        } else {
        
            switch (arrayIndex){
            case 0: 
                amountRequired = 15.00*quantity;
            break;
            case 1:
                amountRequired = 10.20*quantity;
            break;
            case 2:
                amountRequired = 5.70*quantity;
             break;
            case 3:
                amountRequired = 30.85*quantity;
            break;
        }  
    
        moneyRequiredTxt.setText(Double.toString(amountRequired));


.....