You don't check if the user's input can be parsed to double, which will cause an exception when you parse. To check if your input is numerical, you can use the following function, which uses a regex :

public static boolean isNumeric(String str)
{
  return str.matches("-?\\d+(\\.\\d+)?");  //match a number with optional '-' and decimal.
}

This example comes from this SO's answer

This way, you can validate your input instead of letting the application crash if I enter non-numerical characters as an input (note that this regex will accept "." and "-" as valid characters).

You need to loop on your input until it is valid, so you could do something like this :

String carCost = JOptionPane.showInputDialog("What is the total cost of the car?");
while(!isNumeric(carCost)){
    carCost = JOptionPane.showInputDialog("Please enter a valid car cost : ");
}

Afterwards, you could extract this logic into a method, let's call it getDoubleInput(String paneText, String invalidInputText)

private String getDoubleInput(String paneText, String invalidInputText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog(invalidInputText);
    }

    return input;
}

Then you could, as @Caridorc's pointed, skip the String variable in your code and use this format :

double carCost = Double.parseDouble(getDoubleInput("What is the total cost of the car?","Please enter a valid car cost : "));
double insCostP = Double.parseDouble(getDoubleInput("What is the yearly cost of the insurance?", "Please enter a valid yearly cost of insurance : "));
//...

If you dislike have 2 String parameter to the method, you could use a generic invalid input message such as : "Please enter a valid input : ". Then the method would look like this :

private String getDoubleInput(String paneText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog("Please enter a valid input : ");
    }

    return input;
}

You wouldn't need the second parameter in the method call :

double carCost = Double.parseDouble(getDoubleInput("What is the total cost of the car?"));

You don't check if the user's input can be parsed to double, which will cause an exception when you parse. To check if your input is numerical, you can use the following function, which uses a regex :

public static boolean isNumeric(String str)
{
  return str.matches("-?\\d+(\\.\\d+)?");  //match a number with optional '-' and decimal.
}

This example comes from this SO's answer

This way, you can validate your input instead of letting the application crash if I enter non-numerical characters as an input (note that this regex will accept "." and "-" as valid characters).

You need to loop on your input until it is valid, so you could do something like this :

String carCost = JOptionPane.showInputDialog("What is the total cost of the car?");
while(!isNumeric(carCost)){
    carCost = JOptionPane.showInputDialog("Please enter a valid car cost : ");
}

Afterwards, you could extract this logic into a method, let's call it getDoubleInput(String paneText, String invalidInputText)

private String getDoubleInput(String paneText, String invalidInputText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog(invalidInputText);
    }

    return input;
}

Then you could, as @Caridorc's pointed, skip the String variable in your code and use this format :

double carCost = Double.parseDouble(getDoubleInput("What is the total cost of the car?","Please enter a valid car cost : "));
double insCostP = Double.parseDouble(getDoubleInput("What is the yearly cost of the insurance?", "Please enter a valid yearly cost of insurance : "));
//...

If you dislike have 2 String parameter to the method, you could use a generic invalid input message such as : "Please enter a valid input : ". Then the method would look like this :

private String getDoubleInput(String paneText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog("Please enter a valid input : ");
    }

    return input;
}

You wouldn't need the second parameter in the method call :

double carCost = Double.parseDouble(getDoubleInput("What is the total cost of the car?"));

You don't check if the user's input can be parsed to double, which will cause an exception when you parse. To check if your input is numerical, you can use the following function, which uses a regex :

public static boolean isNumeric(String str)
{
  return str.matches("-?\\d+(\\.\\d+)?");  //match a number with optional '-' and decimal.
}

This example comes from this SO's answer

This way, you can validate your input instead of letting the application crash if I enter non-numerical characters as an input (note that this regex will accept "." and "-" as valid characters).

You need to loop on your input until it is valid, so you could do something like this :

String carCost = JOptionPane.showInputDialog("What is the total cost of the car?");
while(!isNumeric(carCost)){
    carCost = JOptionPane.showInputDialog("Please enter a valid car cost : ");
}

Afterwards, you could extract this logic into a method, let's call it getNumericInput(String paneText, String invalidInputText)

private String getNumericInput(String paneText, String invalidInputText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog(invalidInputText);
    }

    return input;
}

Then you could, as @Caridorc's pointed, skip the String variable in your code and use this format :

double carCost = Double.parseDouble(getNumericInput("What is the total cost of the car?","Please enter a valid car cost : "));
double insCostP = Double.parseDouble(getNumericInput("What is the yearly cost of the insurance?", "Please enter a valid yearly cost of insurance : "));
//...

If you dislike have 2 String parameter to the method, you could use a generic invalid input message such as : "Please enter a valid input : ". Then the method would look like this :

private String getNumericInput(String paneText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog("Please enter a valid input : ");
    }

    return input;
}

You wouldn't need the second parameter in the method call :

double carCost = Double.parseDouble(getNumericInput("What is the total cost of the car?"));

You don't check if the user's input can be parsed to double, which will cause an exception when you parse. To check if your input is numerical, you can use the following function, which uses a regex :

public static boolean isNumeric(String str)
{
  return str.matches("-?\\d+(\\.\\d+)?");  //match a number with optional '-' and decimal.
}

This example comes from this SO's answer

This way, you can validate your input instead of letting the application crash if I enter non-numerical characters as an input (note that this regex will accept "." and "-" as valid characters).

You need to loop on your input until it is valid, so you could do something like this :

String carCost = JOptionPane.showInputDialog("What is the total cost of the car?");
while(!isNumeric(carCost)){
    carCost = JOptionPane.showInputDialog("Please enter a valid car cost : ");
}

Afterwards, you could extract this logic into a method, let's call it getDoubleInput(String paneText, String invalidInputText)

private String getDoubleInput(String paneText, String invalidInputText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog(invalidInputText);
    }

    return input;
}

Then you could, as @Caridorc's pointed, skip the String variable in your code and use this format :

double carCost = Double.parseDouble(getDoubleInput("What is the total cost of the car?","Please enter a valid car cost : "));
double insCostP = Double.parseDouble(getDoubleInput("What is the yearly cost of the insurance?", "Please enter a valid yearly cost of insurance : "));
//...

If you dislike have 2 String parameter to the method, you could use a generic invalid input message such as : "Please enter a valid input : ". Then the method would look like this :

private String getDoubleInput(String paneText){

    String input = JOptionPane.showInputDialog(paneText);
    while(!isNumeric(input)){
        input = JOptionPane.showInputDialog("Please enter a valid input : ");
    }

    return input;
}

You wouldn't need the second parameter in the method call :

double carCost = Double.parseDouble(getDoubleInput("What is the total cost of the car?"));