public static String getInput()
{
    while(input isn't right)
    {
        input = JOptionPane.showInputDialog(null, "Put shit here: ");
    }
    return input;
}

public static String getInput(int lowerBound, int upperBound)

...program...
...
//need input
int choice = processInput();
if(choice == x)
    blah;
...
...
private static int processInput() {
    //however you've been getting input
    //handle bad input
    //however you've been working out their pick
    int userChoice = whatever they picked;
    return userChoice;
}

if (userInput0.equalsIgnoreCase(a1) && compShieldInt > 0) {
                if (compShieldInt >= 15) {
                    JOptionPane.showMessageDialog(null, "You fire a basic attack.\n\n15 damage dealt to enemy shields!");
                    compShieldInt = compShieldInt - 15;
                } else if (compShieldInt < 15) {
                    JOptionPane.showMessageDialog(null, "You fire a basic attack.\n\nEnemy shields destroyed! "
                            + ((15 - compShieldInt) * 0.75) + " damage dealt to enemy hull.");
                    compHullInt = compHullInt - ((15 - compShieldInt)) * 0.75;
                    compShieldInt = 0;
                }