請幫助轉義 Java 中 String.split() 方法中的 NullPointExeption 錯誤。用戶輸入的字符串。它無法編譯。在變量的硬編碼的情況下，一切都很好。類計算器：public class Calculator {    private String mathExpression;    public void setMathExpression(String mathExpression) {        this.mathExpression = mathExpression;    }    private String[] parts = mathExpression.split(" ");    private String firstNumber1 = parts[0];    //add other elements to the array...    public void calculatorRun() {        //using all the variables    }}類計算器測試：public class CalculatorTest {    public static void main(String[] args) {        Scanner scanner = new Scanner(System.in);        String userAnswer = "y";        Calculator calculator = new Calculator();        while (userAnswer.equals("y")) {            System.out.print("Please put the math expression: ");            calculator.setMathExpression(scanner.nextLine());            calculator.calculatorRun();        }    }}