请帮助转义 Java 中 String.split() 方法中的 NPE 错误

Question

请帮助转义 Java 中 String.split() 方法中的 NullPointExeption 错误。用户输入的字符串。它无法编译。在变量的硬编码的情况下，一切都很好。类计算器：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    }}类计算器测试：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();        }    }}

Answer 1

您需要将您的split调用放入一个方法中，否则它会在您实例化Calculator. 看到这个

private String[] parts = mathExpression.split(" ");

private String firstNumber1 = parts[0];

将两者都放入方法中 calculatorRun

Answer 2

确保 mathExpression 不为空或为空

if(mathExpression!= null && !mathExpression.isEmpty()) { 

 private String[] parts = mathExpression.split(" ");

 private String firstNumber1 = parts[0];

    /* other code */ }