java将公历转换为字符串，但引发异常

Question

运行程序并尝试显示购买日期后，程序会抛出异常 以下是购买日期的getter和setter代码：

public void setPurchaseDate(String purchaseDate){
    String[] dateSplit = purchaseDate.split("/");
    int month, day, year;
    month = Integer.parseInt(dateSplit[0]);
    day = Integer.parseInt(dateSplit[1]);
    year = Integer.parseInt(dateSplit[2]);
    GregorianCalendar date = new GregorianCalendar (year, month-1, day);
}

public String getPurchaseDate(){
    String date;
    SimpleDateFormat sdf = new SimpleDateFormat("MM/DD/YYYY");
    date = sdf.format(this.purchaseDate.getTime());
    return date;
}

这是控制台中的异常：

Exception in thread "main" java.lang.NullPointerException
at jasonsiboleproject5.motorVehicle.getPurchaseDate(motorVehicle.java:88)
at jasonsiboleproject5.car.toString(car.java:69)
at java.lang.String.valueOf(String.java:2847)
at java.lang.StringBuilder.append(StringBuilder.java:128)
at java.util.AbstractCollection.toString(AbstractCollection.java:458)
at java.lang.String.valueOf(String.java:2847)
at java.io.PrintStream.println(PrintStream.java:821)
at jasonsiboleproject5.mainclass.main(mainclass.java:83)

Java结果：1

Answer 1

1.）首先检查您传递的格式，它是有效的"MM/DD/YYYY"还是使用此"MM/dd/yyyy"

(二)同样在你的function setPurchaseDate中，你有not set the date，在getter中我们调用this.purchaseDate.getTime()，其中this.purchaseDate将为空。因此，首先正确设置该值，并添加一个空检查