java.lang.NullPointerException problem in ListView

Question

我是Android开发的新手，我正在尝试将字符串转换为双精度，然后将其四舍五入到小数点后2位，然后将其转换回字符串以显示在ListView中。这是我的代码：

public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent){
String expenseCategory = getItem(position).getExpenseCategory();
String expenseAmount = getItem(position).getExpenseAmount();
String expenseEntryDate = getItem(position).getExpenseTodayEntry();
Double eA = Double.parseDouble(expenseAmount);
String eAnew = String.format("%.2f",eA);
Expenses ex = new Expenses(eAnew, expenseCategory, expenseEntryDate);
LayoutInflater inflater = LayoutInflater.from(mContext);
convertView = inflater.inflate(mResource, parent, false);
TextView expenseCategoryTextView = (TextView) convertView.findViewById(R.id.CategoryHistoryTV);
TextView expenseDateEntryTextView = (TextView) convertView.findViewById(R.id.DateHistoryTV);
TextView expenseAmountTextView = (TextView) convertView.findViewById(R.id.AmountHistoryTV);
expenseCategoryTextView.setText(expenseCategory);
expenseDateEntryTextView.setText(expenseEntryDate);
expenseAmountTextView.setText(eAnew);
expenseCategoryTextView.setTextColor(Color.RED);
expenseDateEntryTextView.setTextColor(Color.RED);
expenseAmountTextView.setTextColor(Color.RED);
return convertView;
}

这是错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.trim()' on a null object reference

它可以正常运行，无需：

Double eA = Double.parseDouble(expenseAmount);
String eAnew = String.format("%.2f",eA);

但相反，它给了我一个输出，比如

1000.0000

我知道这可能是糟糕的设计，但是由于各种原因，将费用金额声明为双倍在我的代码的其他领域不起作用。

我试图使用一个条件来"捕获"空值，因为错误说，但到目前为止没有任何效果。

Answer 1

在指出问题的位置修改代码，并在那里添加 Null 检查。下面我提到了代码示例。

String eAnew = "00.00";
if(expenseAmount != null && !"null".equals(expenseAmount)){
Double eA = Double.parseDouble(expenseAmount);
eAnew = String.format("%.2f",eA);
}

Answer 2

首先，您尚未发布完整的代码，其中您正在从双精度转换为字符串，反之亦然，其次根据错误"java.lang.String java.lang.String.trim(("，这意味着您的字符串为空，您正在调用 trim 函数。因此，在调用此函数 trim 之前，请确保该字符串必须具有一些值。

Answer 3

似乎expenseAmount在这里是空的。尝试为列表中的项目指定默认expenseAmount值或处理null情况。

喜欢：

public class Expense { // whatever your class is
private String expenseAmount = "0"; // set a default value, so it's not null
...
}

或：

public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
...
String expenseAmount = getItem(position).getExpenseAmount();
...
String eAnew;
if (expenseAmount == null) {
eAnew = "I don't have an expenseAmount yet!"; // handle the null case
else {
Double eA = Double.parseDouble(expenseAmount);
eAnew = String.format("%.2f", eA);
}
Expenses ex = new Expenses(eAnew, expenseCategory, expenseEntryDate);
...
}