java.lang.NumberFormatException：对于输入字符串：“”
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java.lang.NumberFormatException: For input string: ""
提问by Jessy
When running this code:
运行此代码时：
JTextField ansTxt;
...
ansTxt = new JTextField(5);
String aString = ansTxt.getText();
int aInt = Integer.parseInt(aString);
Why do I get this error?
为什么我会收到这个错误？
Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: ""
线程“AWT-EventQueue-0”中的异常java.lang.NumberFormatException：对于输入字符串：“”
UPDATE:
更新：
JTextField ansTxt;
ansTxt = new JTextField(5);
ansTxt.addKeyListener(new KeyAdapter() {
public void keyReleased(KeyEvent e) {
ansTxt = (JTextField) e.getSource();
String aString = ansTxt.getText().trim();
int aInt = Integer.parseInt(aString);
}
}
回答by Thomas L?tzer
You're trying to parse an empty string as an int, which does not work. Which int should "" be parsed as? The JTextField needs to have a text that can be parsed.
您正在尝试将空字符串解析为 int，但这是行不通的。哪个 int 应该被解析为？JTextField 需要有一个可以解析的文本。
ansTxt.addKeyListener(new KeyAdapter() {
public void keyReleased(KeyEvent e) {
ansTxt = (JTextField) e.getSource();
try {
int aInt = Integer.parseInt(ansTxt.getText());
//Do whatever you want with the int
} catch(NumberFormatException nfe) {
/*
* handle the case where the textfield
* does not contain a number, e.g. show
* a warning or change the background or
* whatever you see fit.
*/
}
}
}
It is probably also not a good idea to set ansTxt inside the KeyAdapter. I would suggest you use a local variable for this. That also makes it easier to move the adapter into a "real" class instead of an anonymous one.
在 KeyAdapter 中设置 ansTxt 也可能不是一个好主意。我建议您为此使用局部变量。这也使得将适配器移动到“真实”类而不是匿名类中变得更加容易。
回答by aioobe
The integer argument to the JTextField constructoris actually the widthin number of columns. From the docs:
JTextField 构造函数的整数参数实际上是列数的宽度。从文档：
public JTextField(int columns)Constructs a new empty TextField with the specified number of columns. A default model is created and the initial string is set to null.
public JTextField(int columns)构造一个具有指定列数的新空 TextField。创建默认模型并将初始字符串设置为空。
By constructing it with
通过构建它
ansTxt = new JTextField(5);
you'll basically get an empty text-field(slightly widerthan if you constructed it using no-argument constructor). If you want it to contain the string "5" you should write
你基本上会得到一个空的文本字段（比你使用无参数构造函数构造它稍微宽一些）。如果你想让它包含字符串“5”，你应该写
ansTxt = new JTextField("5");
try { ... } catch (NumberFormatException e) { ... }
回答by Sean
Try introducing the Apache's "commons Lang" library into your project and for your last line you can do
尝试将 Apache 的“ commons Lang”库引入您的项目，对于您的最后一行，您可以这样做
int aInt = 0;
if(StringUtils.isNotBlank(aString) && StringUtils.isNumeric(aString) ){
aInt = Integer.parseInt(aString);
}
edit: Not sure why the downvote. The JtextField will take any string. If the text field is listening on each key press, every non-numeric value (including blank) that is entered will generate the NumberFormatException. Best to check if it is Numeric before doing anything with the new value.
编辑：不知道为什么downvote。JtextField 将采用任何字符串。如果文本字段在每次按键时都在侦听，则输入的每个非数字值（包括空白）都将生成 NumberFormatException。最好在对新值执行任何操作之前检查它是否为数字。
edit2: As per Thomas' comments below. I ran a test to compare the try/catch vs the StringUtils way of solving this issue. The test was ran 5million times for each. The average time for the try/catch was 21 seconds. The average time for the StringUtils was 8 seconds. So using StringUtils for heavy load is considerably faster. If the load on the code is small you will notice little to no difference. The test ran was
编辑2：根据托马斯在下面的评论。我运行了一个测试来比较 try/catch 与 StringUtils 解决此问题的方法。每个测试运行了 500 万次。try/catch 的平均时间为 21 秒。StringUtils 的平均时间为 8 秒。因此，使用 StringUtils 进行重负载要快得多。如果代码的负载很小，您会发现几乎没有区别。测试运行是
try{
result = Integer.parseInt(num);
}catch(NumberFormatException ex){
result = -1;
}
vs
对比
if(StringUtils.isNotBlank(num) && StringUtils.isNumeric(num)){
result = Integer.parseInt(num);
}else{
result = -1;
}
each loop through generated a new random string of 10 digits to avoid any optimization in the loops on the if statement. This added 6-7 seconds of overhead.
每次循环都会生成一个新的 10 位数字随机字符串，以避免在 if 语句的循环中进行任何优化。这增加了 6-7 秒的开销。
回答by Ishtar
Your KeyAdapterwill be run before your ansTextwill process the KeyEvent. In fact, you may e.consume()to prevent ansTextfrom processing it at all. So the first time a key is pressed and released, ansText.getText()will still be "". That's why you get the exception first time. Pressing a numerical key twice, should work the second time.
您KeyAdapter将在ansText处理 KeyEvent之前运行。事实上，您可以e.consume()完全阻止ansText处理它。因此，第一次按下和释放某个键时，ansText.getText()仍将是""。这就是你第一次得到异常的原因。按两次数字键，应该第二次工作。
回答by Roodra
Use the method trim().
使用方法trim()。
int m=Integer.parseInt(txtfield.getText().trim());
the trim()method will remove any string attached to the number.
该trim()方法将删除附加到号码的任何字符串。
