java 如何在 System.in 上使用多个 Scanner 对象?
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How to use multiple Scanner objects on System.in?
提问by ghostdog74
what's the correct way to use multiple Scanner objects in my program. For example, I use scanner to read a file, then depending on what is found in the file, i use scanner again to prompt for user input. An extract of my code is shown
在我的程序中使用多个 Scanner 对象的正确方法是什么。例如,我使用扫描仪读取文件,然后根据在文件中找到的内容,我再次使用扫描仪来提示用户输入。显示了我的代码的摘录
....
Scanner f = new Scanner (System.in); //get the file name
String fileName = f.next();
Scanner input = new Scanner( new File( fileName ) );
while ( input.hasNext() )
{
String currentLine = input.nextLine();
if ( some pattern found) {
Scanner getUserInput = new Scanner (System.in);
String userInput = getUserInput.next();
.....
}
}
....
It doesn't seem to work. Do I need to use userInput.close()
? What am i doing wrong. ?
它似乎不起作用。我需要使用userInput.close()
吗?我究竟做错了什么。?
What I don't understand is, the first System.in
is just getting the file name. After that, why does it interfere with the second System.in
.
As for the input
object, its reading from a File and not from System.in
.
我不明白的是,第一个System.in
只是获取文件名。在那之后,为什么它会干扰第二个System.in
. 至于input
对象,它是从 File 而不是从System.in
.
回答by
What am I doing wrong?
我究竟做错了什么?
Using multiple scanners on the same stream is the underlying problem. Scanners can (and will) consume the stream - this may (will) lead to unexpected side-effects. Best not to do it.
在同一流上使用多个扫描器是潜在的问题。扫描器可以(并且将)消耗流 - 这可能(将)导致意外的副作用。最好不要这样做。
If the input is closed, then the input (but Strings have no close
method) is closed for everyone - and that's not much fun for anyone.
如果输入被关闭,那么输入(但字符串没有close
方法)对每个人都是关闭的——这对任何人来说都不是很有趣。
Edit:"Details" on why multiple scanners are bad: Do not create multiple buffered wrappers on an InputStream
编辑:关于为什么多个扫描器不好的“详细信息”:不要在 InputStream 上创建多个缓冲包装器
...any buffered wrapper is unsafe; this condition is also exploitable if a Scanner is used instead...
...任何缓冲包装器都是不安全的;如果使用扫描仪代替,这种情况也可被利用......
See also Java code question ... scanner related?which also talks about some approaches.
另请参阅Java 代码问题 ... 扫描仪相关?这也谈到了一些方法。