java 在 for 循环中使用 Scanner 进行系统输入
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Using Scanner inside a for loop for system input
提问by dawnoflife
I have been struggling with this for a while. I essentially want to loop through and read in as many strings as determined by num_choices. The following code only executes the else condition.
我已经为此苦苦挣扎了一段时间。我基本上想循环并读入由num_choices. 下面的代码只执行else条件。
Scanner s2 = new Scanner(System.in);
for(int i=0; i < this.num_choices; i++)
{
if(s2.hasNext())
{
System.out.println("Enter choice " + (i+1) +":");
String ch = s2.next();
//this.choices.addElement(ch);
}
else
{
System.out.println("Lets end this");
}
}
`
`
I am getting this: Exception in thread "main" java.util.NoSuchElementException. In the main class, this is where the error points to
我得到这个：Exception in thread "main" java.util.NoSuchElementException。在主类中，这是错误指向的地方
choice2 = Integer.parseInt(read_choice2.next());
which is inside a while loop as well. Here is the code for that:
这也在一个 while 循环中。这是代码：
public class Main
{
public static void main(String args[]) throws IOException
{
Vector<Survey> mysurveys = new Vector<Survey>();
boolean carry_on = true;
int choice = 0;
Scanner read_choice = new Scanner(System.in);
System.out.println("Let's begin the Survey/Test application!");
while(carry_on)
{
System.out.println("What would you like to do?");
System.out.println("1. Create a new Survey");
System.out.println("2. Create a new Test");
System.out.println("3. Display a Survey");
System.out.println("4. Display a Test");
System.out.println("5. Save a Survey");
System.out.println("6. Save a Test");
System.out.println("7. Load a Survey");
System.out.println("8. Load a Test");
System.out.println("9. Quit");
System.out.println();
System.out.println("Please enter a number for the operation you want to perform: ");
choice = Integer.parseInt(read_choice.next());
/*try
{
choice = Integer.parseInt(buffer.readLine());
}
catch(InputMismatchException e)
{
System.out.println("Invalid input. Please Enter again.");
System.out.println();
//read_choice.nextInt();
}*/
switch(choice)
{
case 1:
System.out.println("Please Enter a Name for your Survey");
String in = buffer.readLine();
Survey s1 = new Survey();
s1.CreateNew(in);
mysurveys.add(s1);
////
add_question(s1.type);
break;
case 2:
System.out.println("Please Enter a Name for your Test");
//String in = buffer.readLine();
Test t1 = new Test();
//t1.CreateNew(in);
mysurveys.add(t1);
break;
////
//add_question(t1.type);
case 3:
break;
// call Survey.display()
case 4:
break;
case 5:
Survey s = new Survey();
ReadWriteFiles x = new ReadWriteFiles();
x.SaveSurvey(s);
break;
case 6:
Test t = new Test();
//ReadWriteFiles x = new ReadWriteFiles();
//x.SaveSurvey(t);
break;
case 7:
carry_on = false;
break;
default:
System.out.println("Incorrect Input. Try Again");
System.out.println();
break;
}
}
read_choice.close();
}
public static void add_question(String type) throws IOException, NullPointerException
{
Questions q = null;
boolean carry_on2 = true;
int choice2 = 0;
Scanner read_choice2 = new Scanner(System.in);
//BufferedReader buffer2=new BufferedReader(new InputStreamReader(System.in));
while (carry_on2)
{
//
System.out.println("1. Add a new T/F Question");
System.out.println("2. Add a new Multiple Choice Question");
System.out.println("3. Add a new Short Answer Question");
System.out.println("4. Add a new Essay Question");
System.out.println("5. Add a new Ranking Question");
System.out.println("6. Add a new Matching Question");
System.out.println("7. If you want to stop adding more questions, and go back to the main menu.");
System.out.println("Please enter a number for the operation you want to perform: ");
choice2 = Integer.parseInt(read_choice2.next());
/*try
{
choice2 = Integer.parseInt(buffer2.readLine());
}
catch(InputMismatchException e)
{
System.out.println("Invalid input. Please Enter again.");
System.out.println();
//read_choice2.nextInt();
}*/
switch(choice2)
{
case 1:
q = new TrueFalse();
break;
case 2:
q = new MultipleChoice();
break;
case 3:
q = new ShortAnswer();
break;
case 4:
q = new Essay();
break;
case 5:
q = new Ranking();
break;
case 6:
q = new Matching();
break;
case 7:
carry_on2 = false;
break;
default:
System.out.println("Incorrect Input.");
break;
}
q.createQuestion(type);
}
}
}
I realize there is a lot of messy code, and I apologize for that. I just wanted to show the entire thing, so it's easier to spot the problem. Help would be appreciated.
我意识到有很多乱七八糟的代码，对此我深表歉意。我只是想展示整个事情，这样更容易发现问题。帮助将不胜感激。
回答by hqt
In general way, you should add if(read_choice.hasNext())before invoking read_choice.next();You have the exception java.util.NoSuchElementExceptionbecause no elements found to be read. this is a good habit.
一般来说，您应该if(read_choice.hasNext())在调用之前添加read_choice.next();您有异常，java.util.NoSuchElementException因为没有找到要读取的元素。这是一个好习惯。
About your problem, you are getting error because you has closed scanner before finish reading. Put read_choice.close()outside of loop.
关于您的问题，您收到错误消息，因为您在完成阅读之前关闭了扫描仪。放在read_choice.close()循环之外。
Moreover, for simplify, if you want to read integer, just simple : scanner.nextInt().
此外，为了简化，如果您想读取整数，只需简单 : scanner.nextInt()。
回答by Bhesh Gurung
read_choice.close();
Don't close the scanner as long as you are not done reading all the inputs. Doing also closes the underlying input stream (System.in), check the documention;
只要您没有读完所有输入，就不要关闭扫描仪。这样做也会关闭底层输入流（System.in），检查文档；
You don't need to initialize the Scannermultiple times. Just create one instance and pass it around (keep using it).
您不需要Scanner多次初始化。只需创建一个实例并传递它（继续使用它）。
Also,
还，
for(int i=0; i < this.num_choices; i++)
{
//if(s2.hasNext())
//{
System.out.println("Enter choice " + (i+1) +":");
String ch = s2.next();
//this.choices.addElement(ch);
you don't need that condition check. The next()will block until the input is entered.
你不需要那个条件检查。在next()将阻塞，直到输入的输入。
