扫描仪中的next（）方法不等待输入

Question

i wrote this line (String str = sc.next();) like 10 times in my code, and all of them work ok except for one that doesn't wait for my input and immediately throws an exception "java.util.NoSuchElementException" ! does any one have an idea why that could happen ??

private static Matrix GetRationalMatrix(int[] size) throws Exception {
    Scanner sc = new Scanner(System.in);
    MathVector[] matrix = new MathVector[size[0]];
    for(int i = 0 ; i < size[0] ; i++) {
        MathVector vector = null;
        String str = sc.nextLine();
        str = str.replaceAll("[ \t]+", " ");
        String[] splitStr = str.split(" ");
        Scalar[] scalarVector = new Scalar[size[1]];
        for(int j = 0 ; j < splitStr.length ; j++) {
            String[] tmp = splitStr[j].split("/");
            try {
                scalarVector[j] = new Rational(Integer.parseInt(tmp[0]), Integer.parseInt(tmp[1]));
            }
            catch(Exception e){
                sc.close();
                throw new Exception("Please enter values properly: a/b were a,b are integers!");
            }
        }
        vector = new MathVector(size[1], scalarVector);
        matrix[i] = new MathVector(vector);
    }
    Matrix ans = new Matrix(size[0], matrix);
    sc.close();
    return ans;
}

in size[0] the number of rows and in size[1] number of columns , both will be positive integers

Answer 1

nextLine( ) instead of next();

Both hasNext and next methods may block waiting for further input. Whether a hasNext method blocks has no connection to whether or not its associated next method will block.

Answer 2

If you previously used

Scanner sc = new Scanner(System.in);

inside a try-with-resources or closed the Scanner yourself, that would have closed the underlying System.in stream. In that case, trying to read from it again through a Scanner would cause a NoSuchElementException .

Don't close System.in and try to re-use the same Scanner object.

Answer 3

The javadoc for hasNext() specifies:

Returns true if this scanner has another token in its input. This method may block while waiting for input to scan. The scanner does not advance past any input.

The javadoc for next() specifies:

Finds and returns the next complete token from this scanner. A complete token is preceded and followed by input that matches the delimiter pattern. This method may block while waiting for input to scan, even if a previous invocation of hasNext() returned true.

That does not mean next() will always block, but it might block, say if it's waiting for the end of the token, since it promises to return the next complete token from the input. Checking whether the input contains another token is an entirely different operation, performed by hasNext() , which will block because it will wait for input (until EOF is found).

So I think you're getting NoSuchElementException because you are using next() improperly.

Answer 4

java.util.NoSuchElementException is a RuntimeException which can be thrown by different classes in Java like Iterator, Enumerator, Scanner or StringTokenizer. All of those classes has method to fetch next element or next tokens if underlying data-structure doesn't have any element Java throws "java.util.NoSuchElementException".

u can use a string tokenizer in our code..

StringTokenizer st=new StringTokenizer(sc);

u can use hasMoretokens() method....so that it returns true as long as the input u provided have more tokens.....
incase u want to get next token u can nextToken() method...