删除包含 next() 的 println 会导致无限循环？

Question

I must be missing something obvious about how the various Scanner methods work but this really doesn't make any sense:

this code causes an infinite loop:

Scanner valueScan = new Scanner(line);
valueScan.useDelimiter(",");
while(valueScan.hasNext())
{
count++;
}

while this code is valid:

Scanner valueScan = new Scanner(line);
valueScan.useDelimiter(",");
while(valueScan.hasNext())
{
System.out.println("value token: " + valueScan.next());
cellCount++;
}

This is from Oracle's documentation I am not sure if it applies or what it means: "This method may block while waiting for input to scan, even if a previous invocation of hasNext() returned true. " (http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html#next%28%29)

Answer 1

Because hasNext doesn't remove anything from the Scanner , so in your first case, you never change its state, hence the infinite loop. next extracts input, and hence changes the state of the Scanner .

Answer 2

The call to valueScan.next() in your second example is what makes the scanner move forward. hasNext() only checks that there is a next item in your scanner but doesn't do anything else, hence the infinte loop if there is at least one item.

Answer 3

Without the next() the Scanner wont iterate.

Answer 4

hasnext only tells you if next element is present and next goes to that element.

so in your first loop, it stays at the first element and keeps telling you that second element is present and does nothing more.