Cursor functionality in a for loop as opposed to a while loop?

Question

I am printing documents in a MongoDB database with a DBCursor. When I use the while loop, it works fine.

        DBCursor cursor = people.find(new BasicDBObject("isPerson",true));
        while(cursor.hasNext()){
            System.out.println(cursor.next());
        }
        cursor.close();

With a while loop, the documents are printed. When I use a for loop, I get an error.

        cursor = people.find(new BasicDBObject("isPerson",true));
        for(int a=0;a<cursor.length()-1;a++){                       
            System.out.println(cursor.next());                
        }
        cursor.close();

This is the error:

java.lang.IllegalArgumentException: Can't switch cursor access methods
at com.mongodb.DBCursor.checkIteratorOrArray(DBCursor.java:841)
at com.mongodb.DBCursor.next(DBCursor.java:168)
at mongotest.MongoTest.main(MongoTest.java:39)

But even when I get around the error with the following code, nothing is printed.

        cursor = people.find(new BasicDBObject("isPerson",true));
        int b = cursor.length()-1;
        cursor = people.find(new BasicDBObject("isPerson",true));
        for(int a=0;a<b;a++){                       
            System.out.println(cursor.next());                
        }
        cursor.close();

1) while is preferred. 2) save cursor length in variable and try for loop 3) initialize b again with cursor.length(0-1 after re-initialization cursor to people.find() — Ravindra babu
– Ravindra babu, Commented Oct 1, 2015 at 17:46
As a general note, your condition a < b is wrong, should be a < b + 1 or a <= b. There are n elements satisfying 0 <= x && x < n and this is usually what a for looks like — Dici
– Dici, Commented Oct 1, 2015 at 17:48

Rohit Jain · Accepted Answer · 2015-10-02 02:27:55Z

1

Look at next() method inside DBCursor class

@Override

public DBObject next() {
    checkIteratorOrArray(IteratorOrArray.ITERATOR);
    if (!hasNext()) {
        throw new NoSuchElementException();
    }

    return nextInternal();
}

it clearly mentioned if !hasNext() throw an exception

In Addition

If you keep cursor.length() in for loop and keep on iterate by calling cursor.next() , cursor length keep on change and throw an exception.

Look at your alternate way, you have initialize the variable i with cursor length and value of i does not change during execution of FOR loop.

edited Oct 2, 2015 at 2:27

answered Oct 1, 2015 at 18:12

Rohit Jain

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