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Thread: system.out prints twice in a small program

  1. #1
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    Default system.out prints twice in a small program

    hi friends

    i am new to java and started a week back

    even in a simple program shown below

    system.out prints twice i am using netbeans ide

    it prints as bellows

    press a key : a
    press a key : press a key :

    public class DoWhile {
     
        /**
         * @param args the command line arguments
         * @throws java.io.IOException
         */
        public static void main(String[] args)
                throws java.io.IOException
        {
            char ch;
            do {
            System.out.print("press a key : ");
            ch = (char) System.in.read();
            }
            while(ch != 'q');
            }
     
    }

    kindly help
    Last edited by vinaysagar; March 11th, 2014 at 12:47 PM. Reason: missing parts


  2. #2
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: system.out prints twice in a small program

    You have a do while, so it's going to print out at least once. It prints out, then it reads in a single byte, which is a. Notice how after you pushed a, you also pressed enter (the newline character), so that's still "in" System.in.

    It checks whether a is equal to q, and since it's not, it goes through again. It prints out the message, and then reads in the newline character you left in there. It then checks whether the newline character is equal to q, and since it's not, it goes through yet again.

    You should probably be using the Scanner class.
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    Default Re: system.out prints twice in a small program

    Quote Originally Posted by KevinWorkman View Post
    You have a do while, so it's going to print out at least once. It prints out, then it reads in a single byte, which is a. Notice how after you pushed a, you also pressed enter (the newline character), so that's still "in" System.in.

    It checks whether a is equal to q, and since it's not, it goes through again. It prints out the message, and then reads in the newline character you left in there. It then checks whether the newline character is equal to q, and since it's not, it goes through yet again.

    You should probably be using the Scanner class.
    thanx Kevin for the reply

    another thanx in anticipation

    can you change that part in the program and let me know

    as i am totally new i am doing it as said in the book and dont know how to use scanner

    thanx once again

    regards

    Vinaysagar

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    Default Re: system.out prints twice in a small program

    Refer to the Scanner API page and the Java IO tutorial.

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    Default Re: system.out prints twice in a small program

    Quote Originally Posted by vinaysagar View Post
    as i am totally new i am doing it as said in the book and dont know how to use scanner
    Don't be afraid to google your problem. This is the second result for googling "Java Scanner": Scanning (The Java™ Tutorials > Essential Classes > Basic I/O)
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