Java BufferedReader.readLine() Skipped One Line

November 22, 2021

I’m trying to make some distributed java RMI program, where I encountered a problem when I need to take input from the user. In the following code snippet, entering 1 seems to be good, but entering 2 first time cause nothing to print, when I entered 2 again, then it worked.

How did that happened? Checked the BufferedReader.readLine() function documentation but didn’t find anything relate to this. Does this have anything to do with else if?

import java.io.*;
public class test {

    private static BufferedReader lineOfText = new BufferedReader(new InputStreamReader(System.in));

    public test() throws NumberFormatException, IOException {
        System.out.println("1 to create an auction, 2 to close an auction");
        if (Integer.parseInt(lineOfText.readLine()) == 1) {
            System.out.println("1 recieved");
        } else if (Integer.parseInt(lineOfText.readLine()) == 2) {
            System.out.println("2 recieved");
        } else {
            return;
        }
    }
    public static void main(String[] args) throws NumberFormatException, IOException {
        test t = new test();
    }
}

input 1:

1 to create an auction, 2 to close an auction
1 //<- that's the input
1 recieved

input 2:

1 to create an auction, 2 to close an auction
2 //<- that's the input yet nothing happened
2 //<- input again
2 recieved

Also tried the code snippet on some online compiler and the result is the same as my VSCode. This might be a simply problem but I don’t know what happened ;\

>Solution :

Because you are calling "readline()" in both your "if" and "else-if", it will call readline() twice each time it tries to check for the condition: once when it runs the "if" condition, then a second time when running the else-if. When you place a function in the conditional statement like:

if(myFunction() == true){}

it will ALWAYS run that function before comparing its return value to the condition. So when you have:

if (Integer.parseInt(lineOfText.readLine()) == 1) {
        System.out.println("1 recieved");
    } else if (Integer.parseInt(lineOfText.readLine()) == 2) {
        System.out.println("2 recieved");
    } else {
        return;
    }

It will call readLine() both when it checks the "if", and again at the "else-if".

You should call readline BEFORE you begin checking it against conditions to ensure it is only ran once:

import java.io.*;
public class test {

private static BufferedReader lineOfText = new BufferedReader(new InputStreamReader(System.in));

public test() throws NumberFormatException, IOException {
    
    System.out.println("1 to create an auction, 2 to close an auction");
    String myLine = lineOfText.readLine(); //call readLine() before checking conditions so it only runs once
    if (Integer.parseInt(myLine) == 1) {
        System.out.println("1 recieved");
    } else if (Integer.parseInt(myLine) == 2) {
        System.out.println("2 recieved");
    } else {
        return;
    }
}
public static void main(String[] args) throws NumberFormatException, IOException {
    test t = new test();
}

}