i am having problems with java "if statement"

Question

i am a beginner and i was experimenting with nesting if statements in while loops. i made a basic calculator this way but i am having a slight problem with it. i will first share the code and then put forward my problem.

here is the code:

import java.util.Scanner;
class whileloop{
    public static void main(String args[]){
        Scanner scan = new Scanner(System.in);
    System.out.println("This is a basic calculator.");
    System.out.println("To exit, input exit when asked to input the method.");
    System.out.println("----------------------------------------------");
    while (true){
        System.out.println("to add, input '+', to subtract, input '-', to divide, input '/', to multiply, input '*' and to exit, input 'exit.'");
        System.out.println("----------------------------------------------------");
        System.out.print("Please enter a method here:");
        String method = scan.nextLine();
        if (method.equals("exit")){
            System.out.println("You chose to exit.");
            break;
        }
        else if (method.equals("+")){
            System.out.println("You chose to add.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
            double ans = fnum + snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else if(method.equals("-")){
            System.out.println("You chose to subtract.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
            double ans = fnum - snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else if(method.equals("*")){
            System.out.println("You chose to multiply.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
            double ans = fnum * snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else if(method.equals("/")){
            System.out.println("You chose to divide.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
            double ans = fnum / snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else{
            System.out.println("Invalid input. please select a valid input from the list of operators given.");
        }
    }
}
}

as you can see that it asks for a variable method to process the inputs fnum and snum

at the end, i added the else statement. it is supposed to notify the user when he/she isn't entering a valid method for the operation.

the problem is that whenever i execute this code, no matter what i input as the method it will first work perfectly, ask for the fnum and the snum and then output the answer but then, it also executes the block of code in the else statement.

how to prevent this? please help!

Also, this is the kind of output i get: (i put it in a code block just to make it look a bit more organised.)

to add, input '+', to subtract, input '-', to divide, input '/', to multiply, input '*'         and to exit, input 'exit.'
----------------------------------------------------
Please enter a method here:+
You chose to add.
Please enter first number here:10
Please enter second number here:10
The answer is20.0
to add, input '+', to subtract, input '-', to divide, input '/', to multiply, input '*'     and to exit, input 'exit.'
----------------------------------------------------
Please enter a method here:Invalid input. please select a valid input from the list of     operators given.
to add, input '+', to subtract, input '-', to divide, input '/', to multiply, input '*'   and to exit, input 'exit.'
----------------------------------------------------
Please enter a method here:

This question has been asked so many times. Take a look at [`Scanner issue when using nextLine after nextInt`](http://stackoverflow.com/questions/7056749/scanner-issue-when-using-nextline-after-nextint). — Eng.Fouad, Jul 27 '12 at 10:27

Pramod Kumar · Accepted Answer · 2012-07-27T10:39:30.937

use scan.nextLine() immediately after scan.nextInt().

whenever we enter a number and press Enter, scan.nextInt() reads only the number not the "end of line". When we do scan.nextLine(); it reads the "end of line(\n)" which is still in the buffer from the first input.

import java.util.Scanner;
class whileloop{
    public static void main(String args[]){
        Scanner scan = new Scanner(System.in);
    System.out.println("This is a basic calculator.");
    System.out.println("To exit, input exit when asked to input the method.");
    System.out.println("----------------------------------------------");
    while (true){
        System.out.println("to add, input '+', to subtract, input '-', to divide, input '/', to multiply, input '*' and to exit, input 'exit.'");
        System.out.println("----------------------------------------------------");
        System.out.print("Please enter a method here:");
        String method = scan.nextLine();
        if (method.equals("exit")){
            System.out.println("You chose to exit.");
            break;
        }
        else if (method.equals("+")){
            System.out.println("You chose to add.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
        scan.nextLine();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
        scan.nextLine();
            double ans = fnum + snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else if(method.equals("-")){
            System.out.println("You chose to subtract.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
        scan.nextLine();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
        scan.nextLine();
            double ans = fnum - snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else if(method.equals("*")){
            System.out.println("You chose to multiply.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
        scan.nextLine();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
        scan.nextLine();
            double ans = fnum * snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else if(method.equals("/")){
            System.out.println("You chose to divide.");
            System.out.print("Please enter first number here:");
            double fnum = scan.nextInt();
        scan.nextLine();
            System.out.print("Please enter second number here:");
            double snum = scan.nextInt();
        scan.nextLine();
            double ans = fnum / snum;
            System.out.print("The answer is");
            System.out.println(ans);
        }
        else{
            System.out.println("Invalid input. please select a valid input from the list of operators given.");
        }
    }
}
}

thanks a ton! but will you please tell me why this happens? i could memorize it easily if i know its cause. — Aayush Mahajan, Jul 27 '12 at 10:39
@user whenever we enter a number and press Enter, scan.nextInt() reads only the number not the "end of line". When we do scan.nextLine(); it reads the "end of line(\n)" which is still in the buffer from the first input. — Pramod Kumar, Jul 27 '12 at 10:40
ok! thanks a ton! this problem was literally eating my brain. — Aayush Mahajan, Jul 27 '12 at 10:42

score 1 · Answer 2 · answered Jul 27 '12 at 10:33

1

String method = scan.nextLine(); returns "" after the calculation. Since a blank string is not matched in any of the if statements else will be evaluated

answered Jul 27 '12 at 10:33

Jonas Adler

906
5
9

but why does this happen? haven't we stored its value in a variable? it shouldn't get empty untl we modify it in the next loop. – Aayush Mahajan Jul 27 '12 at 10:40

score 0 · Answer 3 · answered Jul 27 '12 at 10:40

This is because scan.nextLine() returns "" after the calculation. So you can just add this line .

     System.out.println("to add, input '+', to subtract, input '-', to divide, input '/', to multiply, input '*' and to exit, input 'exit.'");
     System.out.println("----------------------------------------------------");
     System.out.print("Please enter a method here:");        
     String method = scan.nextLine();
     // add this line
     if (method.equals(""))
     {
        continue; // no "" will be there 
     }

i am having problems with java "if statement"

3 Answers3