尝试编译简单的Java程序时出现NumberFormatException

Question

This is my code:

public class RetailMarket {
    public static void main(String[] args) throws IOException {
        int i,ch1,ch2,type,total=0,kg=0;
        System.out.println("What would you like to buy::");
        System.out.println("(1)Grains  (2)Oil");
        BufferedReader br1 = new BufferedReader(new InputStreamReader(System.in));
        ch1 = Integer.parseInt(br1.readLine()); // exception thrown here
    }
}

Every time I run it, this is my output:

What would you like to buy::
(1)Grains  (2)Oil
Exception in thread "main" java.lang.NumberFormatException: null
    at java.lang.Integer.parseInt(Integer.java:454)
    at java.lang.Integer.parseInt(Integer.java:527)
    at RetailMarket.main(RetailMarket.java:16)

I am confused as to why this NumberFormatException occurred; could anyone explain why?

Answer 1

You are passing null to Integer.parseInt() . This means that br1.readLine() is returning null . The only way this happens is if the end of stream has been reached. This means that you have somehow managed to not initialise System.in properly. Try running the program from the command line with java RetailMarket in the same directory as the compiled class. This code works fine when I run it.

Also, you should consider catching NumberFormatException and keep the program running if the input is invalid eg

try {
    ch1 = Integer.parseInt(br1.readLine());
} catch (NumberFormatException e) {
    // invalid input
}

Answer 2

You'll get that exception if br1.readLine() cannot be parsed as an int. You should either validate that this String contains only digits, or you should catch the exception.

Answer 3

No need to use BufferedReader, use a Scanner and read the Integer direcly without having to parse.

Try this :

 public class RetailMarket{

 public static void main(String[] args) throws IOException {
 int i,ch1,ch2,type,total=0,kg=0;
 System.out.println("What would you like to buy::");
 System.out.println("(1)Grains  (2)Oil");
 Scanner sc = new Scanner(System.in); 
 ch1 = sc.nextInt();
      }
   }