String to BigInteger in Java

Question

This is my code:

public class sample {
   public static void main(String []args) throws Exception {

      System.out.println("Enter from file:");

      BufferedReader br=new BufferedReader(new FileReader("C:\\Users\\KK\\A Key.txt"));

      String currentline;

      while((currentline=br.readLine())!=null){

         System.out.println(currentline);
      }

      BigInteger a = new BigInteger(currentline); 

      System.out.println(a);

  }

I want to read from the text document , convert it into big integer from a string, I tried this but i get a run time error , How to convert String into corresponding Big integer Ascii value.

If you are running into an Excpetion and are asking what to do, it wont harm to include the exception in the question. Please do so. — SomeJavaGuy, Aug 12 '16 at 07:40
The process is correct but I guess when you try to convert the String the value of currentline is null and you might be getting a NullPointerException there. — Piyush Bhasin, Aug 12 '16 at 07:41
@DurgpalSingh What is that supposed to mean? What constructor?! — GhostCat, Aug 12 '16 at 07:42
You are not appending to the currentline variable, and when loop exits currentline variable will have "null" value and I am assuming you are getting NullPointerException. Try to append with new StringBuilder and then you the BigInteger class. — zstring, Aug 12 '16 at 07:44
@DurgpalSingh: Krishna is *using* the constructor. It's right there in the code, big as life. — T.J. Crowder, Aug 12 '16 at 07:44
The clever thing here: he avoided the immediate "close as duplicate" to "whats a NPE and how do i fix it" by not posting his exception stack trace. — GhostCat, Aug 12 '16 at 07:50
Possible duplicate of [Java: How to use BigInteger?](http://stackoverflow.com/questions/1783912/java-how-to-use-biginteger) — AesSedai101, Aug 12 '16 at 08:35

GhostCat · Accepted Answer · 2016-08-12T07:52:21.717

Your problem is very simple: you build a BigInteger from a null string!

You are looping until currentLine is null.

Afterwards you try to create a BigInteger from that!

Simply move things into your loop:

while((currentLine=br.readLine())!=null) {           
 System.out.println(currentLine);
 BigInteger a = new BigInteger(currentline); 
 System.out.println(a);
}

Et voilà, things are working (assuming that you expect each line in your file to be a number). If the whole file represents one number, then you must do as zstring suggests in his comment: you would have to use a StringBuilder for example to "collect" all lines into a single string that you then use to create the BigInteger object.

And please note java coding styles: class names start Uppercase; and variable names use camelCase (thus I changed to currentLine).

But just for the record: you wrote a low quality question. You should always include compiler error messages or stack traces when asking "why is my code not working".

String to BigInteger in Java

1 Answers1