How to convert BigInteger to String in java

Question

I converted a String to BigInteger as follows:

Scanner sc=new Scanner(System.in);
System.out.println("enter the message");
String msg=sc.next();
byte[] bytemsg=msg.getBytes();
BigInteger m=new BigInteger(bytemsg); 

Now I want my string back. I'm using m.toString() but that's giving me the desired result.

Why? Where is the bug and what can I do about it?

Answer 1

You want to use BigInteger.toByteArray()

String msg = "Hello there!";
BigInteger bi = new BigInteger(msg.getBytes());
System.out.println(new String(bi.toByteArray())); // prints "Hello there!"

The way I understand it is that you're doing the following transformations:

  String  -----------------> byte[] ------------------> BigInteger
          String.getBytes()         BigInteger(byte[])

And you want the reverse:

  BigInteger ------------------------> byte[] ------------------> String
             BigInteger.toByteArray()          String(byte[])

Note that you probably want to use overloads of String.getBytes() and String(byte[]) that specifies an explicit encoding, otherwise you may run into encoding issues.

Answer 2

Why don't you use the BigInteger(String) constructor ? That way, round-tripping via toString() should work fine.

(note also that your conversion to bytes doesn't explicitly specify a character-encoding and is platform-dependent - that could be source of grief further down the line)

Answer 3

Use m.toString() or String.valueOf(m). String.valueOf uses toString() but is null safe.

Answer 4

You can also use Java's implicit conversion:

BigInteger m = new BigInteger(bytemsg); 
String mStr = "" + m;  // mStr now contains string representation of m.

Answer 5

When constructing a BigInteger with a string, the string must be formatted as a decimal number. You cannot use letters, unless you specify a radix in the second argument, you can specify up to 36 in the radix. 36 will give you alphanumeric characters only [0-9,a-z], so if you use this, you will have no formatting. You can create: new BigInteger("ihavenospaces", 36) Then to convert back, use a .toString(36)

BUT TO KEEP FORMATTING: Use the byte[] method that a couple people mentioned. That will pack the data with formatting into the smallest size, and allow you to keep track of number of bytes easily

That should be perfect for an RSA public key crypto system example program, assuming you keep the number of bytes in the message smaller than the number of bytes of PQ

(I realize this thread is old)

Answer 6

To reverse

byte[] bytemsg=msg.getBytes(); 

you can use

String text = new String(bytemsg); 

using a BigInteger just complicates things, in fact it not clear why you want a byte[]. What are planing to do with the BigInteger or byte[]? What is the point?

Answer 7

http://java.sun.com/j2se/1.3/docs/api/java/lang/Object.html

every object has a toString() method in Java.

Answer 8

String input = "0101";
BigInteger x = new BigInteger ( input , 2 );
String output = x.toString(2);

Answer 9

//How to solve BigDecimal & BigInteger and return a String.

  BigDecimal x = new BigDecimal( a );
  BigDecimal y = new BigDecimal( b ); 
  BigDecimal result = BigDecimal.ZERO;
  BigDecimal result = x.add(y);
  return String.valueOf(result);