java BigInteger manipulation

Question

The following code is for debugging:

public static void main(String[] args) {
    BigInteger n = new BigInteger("10000000001");
    String sn = n.toString();
    char[] array = sn.toCharArray();
    //intend to change value of some char in array
    //not standard math operation
    BigInteger result = new BigInteger(array.toString());
}

It gives me error:

    Exception in thread "main" java.lang.NumberFormatException: For input string: "[C@86c347"
        at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
        at java.lang.Integer.parseInt(Integer.java:449)
        at java.math.BigInteger.<init>(BigInteger.java:316)
        at java.math.BigInteger.<init>(BigInteger.java:451)
        at debug.main(debug.java:14)

But it was working fine, until this case, I'm not quite sure what went wrong.

Try to print array.toString() since it seems to be an invalid input string for the BigInteger constructor. — effeffe
– effeffe, Commented Apr 13, 2013 at 16:15
it was working fine, until this case could you show some other cases where it worked fine? — Pshemo
– Pshemo, Commented Apr 13, 2013 at 16:15
I swear it worked for a while..thank you for all of your efforts, it also puzzled me that somehow it worked for a few inputs... — Mzq
– Mzq, Commented Apr 13, 2013 at 16:24

Jon Skeet · Accepted Answer · 2013-04-13 16:14:34Z

4

When in doubt, add more diagnostics... taking out statements which do two things. This:

array.toString()

won't do what you expect it to, because arrays don't override toString(). It'll be returning something like "[C@89ffb18". You can see this by extracting an extra local variable:

BigInteger n = new BigInteger("10000000001");
String sn = n.toString();
char[] array = sn.toCharArray();
String text = array.toString();
BigInteger result = new BigInteger(text);

Then in the debugger you should easily be able to look at the value of text before the call to BigInteger - which would show the problem quite clearly.

To convert a char array to a string containing those characters, you want:

new String(array)

answered Apr 13, 2013 at 16:14

Jon Skeet

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