java - Checking if String entered is a binary numer, getting incorrect output -

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i trying write program check if user-entered string binary number, , if is, output number of 1s in number. had working fine integer value, since int can't hold more 2 billion or whatever max value is, trying rewrite work strings.

as of right now, number enter output "number entered not binary." , when enter 0, stringindexoutofboundsexception. novice programmer, forgive obvious errors may have missed, asking possible solution problem or push in right direction. here code (after trying make work strings rather integers):

    import java.util.*;      public class binaryhw {     public static void main(string[] args) {    scanner kb = new scanner(system.in);   system.out.println("enter binary number: ");   string bin = kb.nextline();    //the method used check whether or not user entered binary   //number requires changing value of 'bin'.   //therefore, 'origbin' set equal 'bin' later use.    string origbin = bin;   int count = 0;    boolean isbinary = true;   /* if bin = 0, loop skipped because     * 0 binary number , need not checked.    */    while (integer.parseint(bin) != 0) {       int lastdigit = bin.charat(bin.length() - 1);       if (lastdigit > 1) {         system.out.println("number entered not binary.");         isbinary = false;         break;       } else {         bin = bin.substring(bin.length() - 2);      }   }     //again, method used count 1s in bin number   //requires changing value of origbin, origbin2 introduced     string origbin2 = origbin;    (int = 0; < origbin.length(); i++) {       if (origbin.charat(origbin.length() - 1) == 1) {         count ++;         origbin2 = origbin.substring(origbin2.length() - 2);       } else {         origbin2 = origbin.substring(origbin2.length() - 2);       }    }      if (isbinary)      if (count == 1)      system.out.println("there "          + count + " 1 in binary number entered.");      else       system.out.println("there "          + count + " 1s in binary number entered.");       }  }

i think overcomplicating things... iterate through binary string, , keep track of number of 1's reached. if number other 0 or 1 found, report input non-binary number. below snippet accomplishes this:

public static void main(string[] args) {        scanner kb = new scanner(system.in);       system.out.println("enter binary number: ");       string bin = kb.nextline();       int onecount = 0;       boolean validbinarynum = true;        (int = 0; < bin.length() && validbinarynum; i++) {           char currentchar = bin.charat(i);           if (currentchar == '1') onecount++;           else if (currentchar != '0') {               validbinarynum = false;           }       }        if (validbinarynum && bin.length() != 0) {           system.out.println("number of 1's: " + onecount);       } else {           system.out.println("number not binary");       }  }

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