Specify Valid Characters in a String

Question

I'm a beginner at Java.
I need to create a program that will validate only binary characters in a string and count the number of 0s in the string. For some reason I keep getting an error message. For the life of me, I can't figure it out.
char a = '1';
char b = '0';
int hmo;
int totalzeroes=0;
String input = JOptionPane.showInputDialog(null, "Please Enter a Binary Number");
for (hmo=0; hmo<input.charAt(hmo);hmo++)
System.out.println(input.charAt(hmo));
totalzeroes++;
OUTPUT:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range:
at java.lang.String.charAt(Unknown Source)
at cop.main(cop.java:19)>

Answer 1

Just a suggestion.. for this kind of stuff you should probably use regular expressions.. You can easily validate that a string contains only 0 and 1 with something like this:
public boolean validate(String s)
     String PATTERN = "[01]+";
     Pattern pattern = Pattern.compile(PATTERN, Pattern.CANON_EQ);
     Matcher matcher = pattern.matcher(s);
     if (matcher.matches())
          return true;
     else
          return false;
Here, you can easily change the PATTERN if you want the string to match something else..