Determine whether it is a palindrome string?

Determine whether it is a palindrome string?

 public static boolean judge (String str) {
        // write code here
	if(str.equals(""))
		return true;
	str = str.toLowerCase();
	int start = 0;
	int end = str.length()-1;
	while(start<end) {
		while(!(str.charAt(start)>='a'&&str.charAt(start)<='z')||str.charAt(start)<='9'&&str.charAt(start)>='0') {
			start++;
		}
		while(!(str.charAt(end)>='a'&&str.charAt(end)<='z')||str.charAt(end)<='9'&&str.charAt(end)>='0') {
			end--;
		}
		if(str.charAt(start)!=str.charAt(end))
			return false;
			start++;
			end--;
	}
	return true;		
    }

}