charAt cannot be resolved or is not a field. Did I missed some imports?

charAt cannot be resolved or is not a field. Did I missed some imports?

String word = JOptionPane.showInputDialog("Enter String");
for(int x = 0 ; x <= word.length() ; x++) {
for( ch = 'a' ; ch <= 'z' ; ch++) {
num++;
if(word.charAt[x].equalsIgnoreCase(ch)) {
int z += num;
num = 0;
}
}
}