How do i ignore invalid samples?

Question

In my application I read numbers off of a text file. What can i do to my code so that it ignores invalid data e.g negative numbers or letters
import java.io.*;
import java.net.*;
import java.util.StringTokenizer;
public class SurveyApp {
     public static void main(String[] args)     {
     try{
          String urlName = "";
          switch (args.length) {
          case 0: urlName = getURL();
          break;
          case 1: urlName = args[0];
     URL url = new URL(urlName);
     URLConnection connection = url.openConnection();
     connection.connect();
     BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
     String line;
     line=in.readLine();
     int noOfPeople = Integer.parseInt(line);
     People people = new People(noOfPeople);
     while ((line = in.readLine()) != null)     {
     StringTokenizer st = new StringTokenizer(line,",");
     while (st.hasMoreTokens())     {  
     int age = Integer.parseInt( st.nextToken().trim() );
     float height = Float.parseFloat( st.nextToken().trim() );
     float weight = Float.parseFloat( st.nextToken().trim() );
     Person p = new Person(age, height, weight);
     people.add(p);
          if (line != null) System.out.println(". . .");
          System.out.println("The file used to supply the data was " +urlName);
          System.out.println("The total number of people surveyed was "+noOfPeople);
          System.out.println("The average age was "+people.mediumAge());
          System.out.println("The average height was "+people.mediumHeight());
          System.out.println("The average weight was "+people.mediumWeight());
          System.out.println();
     catch (FileNotFoundException e){
     System.err.println("This file cannot be accessed");
     catch (IOException e){
          System.err.println("Error: " + e);
     private static String getURL () throws IOException {
          urlInfo();
          String input;
          BufferedReader kbd = new BufferedReader(new InputStreamReader(System.in));
          System.out.println();
          System.out.print("Please enter a valid URL: ");
          input = kbd.readLine().trim();
          return input;     
private static void urlInfo () {
System.out.println();
System.out.println("The URL should be of the format <protocl:resource>");
System.out.println("Possible protocols are http: ftp: file:");
System.out.println("E.g. for a web page: http://java.sun.com");
System.out.println(" for a local file: file:c:/dir1/dir2/file.ext");
} // end urlInfo function
class People {
     private Person[] thePeople;
     private int noOfPeople;
     public People (int size) {
     thePeople = new Person[size];
     noOfPeople = 0;
     public void add (Person p) {
     if (noOfPeople < thePeople.length) {
     thePeople[noOfPeople] = p;
     noOfPeople++;
     public double mediumAge(){          
     int sumAge = 0;
     int numPeople = 0;
     for (int j = 0; j < thePeople.length; ++j)     {     
     if (thePeople[j] == null) continue;
     ++numPeople;
     sumAge += thePeople[j].Age();
     return ( (double) sumAge / numPeople);
     public double mediumHeight(){
     float sumHeight = 0;
     int numPeople = 0;
     for (int j = 0; j < thePeople.length; ++j)     {     
     if (thePeople[j] == null) continue;
     ++numPeople;
     sumHeight += thePeople[j].Height();
     return ( (double) sumHeight / numPeople);
     public double mediumWeight(){
     float sumWeight = 0;
     int numPeople = 0;
     for (int j = 0; j < thePeople.length; ++j)     {     
     if (thePeople[j] == null) continue;
     ++numPeople;
     sumWeight += thePeople[j].Weight();
     return ( (double) sumWeight / numPeople);
class Person {
     private int age;          
     private float height;          
     private float weight;          
     public Person (int a, float h, float w) {
     age = a; height = h; weight =w;
     public Person (Person p) {
     age = p.age; height=p.height;
     weight=p.weight;
     public int Age()
     { return age; }
     public float Height()
     { return height; }
     public float Weight()
     { return weight; }
     public void print() {
     System.out.println("Age: "+age);
     System.out.println("Height: "+height);
     System.out.println("Weight : "+weight);

Answer 1

I'm assuming you're talking about the following lines?
int age = Integer.parseInt( st.nextToken().trim() );
float height = Float.parseFloat( st.nextToken().trim() );
float weight = Float.parseFloat( st.nextToken().trim() );
Put a try-catch block around each. If/when you parse something with a letter, you'll get an error. After you determine that you have a number, then you can test it:
if(age < 0)
  System.err.println("Age is not valid "+age);
}