how to get the values of parameterized variables from a config.properties in java?

I have a mvn project, I have the config.properties in the resources and I want to call it in the main

public class GetProperties {
    private Properties prop = new Properties();

    public GetProperties() {

    InputStream in = 
    getClass().getResourceAsStream("/config.properties");


try {
  prop.load(in);
} catch (FileNotFoundException e) {
  System.out.println("The property file was not found");
} catch (IOException e) {
  System.out.println("Can not read property file");
}
}

  public String getPropValues( String propertyName){
  return prop.getProperty(propertyName);
   }
  }

That's my GetProperties class, but it gives me Null Pointer Exception