Example of NullPointerException cause by failure to instantiate objects

test.java

//
// An example of a NullPointerException caused by a not initializing objects before using them
//

import java.util.ArrayList;

class test {
  // An ArrayList is like a vector in c++
  // This ArrayList hold Strings
  public ArrayList<String> listOfStrings;

  public static void main(String[] args) {
    // Create a new instance of the test class
    test instanceOfTest = new test();

    // EXTREMELY IMPORTANT: Make sure to initialize objects before calling the onjects methods (such as add)
    // The code will compile and run with this line commented by will always throw a NullPointerException
    //instanceOfTest.listOfStrings = new ArrayList<String>();

    // Add a String to a Arraylist
    instanceOfTest.listOfStrings.add("test string");

    // Print out the ArrayList
    System.out.println(instanceOfTest.listOfStrings);
  }
}