Code samples for a quick demo on Java NullPointerException https://clsc.be/12

Demo.java

import java.util.ArrayList; 

public class Demo {
  private ArrayList<String> words; 
  
  public Demo( String[] theWords ) {
      for (String s: theWords) {
        words.add(s);  
      }
  }
  
  public static void main( String... args ) {
    Demo d = new Demo( "here are some words" ); 
    System.out.println( words ); 
  }
}

DemoFixed.java

import java.util.ArrayList; 

public class DemoFixed {
  private ArrayList<String> words; 
  
  public Demo( String[] theWords ) {
      words = new ArrayList<>();
      for (String s: theWords) {
        words.add(s);  
      }
  }
  
  public static void main( String... args ) {
    DemoFixed d = new DemoFixed( "here are some words" ); 
    System.out.println( words ); 
  }
}