97 Things Every Java Programmer Should Know

Chapter 32. How to Avoid Null

Carlos Obregón

Tony Hoare calls null the “billion-dollar mistake.” It’s a mistake, and that’s why you should get in the habit of forbidding code from using null. If you have a reference to an object that might be null, you have to remember to do a null check before trying to call any method of it. But since there’s no obvious difference between a null reference and a non-null one, it’s too easy to forget and get a NullPointerException. 

The most future-proof way to avoid issues is to use an alternative when possible.

Avoid Initializing Variables to Null

It is usually not a good idea to declare a variable until you know what value it should hold. For complex initialization, move all the initialization logic to a method. For example, instead of doing this:

public String getEllipsifiedPageSummary(Path path) {
   String summary = null;
   Resource resource = this.resolver.resolve(path);
   if (resource.exists()) {
       ValueMap properties = resource.getProperties();
       summary = properties.get("summary");
   } else {
       summary = "";
   }
   return ellipsify(summary);
}

Do the following:

 public String getEllipsifiedPageSummary(Path path) { var summary = getPageSummary(path); return ellipsify(summary); } public String getPageSummary(Path path) { var resource = this.resolver.resolve(path); if (!resource.exists()) { return ""; } var properties = resource.getProperties(); return properties.get("summary"); ...