what are the advantages of using Null object pattern?

[shreyas Kulkarni]

Can anyone please tell me what are the advantages of using Null object pattern? I tried to search too. I used it in my doubly linked list as first and last node(head and tail) but I am not able to figure out its use.
Thanks

[Andi Eska]

shreyas Kulkarni wrote:Can anyone please tell me what are the advantages of using Null object pattern? I tried to search too. I used it in my doubly linked list as first and last node(head and tail) but I am not able to figure out its use.
Thanks

Using the Null Object Pattern you can avoid null checks and NullPointerExceptions

The Null Object pattern is described in
Robert C. Martins book Agile Software Development, Principles, Patterns, and Practices

Here is the example - let's assume that you have an application that check whether the user is authenticated:

public class User {
  private String username;
  private boolean authenticated;
  // remainder omitted

  public boolean isAuthenticated() {
    return authenticated;
  }
  // remainder omitted
}

and the code that returns the reference to the User object looks like this:

public User getUser() {
  if (/*some condition*/) {
    return user;
  } else {
    return null;
  }
}

This way the code that checks whether our user is authenticated should look like the following snippet:

if (obj.getUser() != null && obj.getUser().isAuthenticated() {
  // allow
}
// remainder omitted

Checking whether the object is null is not only a boilerplate code but it can also give you a lot of bugs e.g. if you forget to check whether the object is null.

And here the Null Object can help you:

public class NullUser extends User {
  
  public static final NullUser INSTANCE = new NullUser();

  public static NullUser getInstance() {
    return INSTANCE;
  }

  @Override
  public boolean isAuthenticated() {
    return false;
  }

  private NullUser() {
  }
}

plus:

public User getUser() {
  if (/*some condition*/) {
    return user;
  } else {
    return NullUser.getInstance();
  }
}

plus cleaner client code:

if (obj.getUser().isAuthenticated() {
  // allow
}
// remainder omitted

andi