Use @Nullable

To eliminate NullPointerExceptions in your codebase, you must be disciplined about null references. We've been successful at this by following and enforcing a simple rule:

Every parameter is non-null unless explicitly specified.

The Guava: Google Core Libraries for Java and JSR-305 have simple APIs to get a nulls under control. Preconditions.checkNotNull can be used to fast-fail if a null reference is found, and @Nullable can be used to annotate a parameter that permits the null value:

import static com.google.common.base.Preconditions.checkNotNull;
import static javax.annotation.Nullable;

public class Person {
  ...

  public Person(String firstName, String lastName, @Nullable Phone phone) {
    this.firstName = checkNotNull(firstName, "firstName");
    this.lastName = checkNotNull(lastName, "lastName");
    this.phone = phone;
  }

Guice forbids null by default. It will refuse to inject null, failing with a ProvisionException instead. If null is permissible by your class, you can annotate the field or parameter with @Nullable. Guice recognizes any @Nullable annotation, like edu.umd.cs.findbugs.annotations.Nullable or javax.annotation.Nullable.