I tend to follow the design of JDK libraries, especially Collections and Concurrency (Joshua Bloch, Doug Lea, those guys know how to design solid APIs). Anyway, many APIs in the JDK pro-actively throws NullPointerException.
For example, the Javadoc for Map.containsKey states:
@throws NullPointerException if the key is null and this map
does not permit null keys (optional).
It's perfectly valid to throw your own NPE. The convention is to include the parameter name which was null in the message of the exception.
The pattern goes:
public void someMethod(Object mustNotBeNull) {
if (mustNotBeNull == null) {
throw new NullPointerException("mustNotBeNull must not be null");
}
}
Whatever you do, don't allow a bad value to get set and throw an exception later when other code attempts to use it. That makes debugging a nightmare. You should always the follow the "fail-fast" principle.
