Check chains of "get" calls for null

Question

Hi.

Let's say I'd like to perform the following command:

house.getFloor(0).getWall(WEST).getDoor().getDoorknob();

To avoid a NullPointerException, I'd have to do the following if:

if (house != null && house.getFloor(0) && house.getFloor(0).getWall(WEST) != null
  && house.getFloor(0).getWall(WEST).getDoor() != null) ...

Is there a way or an already existing Utils class that does this more elegantly, let's say something like the following?

checkForNull(house.getFloor(0).getWall(WEST).getDoor().getDoorknob());

Answer 1

How about try catch around it?

try
{
  house.getFloor(0).getWall(WEST).getDoor().getDoorknob();
}
catch (NullPointerException e)
{
  //something
}

Answer 2

No, only thing that You can is to insert that code in try catch block, and catch that exception.

try
{
  house.getFloor(0).getWall(WEST).getDoor().getDoorknob();
}
catch (NullPointerException e)
{

}

But this is not recommended solution.

Answer 3

You could of course simply wrap the whole expression up in a try-catch block, but that's a bad idea. Something cleaner is the Null Object pattern. With that, if your house doesn't have floor 0, it just returns a Floor that acts like a regular Floor, but has no real content; Floors, when asked for Walls they don't have, return similar "Null" Walls, etc, down the line.

Answer 4

The best way would be to avoid the chain. If you aren't familiar with the Law of Demeter (LoD), in my opinion you should. You've given a perfect example of a message chain that is overly intimate with classes that it has no business knowing anything about.

Law of Demeter: http://en.wikipedia.org/wiki/Law_of_Demeter

Answer 5

Make sure things that can't logically be null are not. For example - a house always has a West wall. In order to avoid such exceptions in state, you can have methods to check whether the state you expect is present:

if (wall.hasDoor()) {
   wall.getDoor().etc();
}

This is essentially a null-check, but might not always be.

The point is that you should do something in case you have a null. For example - return or throw an IllegalStateException

And what you shouldn't do - don't catch NullPointerException. Runtime exceptions are not for catching - it is not expected that you can recover from them, nor it is a good practice to rely on exceptions for the logic flow. Imagine that you actually don't expect something to be null, and you catch (and log) a NullPointerException. This will not be very useful information, since many things can be null at that point.

Answer 6

There is no checkForNull method that you can write that will facilitate this (that's simply not how method invokation and argument evaluation works in Java).

You can break down the chained statements into multiple statements, checking at every step. However, perhaps a better solution is to not have these methods return null in the first place. There is something called the Null Object Pattern that you may want to use instead.

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