Question

I’d like to know if using NULL as a key in a Map object is considered good style and if not what are the alternatives?

Answer 1

This is generally not considered good style.

Since NULL typically indicates an undetermined or unset value, it is generally confusing to use as a key to a map. (although, there may be specific circumstances where it makes sense)

The alternatives depend on the situation, so let me use an example. Let’s say that we want color specific strings in a text, and we also want a default color for text which doesn’t match any of the strings.

private HashMap<String,Color> stringColors;
private Color defaultColor;

Rather than storing the default color in the HashMap using a NULL key, put the default color in the specific variable. This makes it really clear to anyone viewing the code exactly what this color means.

The driving factor, I would say, is if you will actually have a NULL value somewhere that can be directly looked up in the map. (this doesn’t happen often, in my experience) In this specific example I gave, the default color would be used for any strings which aren’t in the map. In this case, there isn’t a NULL string that you want the color for.