Why is using a wild card with a Java import statement bad?

Question

It is much more convenient and cleaner to use a single statement like

import java.awt.*;

then to import a bunch of individual classes

import java.awt.Panel;
import java.awt.Graphics;
import java.awt.Canvas;
...

What is wrong with using a wild card in the import statement?

Answer 1

The only problem with it is that it clutters your local namespace. For example, let's say that you're writing a Swing app, and so need java.awt.Event, and are also interfacing with the campany's calendaring system, which has com.mycompany.calendar.Event. If you import both using the wildcard method, one of two things happens:

1. You have an outright naming conflict between java.awt.Event and com.mycompany.calendar.Event, and so you can't even compile
2. You actually manage only to import one (only one of your two imports does .*), but it's the wrong one, and you struggle to figure out why your code is claiming the type is wrong
3. In actuality, when you start your code, there is no com.mycompany.calendar.Event, but when they later add one, your previously valid code suddenly stops working (thanks to @Stanchfield for pointing that out)

The advantage of explicitly listing all imports is that I can tell at a glance which class you meant to use, which simply makes reading the code that much easier. If you're just doing a quick one-off thing, there's nothing explicitly wrong, but future maintainers will thank you for your clarity otherwise.

Answer 2

I prefer specific imports, because it allows me to see all the external references used in the file without looking at the whole file. (Yes, I know it won't necessarily show fully qualified references. But I avoid them whenever possible.)

Answer 3

It clutters your namespace, requiring you to fully specify any classnames that are ambiguous. The most common occurence of this is with:

import java.util.*;
import java.awt.*;

...
List blah; // Ambiguous, needs to be qualified.

It also helps make your dependencies concrete, as all of your dependencies are listed at the top of the file.

Answer 4

1. It helps to identify classname conflicts: two classes in different packages that have the same name. This can be masked with the * import.
2. It makes dependencies explicit, so that anyone who has to read your code later knows what you meant to import and what you didn't mean to import.
3. It can make some compilation faster because the compiler doesn't have to search the whole package to identify depdencies, though this is usually not a huge deal with modern compilers.
4. The inconvenient aspects of explicit imports are minimized with modern IDEs. Most IDEs allow you to collapse the import section so it's not in the way, automatically populate imports when needed, and automatically identify unused imports to help clean them up.

Most places I've worked that use any significant amount of Java make explicit imports part of the coding standard. I sometimes still use * for quick prototyping and then expand the import lists (some IDEs will do this for you as well) when productizing the code.

Answer 5

Here's a vote for star imports. An import statement is intended to import a package, not a class. It is much cleaner to import entire packages; the issues identified here (e.g. java.sql.Date vs java.util.Date) are easily remedied by other means, not really addressed by specific imports and certainly do not justify insanely pedantic imports on all classes. There is nothing more disconcerting than opening a source file and having to page through 100 import statements.

Doing specific imports makes refactoring more difficult; if you remove/rename a class, you need to remove all of its specific imports. If you switch an implementation to a different class in the same package, you have to go fix the imports. While these extra steps can be automated, they are really productivity hits for no real gain.

If Eclipse didn't do class imports by default, everyone would still be doing star imports. I'm sorry, but there's really no rational justification for doing specific imports.

Here's how to deal with class conflicts:

import java.sql.*;
import java.util.*;
import java.sql.Date;

Answer 6

please see my article Import on Demand is Evil

In short, the biggest problem is that your code can break when a class is added to a package you import. For example:

import java.awt.*;
import java.util.*;

// ...

List list;

In Java 1.1, this was fine; List was found in java.awt and there was no conflict.

Now suppose you check in your perfectly working code, and a year later someone else brings it out to edit it, and is using Java 1.2.

Java 1.2 added an interface named List to java.util. BOOM! Conflict. The perfectly working code no longer works.

This is an EVIL language feature. There is NO reason that code should stop compiling just because a type is added to a package...

In addition, it makes it difficult for a reader to determine which "Foo" you're using.

Answer 7

In a previous project I found that changing from *-imports to specific imports reduced compilation time by half (from about 10 minutes to about 5 minutes). The *-import makes the compiler search each of the packages listed for a class matching the one you used. While this time can be small, it adds up for large projects.

A side affect of the *-import was that developers would copy and paste common import lines rather than think about what they needed.

Answer 8

There is nothing wrong with using the wildcard import statement.

In Clean Code, Robert C. Martin makes a actually recommends using them to avoid long import lists.

Here is the recommendation:

J1: Avoid Long Import Lists by Using Wildcards

If you use two or more classes from a package, then import the whole package with

import package.*;

Long lists of imports are daunting to the reader. We don’t want to clutter up the tops of our modules with 80 lines of imports. Rather we want the imports to be a concise statement about which packages we collaborate with.

Specific imports are hard dependencies, whereas wildcard imports are not. If you specifically import a class, then that class must exist. But if you import a package with a wildcard, no particular classes need to exist. The import statement simply adds the package to the search path when hunting for names. So no true dependency is created by such imports, and they therefore serve to keep our modules less coupled.

There are times when the long list of specific imports can be useful. For example, if you are dealing with legacy code and you want to find out what classes you need to build mocks and stubs for, you can walk down the list of specific imports to find out the true qualified names of all those classes and then put the appropriate stubs in place. However, this use for specific imports is very rare. Furthermore, most modern IDEs will allow you to convert the wildcarded imports to a list of specific imports with a single command. So even in the legacy case it’s better to import wildcards.

Wildcard imports can sometimes cause name conflicts and ambiguities. Two classes with the same name, but in different packages, will need to be specifically imported, or at least specifically qualified when used. This can be a nuisance but is rare enough that using wildcard imports is still generally better than specific imports.