为什么在 Java 导入语句中使用通配符不好？

Question

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

import java.awt.*;

than 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 wildcard 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 company's calendaring system, which has com.mycompany.calendar.Event . If you import both using the wildcard method, one of these three 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. When you compile your code there is no com.mycompany.calendar.Event , but when they later add one your previously valid code suddenly stops compiling.

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

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 (eg 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 specific 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 3

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 4

It's not bad to use a wild card with a Java import statement.

In Clean Code , Robert C. Martin 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.

Answer 5

Performance : No impact on performance as byte code is same. though it will lead to some compile overheads.

Compilation : on my personal machine, Compiling a blank class without importing anything takes 100 ms but same class when import java.* takes 170 ms.

Answer 6

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 7

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 8

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 9

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 10

In DDD book

In whatever development technology the implementation will be based on, look for ways of minimizing the work of refactoring MODULES . In Java, there is no escape from importing into individual classes, but you can at least import entire packages at a time, reflecting the intention that packages are highly cohesive units while simultaneously reducing the effort of changing package names.

And if it clutters local namespace its not your fault - blame the size of the package.

Answer 11

Here are the few things that I found regarding this topic.

• During compilation, the compiler tries to find classes that are used in the code from the .* import and the corresponding byte code will be generated by selecting the used classes from .* import. So the byte code of using .* import or .class names import will be same and the runtime performance will also be the same because of the same byte code.

• In each compilation, the compiler has to scan all the classes of .* package to match the classes that are actually used in the code. So, code with .* import takes more time during the compilation process as compared to using .class name imports.

• Using .* import helps to make code more cleaner

• Using .* import can create ambiguity when we use two classes of the same name from two different packages. Eg, Date is available in both packages.

   import java.util.*; import java.sql.*; public class DateDemo { private Date utilDate; private Date sqlDate; }

Answer 12

The most important one is that importing java.awt.* can make your program incompatible with a future Java version:

Suppose that you have a class named "ABC", you're using JDK 8 and you import java.util.* . Now, suppose that Java 9 comes out, and it has a new class in package java.util that by coincidence also happens to be called "ABC". Your program now will not compile on Java 9, because the compiler doesn't know if with the name "ABC" you mean your own class or the new class in java.awt .

You won't have that problem when you import only those classes explicitly from java.awt that you actually use.

Resources:

Java Imports

Answer 13

Among all the valid points made on both sides I haven't found my main reason to avoid the wildcard: I like to be able to read the code and know directly what every class is, or if it's definition isn't in the language or the file, where to find it. If more than one package is imported with * I have to go search every one of them to find a class I don't recognize. Readability is supreme, and I agree code should not require an IDE for reading it.

Answer 14

• There is no runtime impact, as compiler automatically replaces the * with concrete class names. If you decompile the .class file, you would never see import ...* .

• C# always uses * (implicitly) as you can only using package name. You can never specify the class name at all. Java introduces the feature after c#. (Java is so tricky in many aspects but it's beyond this topic).

• In Intellij Idea when you do "organize imports", it automatically replaces multiple imports of the same package with *. This is a mandantory feature as you can not turn it off (though you can increase the threshold).

• The case listed by the accepted reply is not valid. Without * you still got the same issue. You need specify the pakcage name in your code no matter you use * or not.

Answer 15

For the record: When you add an import, you are also indicating your dependencies.

You could see quickly what are the dependencies of files (excluding classes of the same namespace).

Answer 16

Forget about cluttered namespaces... And consider the poor soul who has to read and understand your code on GitHub, in vi, Notepad++, or some other non-IDE text editor.

That person has to painstakingly look up every token that comes from one of the wildcards against all the classes and references in each wildcarded scope... just to figure out what in the heck is going on.

If you're writing code for the compiler only - and you know what you're doing - I'm sure there's no problem with wildcards.

But if other people - including future you - want to quickly make sense of a particular code file on one reading, then explicit references help a lot.

Answer 17

Using wild card imports is not bad because Java tutorials by Oracle use wild card imports. I don't think that Java folks at Oracle would do wrong thing. Please look here: https://docs.oracle.com/javase/tutorial/uiswing/examples/components/CustomComboBoxDemoProject/src/components/CustomComboBoxDemo.java and here: https://docs.oracle.com/javase/tutorial/uiswing/examples/components/ .

Answer 18

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

If you're using an IDE (which you should be doing), and there are more code owners than just you, using wildcard imports is bad because it:

• conceals information from the rest of the team
• provides only false benefits (things which are better-solved using IDE functionality than with wildcard imports) to you as an individual

Most of the "use wildcards" proponents have a focus on the individual: I don't want to maintain the list, I don't want see the clutter, etc. Here are several of the common examples:

• maintenance is harder – when you want to introduce a new class into your source code, you have to manually add the import statement
• refactoring is more difficult – if code is moved around, then import statements have to be updated
• reduce clutter, tidy up file contents – goal here is something along the lines of "removing distractions"

These arguments were more convincing before IDEs did all of that automatically. If you're using a plain text editor instead of an IDE, then these arguments have some merit. But if you're using a plain text editor, you are already subjecting yourself to a number of other much more significant inefficiencies, and managing import statements is just one among many things that you should stop doing by hand. IDEs offer automatic management of imports, powerful refactoring tools, and folding (hiding) of any parts of the code you don't want to see.

For the "avoid wildcards" proponents, there are many examples, but I'll point out only one:

• clarity – specifically, when someone new enters the codebase. They will arrive with questions, and continue to discover new questions as they explore the code. For this new code contributor, wildcard import statements do not answer any questions, and at worst can produce confusion, misunderstanding, new questions. In contrast, with explicit imports (and using an IDE) the worst case is neutral: no new info provided; at best, it not only reduces ambiguity but it can also provide answers.

At the end of the day, it helps the entire team to reduce (albeit in a small way) code complexity, to reduce confusion, to add clarity.

Answer 19

Importing all the classes in a package is considered a blind approach. A major reason for this is that it clutters the class namespace and could lead to conflicts between classes in different packages with the same name.

Specifically populating the necessary classes avoids that problem and clearly shows which versions were wanted. This is good for code maintainability.