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      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

User · Answer

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;
  }
```

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

It helps to identify classname conflicts  two classes in different packages that have the same name   This can be masked with the   import  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  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  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

User · Answer

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

User · Answer

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

User · Answer

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    You have an outright naming conflict between java awt Event and com mycompany calendar Event  and so you can t even compile  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  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

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

Examples related to java

Examples related to import

Examples related to wildcard