Don’t import wildcards from non-standard Java packages

Thursday, 22 July 2004

Import statements provide good documentation, but not if they contain wildcards. For example:

import com.zikzak.db.*;
import com.zikzak.util.*;

   ...code...

WidgetId = new WidgetId();

There’s no way to find out what package the WidgetId class belongs to without searching through the source tree. Much better to do this instead:

import com.zikzak.db.Table;
import com.zikzak.util.WidgetId;

   ...code...

WidgetId = new WidgetId();

Now it’s clear what and where WidgetId is.

This is not such a problem with well-known packages. For example, everybody knows what classes are in the java.util package, so this probably won’t cause any confusion:

import java.util.*;

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2 comments

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  1. Gary Nair
    25 July 2008 at 5:43 am

    It appears as if, an explicit import without the wildcard is now REQUIRED for user defined classes(not using JAR files here). Why ? who knows ? Maybe SUN changed stuff in jdk 6.X

    So here:
    Even if there is no ambiguity and you reference WidgetId, as a wildcard

    import com.zikzak.db.*;
    …code…
    //javac: won’t find the package //com.zikzak.db even if correctly written in //the javac -classpath methodology
    WidgetId = new WidgetId();//FAILS

    this will not compile(used javac -classpath properly).
    I am able to use 2 workarounds(still not using JAR methodology):

    1) You must restate the import as
    import com.zikzak.db.WidgetId;
    WidgetId = new WidgetId();//works

    OR explicitly in the instantiation declaration, as:

    2. //doesn’t make a difference now, if you //import it or not
    com.zikzak.db.WidgetId =
    new com.zikzak.db.WidgetId();

    I am compiling using the java compiler javac -classpath switch and it rejects any wildcard references to properly packaged up user defined class files, unless I did one of the above workarounds. The fact that these code workarounds actually work, is sufficient to let you know, there’s nothing wrong with my syntax of using the javac -classpath switch as they work, the moment I explicitly put the imports in the source code. Unfortunately, that also means you can’t use wild cards, on several classes belonging to your user defined package.

  2. Amanda
    10 July 2010 at 4:15 am

    Thank you. I looked EVRYWHERE to find out why my wildcard wasn’t working.

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