How does Java import work

Question

I would like to know how the import statement works   I m asking this because I have the following imports in my project   import static com googlecode javacv jna highgui cvCreateCameraCapture  import static com googlecode javacv jna highgui cvGrabFrame  import static com googlecode javacv jna highgui cvReleaseCapture  import com googlecode javacv CanvasFrame  import com googlecode javacv FrameGrabber  import com colorfulwolf webcamapplet gui ImagePanel  import com googlecode javacv OpenCVFrameGrabber  import com googlecode javacv jna cxcore IplImage    I don t have these packages in my project  so  how will this be imported   If I create a JAR file with all my classes  my server where will host this JAR file  has to be free Internet access to get these package   I got some problem in my Applet that has these imports  and I m asking this question  to understand if can be an Internet rule    lt applet code  com colorfulwolf webcamapplet WebcamApplet  archive  http   san redenetimoveis com teste jar  http   san redenetimoveis com core jar  http   san redenetimoveis com javacv jar  http   san redenetimoveis com javase jar  http   san redenetimoveis com jna jar  http   san redenetimoveis com customizer jar  http   san redenetimoveis com jmf jar  http   san redenetimoveis com mediaplayer jar  http   san redenetimoveis com multiplayer jar  http   san redenetimoveis com sound jar      height  550  width  550  gt   lt  applet gt

User · Accepted Answer

In dynamic languages  when the interpreter imports  it simply reads a file and evaluates it   In C  external libraries are located by the linker at compile time to build the final object if the library is statically compiled  while for dynamic libraries a smaller version of the linker is called at runtime which remaps addresses and so makes code in the library available to the executable   In Java  import is simply used by the compiler to let you name your classes by their unqualified name  let s say String instead of java lang String  You don t really need to import java lang   because the compiler does it by default  However this mechanism is just to save you some typing  Types in Java are fully qualified class names  so a String is really a java lang String object when the code is run  Packages are intended to prevent name clashes and allow two classes to have the same simple name  instead of relying on the old C convention of prefixing types like this  java lang String  This is called namespacing   BTW  in Java there s the static import construct  which allows to further save typing if you use lots of constants from a certain class  In a compilation unit  a  java file  which declares  import static java lang Math      you can use the constant PI in your code  instead of referencing it through Math PI  and the method cos   instead of Math cos    So for example you can write  double r   cos PI   theta     Once you understand that classes are always referenced by their fully qualified name in the final bytecode  you must understand how the class code is actually loaded  This happens the first time an object of that class is created  or the first time a static member of the class is accessed  At this time  the ClassLoader tries to locate the class and instantiate it  If it can t find the class a NoClassDefFoundError is thrown  or a a ClassNotFoundException if the class is searched programmatically   To locate the class  the ClassLoader usually checks the paths listed in the  CLASSPATH environment variable   To solve your problem  it seems you need an applet element like this   lt applet   codebase    http   san redenetimoveis com    archive  test jar  core jar    code  com colorfulwolf webcamapplet WebcamApplet          width  550  height  550   gt    BTW  you don t need to import the archives in the standard JRE

User · Answer

Import in Java does not work at all  as it is evaluated at compile time only   Treat it as shortcuts so you do not have to write fully qualified class names   At runtime there is no import at all  just FQCNs   At runtime it is necessary that all classes you have referenced can be found by classloaders   classloader infrastructure is sometimes dark magic and highly dependent on environment   In case of an applet you will have to rig up your HTML tag properly and also provide necessary JAR archives on your server   PS  Matching at runtime is done via qualified class names - class found under this name is not necessarily the same or compatible with class you have compiled against

User · Answer

Java s import statement is pure syntactical sugar  import is only evaluated at compile time to indicate to the compiler where to find the names in the code   You may live without any import statement when you always specify the full qualified name of classes  Like this line needs no import statement at all   javax swing JButton but   new  javax swing JButton      The import statement will make your code more readable like this   import javax swing     JButton but   new JButton

User · Answer

javac  or java during runtime  looks for the classes being imported in the classpath  If they are not there in the classpath then classnotfound exceptions are thrown   classpath is just like the path variable in a shell  which is used by the shell to find a command or executable   Entire directories or individual jar files can be put in the classpath  Also  yes a classpath can perhaps include a path which is not local but is somewhere on the internet  Please read more about classpath to resolve your doubts

User · Answer

The classes which you are importing have to be on the classpath  So either the users of your Applet have to have the libraries in the right place or you simply provide those libraries by including them in your jar file  For example like this  Easiest way to merge a release into one JAR file

[java] How does Java import work?

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