Java Classes

1. Java Classes How does it find them?

2. Basic Approach • Different from c++. • Uses header files for prototypes • uses INCLUDE variable to indicate location of .h files not in system directories • libraries are stored elsewhere and located by LIB variable • Java uses only the java source. • No prototypes • uses a more complex naming/search scheme • details follow

3. C++ #include “myclass.h” void main () { myclass x; ….. } public class myclass() { int a; int b; public: myclass(); ~myclass(); ….. } myclass.cpp How does c++ find the header files? .. Searching the “include” path. myclass.h

4. No “include” in Java! Only one file!

5. Java connects files with“package” and “import” • package • Groups class definitions together for privileged access, lets other classes in the package see more details • Generally a way of grouping a set of library routines • import • Very similar to include, but there is no include file counterpart • Identifies all packages OTHER THAN the one that class is in, from which this class needs access

6. Directory location is important • Classes in the same package are stored in directory hierarchies to help import find the files. • These directories also aid in resolution of ambiguous naming problems. • More later….

7. Two classes defined to be in package first package first; class classA { classB b = … new classB(); } classA.java Being declared in the same package will allow each class to access each other without extra work. classA using classB package first; classclassB { // most of your code } classB.java

8. Two classes in different packages! packagefirst; import second; class classA { classB b = … new classB(); } classA.java Being declared in the different package will require the use of import to access each other. packagesecond; class classB { // most of your code } classB.java

9. Package names • Can be dotted to show directory organization • Example • math.algebra.sets • math.calculus.integration • math.calculus.differentiation import math.*; or import math.algebra.sets; math algebra calculus differentiation integration sets

10. Example Format of a file package com.myplace.mygroup.myclass; // OPTIONAL import java.io.*; // OPTIONAL import anotherclass.mathlib.vectors; // OPTIONAL public class thisclass { // most of your code } So let’s examine this one element at a time.

11. package com.myplace.mygroup.myclass; // OPTIONAL import java.io.*; // OPTIONAL import anotherclass.mathlib.vectors; // OPTIONAL public class thisclass { // most of your code } • Provides a solution to the problem of generating a unique package name • Groups classes for more convenient use of related classes. • Defines the package in which a class is placed • Helps to define the directory in which the compiled file will be placed • Only ONE package statement per file because a class can only be in a single package. • If no package, class is considered to be in defaultpackage.

12. Unique package names • Lots of programmers in the world • Significant opportunity for redundant naming of classes • General principle -> namespaces • Here, classes are grouped in packages to provide a means to give uniqueness to name • two programmers name the routine x • com.mycompany.billy.io.x • com.mycompany.sam.io.x • x can be uniquely identified by qualification

13. Package name -> Directory name • A package name dictates the directory structure in which class is stored. • Does NOT indicate the complete path but a path that exists somewhere. • E.g., com.mycompany.billy.io.x / ? It DOES provide a unique name to use in accessing x! bin home com company sam billy io io x x

14. package com.myplace.mygroup.myclass; // OPTIONAL import java.io.*; // OPTIONAL import anotherclass.mathlib.vectors; // OPTIONAL public class thisclass { // most of your code } • Directly related to packages • Used to provide a shorthand • Used for accessing OTHER classes NOT related to the package statement (if it exists in the file) • CAN have import without package and vice versa. • While they are related, the use of the use of them in the SAME file is typically independent/coincidental • package defines the package THIS class is IN • import used to ease USE of classes in OTHER packages

15. import just makes is easier! • The names have a tendency to be long in order to be unique • import lets you use a shorthand for qualification. • Like “with” in pascal to avoid listing all substructures in a record to access names. • Example follows:

16. How to define the CLASSPATH Using a unix example, the code references “com..” In order to reference com.company.billy.io.x the CLASSPATH must include /dev, everything up to but not including com. / bin develop home com company sam billy io io x x CLASSPATH= /lib/java;/develop

17. What can one import access?import examples com // can’t access ANY classes import com; company // can’t access ANY classes import com.*; logo billy organization // can access // logo and organization import com.company.*; io x y // can access only x import com.company.billy.io.x; red - directories black -> classes

18. Which x? package com.myplace.mygroup.myclass; // OPTIONAL import com.mycompany.sam.io.x; public class thisclass { // most of your code x thisx = new x; } Here x can be referenced in shorthand and is uniquely defined to be the x in the io package defined by sam. This also emphasizes disconnect between the package in the package statement and the package in the import.

19. What about * in import? package com.myplace.mygroup.myclass; // OPTIONAL import com.mycompany.sam.io.*; public class thisclass { // most of your code x thisx = new x; y thisy = new y; } * indicates that you can shorthand any class in the package. In this example, y is considered to be in the same io package and can be accessed in the same manner as x (shorthand).

20. So how does java figure outEXACTLY where to put package? • We used a ‘?’ in directory to indicate it was not known where. • Determined at compile time • uses -d option to say where • E.g. • javac -d /project1/devstuff thisclass • creates the package and the com.mycompany… directory under /project/devstuff • /project/devstuff/com/mycompany/mygroup/...

21. Now how does it find the class when it compiles the program? • 1. Look in java system class directories • 2. Look in extensions directory (???) • 3. Look in CLASSPATH. • Environment variable • default is current directory • unix example: .:/project/dev/java:/home/dgame/611 • current directory first • then /project/dev/java • and lastly /home/dgame/611

22. Confusion • Search of path is documented in contradicting manner • O’reilly Java in a Nutshell: • “The interpreter always appends the location of the system classes to the end of the path specified by the environment variable.” • other references indicate it searches in the system first. • Regardless, there can be no naming ambiguity

23. What if there are 2 classes with the same name? • Compiler will alert you if there are multiple names in the path AND no means to resolve the ambiguity (import) is coded. • Using import resolves which one you are referencing. • If error message occurs, use more specific reference to the class.