---

The "default" access specifier: package access

---

• Review of access specifiers
  • public

    public members (variables and methods) are accessible in every method of every class I.e.: the world.

    ---

  • private

    private members (variables and methods) are accessible in methods in the defining class I.e.: the class where the member is defined.

  ---

  ---

  ---

  ---

  ---

  ---

• The default (= omitted) access specifier
  • Default access specifier:

    If an instance member (variable or method) or a class member (variable or method) is defined without any access specfier, the member has the default access specifier

    ---

  • Example:

    package myLib; public class ClassA { /* ---------------------------------------- Default (no access specfier) access ---------------------------------------- */ static void Method81() { System.out.println( "*** This is (no specifier) Method81() of ClassA in PACKAGE myLib"); } }

    ---

    ---

  • Meaning of the default access speficier:

    default access specifier (no specifier used): package access default (package) access members are accessible by statement in methods that are contained inside: the defining class (= the class that contains the variable or method with default access) Any class inside the same package as the defining class

    ---

    ---

    ---

  • Experiment 1:

    Show accessing Method81() from a class that is not inside package myLib is not permitted

    We will set up the following scenario:

    The method main() in myProg1.java is not inside package myLib:

    /* NOT a class inside package myLib */ import myLib.*; public class myProg1 { public static void main(String[] args) { ClassA.Method81(); // Try to invoke the PACKAGE access method (fail) } }

    Result: compile error

    >>> javac myProg1.java myProg1.java:9: Method81() is not public in myLib.ClassA; cannot be accessed from outside package ClassA.Method81(); // Try to invoke the PACKAGE access method (fail) ^ 1 error

    ---

  • Example Program: (Demo above code)                                                
    • The myProg1.java Prog file: click here
    • The ClassA.java Prog file: click here

    How to run the program:

    mkdir ~/demo mkdir ~/demo/myLib Right click on link ClassA.java and save ClassA.java in the directory ~/demo/myLib. Right click on link myProg1.java and save myProg1.java in directory ~/demo. To compile ClassA.java:   (cd ~/demo; javac myLib/ClassA.java) To compile myProg1.java:   (cd ~/demo; javac myProg1.java) Replace /home/cs170000 with your own home directory !!! You will see the error message...

    Note:

    I am taking advantage of the fact that "." is the default search path (class path) The program file is inside /home/cs170000/demo The package myLib is inside the sub-directory myLib inside /home/cs170000/demo

    ---

    ---

    ---

  • Experiment 2:

    Show accessing Method81() from a class that is inside package myLib is permitted

    We will set up the following scenario:

    The method helper() inside ClassB is inside package myLib and has public access

    package myLib; public class ClassB { /* ------------------------------ PUBLIC access ------------------------------ */ public static void helper() { System.out.println( "*** This is PUBLIC helper() of ClassB in PACKAGE myLib"); System.out.println( "--- I will invoke the PACKAGE access Method81() in ClassA"); ClassA.Method81(); // invoke (default access) method ! } }

    The method main() will invoke ClassB.helper() (which in turn will invoke ClassA.Method81()):

    import myLib.*; public class myProg2 { public static void main(String[] args) { ClassB.helper(); } }

    Result:

    *** This is PUBLIC helper() of ClassB in PACKAGE myLib --- I will invoke the PACKAGE access Method81() in ClassA *** This is (no specifier) Method81() of ClassA in PACKAGE myLib

    This experiment shows that the method helper() (that is inside the same package) can invoke the method ClassA.Method81() !!

    ---

  • Example Program: (Demo above code)                                                
    • The myProg2.java Prog file: click here
    • The ClassA.java Prog file: click here
    • The ClassB.java Prog file: click here

    How to run the program:

    mkdir ~/demo mkdir ~/demo/myLib Right click on link ClassA.java and save ClassA.java in the directory /home/cs170000/demo/myLib. Right click on link ClassB.java and save ClassB.java in the directory /home/cs170000/demo/myLib. Right click on link myProg1.java and save myProg1.java in directory /home.cs170000/demo. To compile ClassA.java:   (cd ~/demo; javac myLib/ClassA.java) To compile ClassB.java:   (cd ~/demo; javac myLib/ClassB.java) To compile myProg1.java:   (cd ~/demo; javac myProg2.java) Replace /home/cs170000 with your own home directory !!! To run:          (cd ~/demo; java myProg2)