Seminar on java
- 2. INHERITANCE
- 3. THE MECHANISM OF DERIVING A NEW CLASS FROM AN OLD ONE IS CALLED INHERITANCE.
- 4. •THE OLD CLASS IS KNOWN AS BASE CLASS OR SUPER CLASS OR PARENT CLASS THE NEW ONE IS CALLED THE SUB CLASS OR DERIVED CLASS OR CHILD CLASS
- 6. SINGLE INHERITANCE •ONLY ONE BASE CLASS
- 7. MULTILEVEL INHERITANCE GRAND FATHER SUPER CLASS FATHER INTERMEDIATE SUPERCLASS CHILD SUB CLASS
- 8. A DERIVED CLASS WITH MULTILEVEL BASE CLASSES IS DECLARED AS FOLLOWS CLASS A { ………… ………… } CLASS B extends A // first level { …………. …………. } CLASS C //second level { …………. …………. }
- 10. EXAMPLE
- 11. HERE CERTAIN FEATURES OF ONE LEVEL ARE SHARED BY MANY OTHER BELOW THE LEVEL.
- 13. OVERRIDING CAN BE DONE BY DEFINING A METHOD IN THE SUB CLASS THAT HAS THE SAME NAME,SAME ARGUMENTS AND SAME RETURN TYPE AS A METHOD IN THE SUPER CLASS. THIS IS KNOWN AS OVERRIDING.
- 14. FINALIZER METHODS FINILIZATION IS JUST OPPOSITE TO INITIALASATION. JAVA RUN-TIME IS AN AUTOMATIC GARBAGE COLLECTING SYSTEM. IT FREES THE MEMORY RESOURCES USED BY OBJECTS. BUT OBJECTS MAY HOLD OTHER NON- OBJECT RESOURCES SUCH AS FILE DESCRIPTORS OR WINDOW SYSTEM FONTS.
- 15. THE GARBAGE COLLECTOR CANNOT FREE THESE RESOURCES. IN ORDER TO FREE THIS WE USE FINALIZER METHOD. THIS IS SIMILAR TO DESTRUCTORS. THE FINALIZER METHOD IS SIMPLY FINALIZE()
- 16. ABSTRACT METHODS FINALIZER HELPS US NOT TO RE- DEFINE THE METHOD IN SUB CLASS. BUT TO DO THE OPPOSITE, i.e. REDEFINE THE METHOD IN SUB CLASS,WE USE THE MODIFIER KEYWORD abstract IN THE METHOD DEFINITION
- 17. EXAMPLE abstract class shape { …………. …………. abstract void draw(); …………. …………. }
- 18. VISIBILITY CONTROL IT IS ALSO KNOWN AS ACCESS MODIFIERS.THIS PROVIDES THREE TYPES OF MODIFIERS: PUBLIC PRIVATE PROTECTED
- 19. By simply declaring the variable or method as public it is visible to entire class in which it is defined. A variable or method which is declared as public has the widest possible visibility and accessible everywhere.
- 21. FRIENDLY ACCESS • When no access modifier is specified,the member defaults to a limited version of public accessibility known as “friendly” level of access. • this makes fields visible only in the same package,but not in other packages.
- 22. The visibility lies in between the public access and friendly access. The protected modifier makes the fields visible not only to all classes and sub classes in the same package but also to sub classes in other packages.
- 23. PRIVATE ACCESS They enjoy the highest degree of protection. They are accessible only with their own class. They cannot be accessed and inherited by sub classes. The method declared as private behaves like a method declared as final. It prevents the method from being sub classed.
- 24. A field can be declared with two key words private and protected together like: Private protected int codenumber; the visibility level is between the “protected” access and private access. This makes the fields visible in all subclasses regardless of what package they are in
- 25. USE PUBLIC IF THE FIELD IS TO BE VISIBLE EVERYWHERE. USE PROTECTED IF THE FIELD IS TO BE VISIBLE EVERYWHERE IN THE CURRENT PACKAGE AND ALSO SUBCLASSES IN OTHER PACKAGES. USE “DEFAULT” IF THE FIELD IS TO BE VISIBLE EVERYWHERE IN THE CURRENT PACKAGE ONLY. USE PRIVATE PROTECTED IF THE FIELD IS
- 27. thank you