Object Orientation

Class Modifiers

Regular Classes:

• Access Modifiers: public and default only
• Non access: strictfp, abstract and final only
• Adding any other modifier will result in a compiler error.

• A final class cannot be overriden (e.g. String)
• An abstract class cannot be instantiated (e.g. DateFormat)

Class Members

• Members can use all the access modifiers: public, private, default(package), protected
• Private members are not inherited.
• This means private members can be redeclared in subclasses.
• Instance level variables cannot be synchronized, abstract, strictfp, native.

Protected

• Protected is wider than package. Protected = package + subclasses.
• Only accessible through inheritance in the subclass.
• The subclass cannot use a reference to super to access the protected member.

package pkg1;

public class Foo {
  protected String str = "Hello";
}

package pkg2;

public class Bar extends Foo {
  public void go() {
     Foo f = new Foo();
     String s = f.str; //Won't work ! Compiler error
     System.out.println(str); //Will work - through inheritance
  }
}

package pkg2;

public class Fubar {
   public void goo() {
        Bar b = new Bar();
        System.out.println(b.str); //wont'w work - str only accessible through inheritance
   }
}

Abstract Methods

As they go against the logic of overriding :

• abstract methods cannot be static.
• abstract methods cannot be private.