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Abstract Classes and Abstract Methods

  • abstract is a keyword.
  • abstract keyword can be used in two ways:
    • with classes
    • with methods
  • Classes defined without abstract modifier are called as concrete class. Concrete class can be instantiated.
  • Classes defined with abstract modifier are called as abstract class. Abstract class cannot be instantiated but can be subclassed.
  • Methods defined without abstract modifier are called as concrete method. Concrete method must have body.
  • Methods defined with abstract modifier are called as abstract method. Abstract method must not have body.
  • Abstract class can be used when you want to define default behavior for subclasses.

Lab529.java​

classShape{
voidarea(){ }
}

classSquareextendsShape{ }

classLab529{
publicstaticvoidmain(Stringargs[]){
Shapesp=newSquare();
sp.area();
    }
}

Lab530.java​

classShape{
voidarea();// Error
}

classSquareextendsShape{ }

classLab530{
publicstaticvoidmain(Stringargs[]){
Shapesp=newSquare();
sp.area();
    }
}

Lab531.java​

classShape{
abstractvoidarea();
}

classSquareextendsShape{ }

classLab531{
publicstaticvoidmain(Stringargs[]){
Shapesp=newSquare();
sp.area();
    }
}

Lab532.java​

abstractclassShape{
abstractvoidarea();
}

classLab532{
publicstaticvoidmain(Stringargs[]){
Shapesp=newShape();
sp.area();
    }
}

Lab533.java​

abstractclassShape{
abstractvoidarea();
}

classSquareextendsShape{ }
classLab533{
publicstaticvoidmain(Stringargs[]){
Shapesp=newSquare();
sp.area();
    }
}

Lab534.java​

abstractclassShape{
abstractvoidarea();
}

classSquareextendsShape{
voidarea(){
System.out.println("Square -> area()");
    }
}
classLab534{
publicstaticvoidmain(Stringargs[]){
Shapesp=newSquare();
sp.area();
    }
}

Lab535.java​

abstractclassPerson{
abstractvoidsleeping();
abstractvoidwalking();
}

abstractclassStudentextendsPerson{
voidsleeping(){
System.out.println("Student -> sleeping()");
    }
}
classCurrentStudentextendsStudent{
voidwalking(){
System.out.println("CurrentStudent -> walking()");
    }
}

classLab535{
publicstaticvoidmain(Stringargs[]){
Personpob=newCurrentStudent();
pob.sleeping();
pob.walking();
    }
}

Lab536.java​

abstractclassPerson{
abstractvoidsleeping();
}

abstractclassStudentextendsPerson{
voidsleeping(){
System.out.println("Student -> sleeping()");
    }
}
classCurrentStudentextendsStudent{ }

classLab536{
publicstaticvoidmain(Stringargs[]){
Personpob=newCurrentStudent();
pob.sleeping();
    }
}

Lab537.java​

abstractclassHello{
staticintA=1234;

staticvoidshow(){
System.out.println("Hello -> show()");
    }
}
classLab537{
publicstaticvoidmain(String[]args){
System.out.println(Hello.A);
Hello.show();
    }
}

Lab538.java​

abstractclassHello{
inta;

Hello(inta){
this.a=a;
System.out.println("Hello (int) Cons");
    }

    {
System.out.println("Hello -> I.B");
    }

voidshow(){
System.out.println("Hello -> show()");
    }
}
classHaiextendsHello{
Hai(inta){
super(a);
    }
}

classLab538{
publicstaticvoidmain(String[]args){
Haihai=newHai(10);
hai.show();
    }
}

Lab539.java​

abstractclassLab539{
publicstaticvoidmain(String[]args){
System.out.println("Lab539 Main Method");
    }
}