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Abstract class-1

Abstract class:-abstract is a  modifier used for class and methods, abstract modifier is used for class than that class must be inherit by other class otherwise that class will be useless and nobody can create object of abstract class directly but we can create object of sub class of abstract class. Abstract modifier is used on methods, those methods called abstract methods. An abstract method shows only requirement specification only and implementation provided by sub class of abstract class, if sub class not provides implementation of abstract methods than that sub class must be declared as an abstract.
Abstract class can contains both concrete methods as well as abstract methods. But normal class or non abstract class cannot contain abstract methods.
If any class is declared as an abstract than that class not compulsorily contains abstract methods but if any class contains abstract methods that class must be declared with abstract modifier.

1)   Abstract class may be empty.
2)   Abstract class can contain concrete methods only.
3)   Abstract class can contain abstract methods only.
4)   Abstract class can contain concrete methods and abstract methods.


Example1:-Student.java


 class Student
{
abstract void disp();
}

Output:-

C:\JAVATECH>javac Student.java
Student.java:1: error: Student is not abstract and does not override abstract method disp() in Student
class Student
^
1 error

Example2:-Student.java

class Student
{
void disp();
}

Output:-

C:\JAVATECH>javac Student.java
Student.java:3: error: missing method body, or declare abstract
 void disp();
      ^
1 error

Example3:-

abstract class Student
{
abstract void disp();
public static void main(String[]args)
{
System.out.println("Well-Come Friends");
}
}

Output:-

C:\JAVATECH>javac Student.java
C:\JAVATECH>java Student
Well-Come Friends

Example4:-

abstract class Student
{
abstract void disp();
public static void main(String[]args)
{
Student s=new Student();
System.out.println("Well-Come Friends");
}
}

Output:-

C:\JAVATECH>javac Student.java
Student.java:6: error: Student is abstract; cannot be instantiated
Student s=new Student();
          ^
1 error

Example5:-Test.java

abstract class Student
{
abstract void disp();
}
class Test extends Student
{
public static void main(String[]args)
{
Test t=new Test();
System.out.println("Well-Come Friends");
}
}

Output:-

C:\JAVATECH>javac Test.java
Test.java:5: error: Test is not abstract and does not override abstract method disp() in Student
 class Test extends Student
 ^
1 error

Example6:- 

abstract class Student
{
abstract void disp();
}
abstract class Test extends Student
{
public static void main(String[]args)
{
Test t=new Test();
System.out.println("Well-Come Friends");
}
}

Output:-

C:\JAVATECH>javac Test.java
Test.java:9: error: Test is abstract; cannot be instantiated
Test t=new Test();
       ^

1 error
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