In C++, if a class has at least one pure virtual function, then the class becomes abstract. Unlike C++, in Java, a separate keyword abstract is used to make a class abstract.
Following are some important observations about abstract classes in Java.
1) Like C++, in Java, an instance of an abstract class cannot be created, we can have references to abstract class type though.
Derived fun() called
2) Like C++, an abstract class can contain constructors in Java. And a constructor of abstract class is called when an instance of an inherited class is created. For example, the following is a valid Java program.
Base Constructor Called Derived Constructor Called
3) In Java, we can have an abstract class without any abstract method. This allows us to create classes that cannot be instantiated but can only be inherited.
Base fun() called
4) Abstract classes can also have final methods (methods that cannot be overridden). For example, the following program compiles and runs fine.
Derived fun() called
5) For any abstract java class we are not allowed to create an object i.e., for abstract class instantiation is not possible.
Compile time error. Test is abstract; cannot be instantiated Test t=new Test();
6) Similar to the interface we can define static methods in an abstract class that can be called independently without an object.
Lets have some fun!!
1. Is it possible to create an abstract and final class in Java?
2. Is it possible to have an abstract method in a final class?
3. Is it possible to inherit from multiple abstract classes in Java?
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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