abstract method

[nirjari patel]

When a class is declared as abstract , is it required that it must contain an abstract method ?

I tried it in eclipse and I did not get any error. Does it mean that a class can be declared abstract without any of its method declared as abstract or any of its methids undefined ? If thats the case, then why do we need abstract modifier for a method ? We can simply not define the method and just declare class as an abstract.

I tried other way also. I kept class declaration as non-abstract and declared a method as abstract, but it gave error. So why do we need abstract modifier for method ? I dont get any error even if I dont use that modifier or define all methods in abstract class ?

Thanks

[Wouter Oet]

nirjari patel wrote:When a class is declared as abstract , is it required that it must contain an abstract method ?

Nope.

nirjari patel wrote:I tried it in eclipse and I did not get any error. Does it mean that a class can be declared abstract without any of its method declared as abstract or any of its methids undefined ? If thats the case, then why do we need abstract modifier for a method ? We can simply not define the method and just declare class as an abstract.

No because an abstract class can contain concrete methods.

[pawan chopra]

Not sure what is the query in this. But if you want to make a class Abstract.

(1) You need to declare abstract keyword with the class declaration. Now your class is an abstract class. By declaring your class abstract you make sure that this class can not be instantiated. Below can not work:

public abstract class AbtsractClass{
}

public class Main{

public static void main(String...args){
AbtsractClass a = new AbstractClass(); // Compiler will give you error at this line.
}

}

(2) You can have abstract methods in that class but they also need to have a abstract keyword in the method signature.

(3) There is no restriction that an abstract class must have abstract methods.

(5) Abstract class can have concrete methods.

(4) You can not have an abstract method in a non-abstract class.

I hope this will clear things a bit. You can go through this link for more information.

[Mohamed Sanaulla]

abstract for a method indicates that the method doesn't have a method-body. And a class that has this method has to be declared as abstract because the other class extending this class have to provide the method body for the abstract method.

And suppose the class has abstract method and the class is not declared as abstract- then if we create an instance of that class and invoke the abstract method- what method body will it execute?

class SupposedToBeAbstract
{
    public void abstract anAbstractMethod();
    public static void main(String[] args)
    {
        SupposedToBeAbstract obj = new SupposedToBeAbstract();
        obj.anAbstractMethod(); //What would this call execute? 
        //Hence the classes enclosing an abstract method have to be declared as abstract 
    }
}

[swapnil kataria]

to having an abstract method a class should be abstract

[Jesper de Jong]

swapnil kataria wrote:to having an abstract method a class should be abstract

Yes, but not the other way around, which is what Nirjari asked.

[nirjari patel]

public abstract class AbstractClass{

public void method1(){}

public void method2(){}

//public String method3(){}
}

In this code none of the methods are abstract. If AbstractSubClass extends AbstractClass, it is not required to implement any of these methods as all the methods are already implemented. Commented line, will give an error if its not implemented as it returns String. If I want to use this method in the class, do I need to declare it as abstract ?

Thanks

[Mohamed Sanaulla]

nirjari patel wrote:public abstract class AbstractClass{

public void method1(){}

public void method2(){}

//public String method3(){}
}

In this code none of the methods are abstract. If AbstractSubClass extends AbstractClass, it is not required to implement any of these methods as all the methods are already implemented.

Yes not required to implement.

nirjari patel wrote:
Commented line, will give an error if its not implemented as it returns String. If I want to use this method in the class, do I need to declare it as abstract ?

Thanks

It gives error because its implementation is wrong. You have a method which returns something, but in the implementation it doesn't return anything. If you want it to work still you would have to declare that method as:

public abstract String method3();

and then make the sub class to implement this method- Remember that the implementation should return a value of type String, otherwise it will be a compiler error.