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Interview: Can we instantiate abstract class?

Posted by: admin November 2, 2017 Leave a comment

Questions:

The interviewer asked – Can we instantiate an abstract class? I said, No. He told me – Wrong, we can.

I argued a bit on this. Then he told me to try this yourself at your home.

abstract class my {
    public void mymethod() {
        System.out.print("Abstract");
    }
}

class poly {
    public static void main(String a[]) {
        my m = new my() {};
        m.mymethod();
    }
}

Here, I’m creating instance of my class and calling method of abstract class. Can anyone please explain this to me? Was I really wrong during my interview?

Answers:

Here, i’m creating instance of my class

No, you are not creating the instance of your abstract class here. Rather you are creating an instance of an anonymous subclass of your abstract class. And then you are invoking the method on your abstract class reference pointing to subclass object.

This behaviour is clearly listed in JLS – Section # 15.9.1: –

If the class instance creation expression ends in a class body, then
the class being instantiated is an anonymous class. Then:

  • If T denotes a class, then an anonymous direct subclass of the class named by T is declared. It is a compile-time error if the
    class denoted by T is a final class.
  • If T denotes an interface, then an anonymous direct subclass of Object that implements the interface named by T is declared.
  • In either case, the body of the subclass is the ClassBody given in the class instance creation expression.
  • The class being instantiated is the anonymous subclass.

Emphasis mine.

Also, in JLS – Section # 12.5, you can read about the Object Creation Process. I’ll quote one statement from that here: –

Whenever a new class instance is created, memory space is allocated
for it with room for all the instance variables declared in the class
type and all the instance variables declared in each superclass of the
class type, including all the instance variables that may be hidden.

Just before a reference to the newly created object is returned as the
result, the indicated constructor is processed to initialize the new
object using the following procedure:

You can read about the complete procedure on the link I provided.


To practically see that the class being instantiated is an Anonymous SubClass, you just need to compile both your classes. Suppose you put those classes in two different files:

My.java:

abstract class My {
    public void myMethod() {
        System.out.print("Abstract");
    }
}

Poly.java:

class Poly extends My {
    public static void main(String a[]) {
        My m = new My() {};
        m.myMethod();
    }
}

Now, compile both your source files:

javac My.java Poly.java

Now in the directory where you compiled the source code, you will see the following class files:

My.class
Poly$1.class  // Class file corresponding to anonymous subclass
Poly.class

See that class – Poly$1.class. It’s the class file created by the compiler corresponding to the anonymous subclass you instantiated using the below code:

new My() {};

So, it’s clear that there is a different class being instantiated. It’s just that, that class is given a name only after compilation by the compiler.

In general, all the anonymous subclasses in your class will be named in this fashion:

Poly$1.class, Poly$2.class, Poly$3.class, ... so on

Those numbers denote the order in which those anonymous classes appear in the enclosing class.

Questions:
Answers:

The above instantiates an anonymous inner class which is a subclass of the my abstract class. It’s not strictly equivalent to instantiating the abstract class itself. OTOH, every subclass instance is an instance of all its super classes and interfaces, so most abstract classes are indeed instantiated by instantiating one of their concrete subclasses.

If the interviewer just said “wrong!” without explaining, and gave this example, as a unique counterexample, I think he doesn’t know what he’s talking about, though.

Questions:
Answers:

= my() {}; means that there’s an anonymous implementation, not simple instantiation of an object, which should have been : = my(). You can never instantiate an abstract class.

Questions:
Answers:

Just observations you could make:

  1. Why poly extends my? This is useless…
  2. What is the result of the compilation? Three files: my.class, poly.class and poly$1.class
  3. If we can instantiate an abstract class like that, we can instantiate an interface too… weird…

Can we instantiate an abstract class?

No, we can’t. What we can do is, create an anonymous class (that’s the third file) and instantiate it.

What about a super class instantiation?

The abstract super class is not instantiated by us but by java.

EDIT: Ask him to test this

public static final void main(final String[] args) {
    final my m1 = new my() {
    };
    final my m2 = new my() {
    };
    System.out.println(m1 == m2);

    System.out.println(m1.getClass().toString());
    System.out.println(m2.getClass().toString());

}

output is:

false
class my$1
class my$2

Questions:
Answers:

You can simply answers, in just one line

No, you can never instance Abstract Class

But, interviewer still not agree, then you can tell him/her

all you can do is, you can create an Anonymous Class.

And, according to Anonymous class, class declared and instantiate at the same place/line

So, it might be possible that, interviewer would be interested to check your confidence level and how much you know about the OOPs .

Questions:
Answers:

The technical part has been well-covered in the other answers, and it mainly ends in:
“He is wrong, he doesn’t know stuff, ask him to join SO and get it all cleared :)”

I would like to address the fact(which has been mentioned in other answers) that this might be a stress-question and is an important tool for many interviewers to know more about you and how do you react to difficult and unusual situations. By giving you incorrect codes, he probably wanted to see if you argued back. To know whether you have the confidence to stand up against your seniors in situations similar to this.

P.S: I don’t know why but I have a feeling that the interviewer has read this post.

Questions:
Answers:

Abstract classes cannot be instantiated, but they can be subclassed. See This Link

The best example is

Although Calender class has a abstract method getInstance(), but when you say Calendar calc=Calendar.getInstance();

calc is referring to the class instance of class GregorianCalendar as “GregorianCalendar extends Calendar

Infact annonymous inner type allows you to create a no-name subclass of the abstract class and an instance of this.

Questions:
Answers:

Technical Answer

Abstract classes cannot be instantiated – this is by definition and design.

From the JLS, Chapter 8. Classes:

A named class may be declared abstract (§8.1.1.1) and must be declared
abstract if it is incompletely implemented; such a class cannot be
instantiated, but can be extended by subclasses.

From JSE 6 java doc for Classes.newInstance():

InstantiationException – if this Class represents an abstract class, an interface, an array
class, a primitive type, or void; or if the class has no nullary constructor; or if the
instantiation fails for some other reason.

You can, of course, instantiate a concrete subclass of an abstract class (including an anonymous subclass) and also carry out a typecast of an object reference to an abstract type.

A Different Angle On This – Teamplay & Social Intelligence:

This sort of technical misunderstanding happens frequently in the real world when we deal with complex technologies and legalistic specifications.

“People Skills” can be more important here than “Technical Skills”. If competitively and aggressively trying to prove your side of the argument, then you could be theoretically right, but you could also do more damage in having a fight / damaging “face” / creating an enemy than it is worth. Be reconciliatory and understanding in resolving your differences. Who knows – maybe you’re “both right” but working off slightly different meanings for terms??

Who knows – though not likely, it is possible the interviewer deliberately introduced a small conflict/misunderstanding to put you into a challenging situation and see how you behave emotionally and socially. Be gracious and constructive with colleagues, follow advice from seniors, and follow through after the interview to resolve any challenge/misunderstanding – via email or phone call. Shows you’re motivated and detail-oriented.

Questions:
Answers:

It is a well-established fact that abstract class can not be instantiated as everyone answered.

When the program defines anonymous class, the compiler actually creates a new class with different name (has the pattern EnclosedClassName$n where n is the anonymous class number)

So if you decompile this Java class you will find the code as below:

my.class

abstract class my { 
    public void mymethod() 
    { 
        System.out.print("Abstract"); 
    }
} 

poly$1.class (the generated class of the “anonymous class”)

class poly$1 extends my 
{
} 

ploly.cass

public class poly extends my
{
    public static void main(String[] a)
    {
        my m = new poly.1(); // instance of poly.1 class NOT the abstract my class

        m.mymethod();
    }
}

Questions:
Answers:

No, you can’t instantite an abstract class.We instantiate only anonymous class.In abstract class we declare abstract methods and define concrete methods only.

Questions:
Answers:

About Abstract Classes

  • Cannot create object of an abstract class
  • Can create variables (can behave like datatypes)
  • If a child can not override at least of one abstract method of the parent, then child also becomes abstract
  • Abstract classes are useless without child classes

The purpose of an abstract class is to behave like a base. In inheritance hierarchy you will see abstract classes towards the top.

Questions:
Answers:

You can say:
we can’t instantiate an abstract class, but we can use new keyword to create an anonymous class instance by just adding {} as implement body at the the end of the abstract class.

Questions:
Answers:

Extending a class doesn’t mean that you are instantiating the class. Actually, in your case you are creating an instance of the subclass.

I am pretty sure that abstract classes do not allow initiating. So, I’d say no: you can’t instantiate an abstract class. But, you can extend it / inherit it.

You can’t directly instantiate an abstract class. But it doesn’t mean that you can’t get an instance of class (not actully an instance of original abstract class) indirectly. I mean you can not instantiate the orginial abstract class, but you can:

  1. Create an empty class
  2. Inherit it from abstract class
  3. Instantiate the dervied class

So you get access to all the methods and properties in an abstract class via the derived class instance.

Questions:
Answers:

It’s impossible to instantiate an abstract class.
What you really can do, has implement some common methods in an abstract class and let others unimplemented (declaring them abstract) and let the concrete descender implement them depending on their needs.
Then you can make a factory, which returns an instance of this abstract class (actually his implementer). In the factory you then decide, which implementer to choose. This is known as a factory design pattern:

   public abstract class AbstractGridManager {
        private LifecicleAlgorithmIntrface lifecicleAlgorithm;
        // ... more private fields

        //Method implemented in concrete Manager implementors 
        abstract public Grid initGrid();

        //Methods common to all implementors
        public Grid calculateNextLifecicle(Grid grid){
            return this.getLifecicleAlgorithm().calculateNextLifecicle(grid);
        }

        public LifecicleAlgorithmIntrface getLifecicleAlgorithm() {
            return lifecicleAlgorithm;
        }
        public void setLifecicleAlgorithm(LifecicleAlgorithmIntrface lifecicleAlgorithm) {
            this.lifecicleAlgorithm = lifecicleAlgorithm;
        }
        // ... more common logic and getters-setters pairs
    }

The concrete implementer only needs to implement the methods declared as abstract, but will have access to the logic implemented in those classes in an abstract class, which are not declared abstract:

public class FileInputGridManager extends AbstractGridManager {

private String filePath;

//Method implemented in concrete Manager implementors 
abstract public Grid initGrid();

public class FileInputGridManager extends AbstractGridManager {

    private String filePath;

    //Method implemented in concrete Manager implementors 
    abstract public Grid initGrid();

    public Grid initGrid(String filePath) {
        List<Cell> cells = new ArrayList<>();
        char[] chars;
        File file = new File(filePath); // for example foo.txt
        // ... more logic
        return grid;
    }
}

Then finally the factory looks something like this:

public class GridManagerFactory {
    public static AbstractGridManager getGridManager(LifecicleAlgorithmIntrface lifecicleAlgorithm, String... args){
        AbstractGridManager manager = null;

        // input from the command line
        if(args.length == 2){
            CommandLineGridManager clManager = new CommandLineGridManager();
            clManager.setWidth(Integer.parseInt(args[0]));
            clManager.setHeight(Integer.parseInt(args[1]));
            // possibly more configuration logic
            ...
            manager = clManager;
        } 
        // input from the file
        else if(args.length == 1){
            FileInputGridManager fiManager = new FileInputGridManager();
            fiManager.setFilePath(args[0]);
            // possibly more method calls from abstract class
            ...
            manager = fiManager ;
        }
        //... more possible concrete implementors
        else{
            manager = new CommandLineGridManager();
        }
        manager.setLifecicleAlgorithm(lifecicleAlgorithm);
        return manager;
    }
}

The receiver of AbstractGridManager would call the methods on him and get the logic, implemented in the concrete descender (and partially in the abstract class methods) without knowing what is the concrete implementation he got. This is also known as inversion of control or dependency injection.

Questions:
Answers:

No, we can’t create the object of abstract class, but create the reference variable of the abstract class. The reference variable is used to refer to the objects of derived classes (Sub classes of Abstract class)

Here is the example that illustrates this concept

abstract class Figure { 

    double dim1; 

    double dim2; 

    Figure(double a, double b) { 

        dim1 = a; 

        dim2 = b; 

    } 

    // area is now an abstract method 

    abstract double area(); 

    }


    class Rectangle extends Figure { 
        Rectangle(double a, double b) { 
        super(a, b); 
    } 
    // override area for rectangle 
    double area() { 
        System.out.println("Inside Area for Rectangle."); 
        return dim1 * dim2; 
    } 
}

class Triangle extends Figure { 
    Triangle(double a, double b) { 
        super(a, b); 
    } 
    // override area for right triangle 
    double area() { 
        System.out.println("Inside Area for Triangle."); 
        return dim1 * dim2 / 2; 
    } 
}

class AbstractAreas { 
    public static void main(String args[]) { 
        // Figure f = new Figure(10, 10); // illegal now 
        Rectangle r = new Rectangle(9, 5); 
        Triangle t = new Triangle(10, 8); 
        Figure figref; // this is OK, no object is created 
        figref = r; 
        System.out.println("Area is " + figref.area()); 
        figref = t; 
        System.out.println("Area is " + figref.area()); 
    } 
}

Here we see that we cannot create the object of type Figure but we can create a reference variable of type Figure. Here we created a reference variable of type Figure and Figure Class reference variable is used to refer to the objects of Class Rectangle and Triangle.