Can we instantiate an abstract class directly?

java android class oop abstract

115,449

Solution 1

You can't directly instantiate an abstract class, but you can create an anonymous class when there is no concrete class:

public class AbstractTest {
    public static void main(final String... args) {
        final Printer p = new Printer() {
            void printSomethingOther() {
                System.out.println("other");
            }
            @Override
            public void print() {
                super.print();
                System.out.println("world");
                printSomethingOther(); // works fine
            }
        };
        p.print();
        //p.printSomethingOther(); // does not work
    }
}

abstract class Printer {
    public void print() {
        System.out.println("hello");
    }
}

This works with interfaces, too.

Solution 2

No, you can never instantiate an abstract class. That's the purpose of an abstract class. The getProvider method you are referring to returns a specific implementation of the abstract class. This is the abstract factory pattern.

Solution 3

No, abstract class can never be instantiated.

115,449

Author by

satheesh.droid

Updated on June 08, 2020

Comments

satheesh.droid almost 4 years
I have read we can only instantiate an abstract class by inheriting it, but we cannot instantiate it directly.
However, I saw we can create an object with the type of an abstract class by calling a method of another class.
For example - LocationProvider is an abstract class, and we can instantiate it by calling getProvider() function in the LocationManager class:
```
LocationManager lm = getSystemService(Context.LOCATION_PROVIDER);
LocationProvider lp = lm.getProvider("gps");
```
How is the abstract class instantiate here?
satheesh.droid over 13 years

Can you please elaborate on factory pattern or give some link about that?
Darin Dimitrov over 13 years

@satheesh.droid, here's an example: javabeat.net/tips/…
satheesh.droid over 13 years

can we have function definition inside abstract class?
kiritsuku over 13 years

You can create methods inside anonymous classes, but you can only call these methods inside of the anonymous class. See the code in my answer, I edited it.
SexyBeast over 9 years

@sschaef, can you explain why the call to p.printSomethingOther works fine from inside the class but not from outside?
kiritsuku over 9 years

@Cupidvogel: because it is not public/not known to type Printer.
James Wierzba over 8 years

What word would you describe this as, if not "instantiation"?
kiritsuku over 8 years

@JamesWierzba it is instantiation, just not of the abstract class but the anonymous subclass.
AnV over 7 years

@sschaef Is it allowed for an abstract class (in this case Printer) to have only concrete method(s) [method(s) with definition]? Any class must have at least one abstract method to become abstract right?
theyuv about 5 years

What's the motivation for placing the create[Object] method in a separate [Object]Factory class rather than in the abstract [Object] class itself? To use the example in your link: What's the motivation for placing the createButton method in a separate ButtonFactory class rather than in the abstract Buttton class itself?
h4nek almost 5 years

@AnV For a class to be abstract, you just need to define it abstract. It can then have 0 abstract methods along with any number of concrete methods. (see: this SO question)