Abstract vs concrete classes in Java - Quick Revision & Key Differences

Recall & Review

beginner

What is an abstract class in Java?

An abstract class in Java is a class that cannot be instantiated directly. It can have abstract methods (without body) that must be implemented by subclasses, and it can also have concrete methods with implementations.

beginner

What is a concrete class in Java?

A concrete class in Java is a class that can be instantiated. It provides implementations for all its methods and can be used to create objects.

beginner

Can you create an object of an abstract class directly?

No, you cannot create an object of an abstract class directly. You must create an object of a subclass that extends the abstract class and implements its abstract methods.

intermediate

Why use abstract classes instead of concrete classes?

Abstract classes are used to define a common template or blueprint for subclasses. They allow you to enforce certain methods to be implemented while sharing common code, promoting code reuse and design clarity.

intermediate

What happens if a concrete class does not implement all abstract methods from its abstract superclass?

If a concrete class does not implement all abstract methods from its abstract superclass, the concrete class must also be declared abstract. Otherwise, the code will not compile.

Which of the following statements is true about abstract classes in Java?

A Abstract classes are the same as interfaces.

B Abstract classes cannot have any methods with implementation.

C You cannot create an instance of an abstract class.

D Abstract classes must be final.

Abstract classes cannot be instantiated directly, but they can have methods with implementation.

What must a concrete subclass do when it extends an abstract class?

A Declare itself abstract.

B Have no constructors.

C Override all concrete methods of the abstract class.

D Implement all abstract methods of the abstract class.

A concrete subclass must implement all abstract methods inherited from the abstract class.

Which keyword is used to declare an abstract class in Java?

A concrete

B abstract

C static

D final

The keyword 'abstract' is used to declare an abstract class.

Can a concrete class extend an abstract class?

A Yes, and it must implement all abstract methods.

B No, only abstract classes can extend abstract classes.

C Yes, but it cannot add new methods.

D No, concrete classes cannot extend any class.

Concrete classes can extend abstract classes but must implement all abstract methods.

If a class has at least one abstract method, what must be true?

A The class must be declared abstract.

B The class must be final.

C The class must be concrete.

D The class must have no constructors.

Any class with abstract methods must be declared abstract.

Explain the difference between abstract and concrete classes in Java.

Think about whether you can create objects and if methods have bodies.

Describe why and when you would use an abstract class instead of a concrete class.

Consider design and code reuse benefits.