Abstract classes in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an abstract class in Java?

An abstract class is a class that cannot be instantiated on its own and is meant to be a base class. It can have abstract methods (without body) that must be implemented by subclasses.

beginner

Can an abstract class have concrete (non-abstract) methods?

Yes, an abstract class can have both abstract methods and concrete methods with full implementations.

beginner

What keyword is used to declare an abstract class and an abstract method in Java?

The keyword <code>abstract</code> is used before the class name to declare an abstract class and before a method signature to declare an abstract method.

beginner

Why can't you create an object of an abstract class?

Because abstract classes are incomplete by design (they may have abstract methods without implementation), Java prevents creating objects from them to avoid errors.

intermediate

How do subclasses relate to abstract classes?

Subclasses must provide implementations for all abstract methods of the abstract class, or they must also be declared abstract.

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

A You cannot create an instance of an abstract class.

B Abstract classes cannot have any methods with implementation.

C Abstract classes are the same as interfaces.

D Abstract classes must have at least one abstract method.

Abstract classes cannot be instantiated directly. They can have methods with implementation, and they are different from interfaces. Also, an abstract class can have zero or more abstract methods.

What happens if a subclass does not implement all abstract methods of its abstract superclass?

A The subclass will compile without errors.

B The subclass must be declared abstract.

C The subclass becomes a final class automatically.

D The program will throw a runtime exception.

If a subclass does not implement all abstract methods, it must also be declared abstract.

Which keyword is used to declare an abstract method?

A abstract

B final

C static

D void

The keyword abstract is used to declare a method without a body in an abstract class.

Can an abstract class have constructors?

A Only if all methods are abstract.

B No, abstract classes cannot have constructors.

C Yes, abstract classes can have constructors.

D Only if the class has no abstract methods.

Abstract classes can have constructors which are called when subclasses are instantiated.

Which statement about abstract classes is false?

A Abstract classes can have fields and methods with implementations.

B Abstract methods must be implemented by subclasses unless the subclass is abstract.

C Abstract classes can be extended by other classes.

D You can create an object of an abstract class using the new keyword.

You cannot create an object of an abstract class directly using the new keyword.

Explain what an abstract class is and why it is useful in Java.

Think about a blueprint that is incomplete but guides other classes.

Describe the rules for subclassing an abstract class in Java.

Focus on what the subclass must do with abstract methods.