Abstract Classes and Methods Quiz

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Questions and Answers

What is an abstract class?

A class that cannot extend other classes

A class that cannot contain methods

A class that may or may not include abstract methods (correct)

A class that can be instantiated

Why can't we instantiate an abstract class in Java?

Because it represents an abstract concept and requires a specific type of instance.

What is an abstract method?

A method that is fully implemented

A method that includes a return type

A method that cannot be overridden

A method declared without an implementation (correct)

What happens when an abstract class is subclassed?

The subclass usually provides implementations for all abstract methods. Signup and view all the answers

What is a similarity between interfaces and abstract classes?

They cannot be instantiated Signup and view all the answers

Match the following features to abstract classes and interfaces:

Abstract Class = Can define access modifiers other than public Interface = Can implement multiple interfaces Signup and view all the answers

When should you use an abstract class?

When you want to share code among closely related classes Signup and view all the answers

When should you use an interface?

When you want to specify behavior of unrelated classes Signup and view all the answers

What is an example of an abstract class in JDK?

AbstractMap Signup and view all the answers

Which subclasses does AbstractMap contain?

HashMap, TreeMap, ConcurrentHashMap Signup and view all the answers

What methods does AbstractMap define?

Methods like get, put, isEmpty, containsKey, and containsValue Signup and view all the answers

What is an example of a class that implements several interfaces?

HashMap Signup and view all the answers

What is an example of an interface in JDK?

Serialization, Cloneable, Map Signup and view all the answers

A class that implements an interface must implement all declared methods.

True Signup and view all the answers

When does an abstract class implement an interface?

When it does not fully implement the methods of the interface. Signup and view all the answers

What is the keyword used to invoke an overridden superclass method?

super Signup and view all the answers

How do you initialize an array in Java?

int data[] = new int[] {10,20,30,40,50,60,71,80,90,91 }; Signup and view all the answers

What is the difference between inheritance and polymorphism?

Inheritance is a compile-time feature, while polymorphism is a runtime feature. Signup and view all the answers

Study Notes

Abstract Class

An abstract class is declared as such and may include abstract methods.
It serves as a blueprint for other classes, defining structure but not allowing instantiation.

Instantiation Restrictions

Abstract classes cannot be instantiated because they represent abstract concepts (e.g., a generic vehicle).
Instantiation requires a concrete subclass that provides specific functionality.

Abstract Method

An abstract method lacks implementation; it is declared with a semicolon, signifying that subclasses must provide the body.
Example: abstract void moveTo(double deltaX, double deltaY);

Method Implementation in Subclasses

When subclassing an abstract class, all abstract methods must typically be implemented.
If a subclass does not implement all abstract methods, it must also be marked as abstract.

Similarities with Interfaces

Both abstract classes and interfaces cannot be instantiated.
They can contain a blend of declared methods, some with and some without implementations.

Differences: Abstract Class vs Interface

Abstract Class:
- Can have non-static, non-final fields.
- Can define access modifiers for methods (public, protected, private).
- Can extend only one class, abstract or not.
Interface:
- Fields are implicitly public, static, and final.
- Methods are public; default methods can be defined.
- Can implement multiple interfaces.

Usage Scenarios

Abstract Class:
- Ideal for sharing code among related classes.
- Useful when expecting related classes to have common methods/fields, or require specific access modifiers.
- Allows declaration of non-static, non-final fields to manage object state.
Interface:
- Suitable for unrelated classes to share a contract (e.g., Comparable, Cloneable).
- Focused on defining behavior without dictating implementations.
- Enables multiple inheritance of type.

JDK Examples

Abstract Class: AbstractMap, part of the Collections Framework.
AbstractMap Subclasses: Include HashMap, TreeMap, and ConcurrentHashMap.
Interface Examples: Serialization, Cloneable, and Map, which are implemented by HashMap.

Implementing Interfaces

Any class implementing an interface must provide implementations for all declared methods in that interface.

Abstract Class Implementing Interface

An abstract class can implement an interface partially, leaving out one or more methods.
Example structure:
- abstract class X implements Y { /* partial implementation */ }
- class XX extends X { /* implements remaining methods of Y */ }

Keyword Usage

super: Used to invoke an overridden method from a superclass using super.method.

Array Initialization

Arrays can be initialized directly or separately:
- Direct: int data[] = new int[] {10,20,30,40,50,60,71,80,90,91};
- Separate:
```
int data[];
data = new int[] {10,20,30,40,50,60,71,80,90,91};
```

Inheritance vs Polymorphism

Inheritance is a mechanism where a new class derives properties and behaviors from an existing class, facilitating code reuse.
Polymorphism allows for methods to be used in different ways depending on the object calling them, enhancing flexibility in programming.

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Test your knowledge on abstract classes and methods in Java with this engaging flashcard quiz. Understand the core concepts behind abstract classes and the reasons we cannot instantiate them. Whether you're a beginner or refining your skills, this quiz will solidify your understanding of abstract concepts in programming.