Abstract Classes and Methods in Java Explained in 7 Minutes

Summary

In this Java tutorial, John, a lead software engineer, demystifies abstract classes by explaining their purpose and usage with an example. He clarifies that abstract classes cannot be instantiated but can be subclassed, illustrating with an 'Animal' class. John also introduces abstract methods, which enforce subclasses to implement specific behaviors. He contrasts abstract classes with interfaces, highlighting that interfaces can't have instance fields and a class can implement multiple interfaces but only extend one class. The tutorial aims to provide a clear understanding of when to use abstract classes and interfaces in Java.

Takeaways

• 📚 Abstract classes in Java cannot be instantiated, meaning you cannot create objects directly from them.
• 🐱 An example given is the 'Cat' class, which if made abstract, cannot be instantiated but can be subclassed.
• 🔑 The 'abstract' keyword is used in the class definition to make a class abstract.
• 🤔 The purpose of an abstract class is to provide a base for subclasses to share fields and methods, without being instantiated themselves.
• 📝 Abstract classes can contain both abstract methods, which have no implementation and must be overridden by subclasses, and concrete methods, which provide default implementations.
• 📣 Abstract methods are declared without a method body and end with a semicolon, requiring subclasses to provide an implementation.
• 🐾 In the example, the 'make noise' method is abstract, so each animal subclass like 'Cat' must implement its own version.
• 🔄 Abstract classes enforce a contract for subclasses, ensuring they provide specific functionality, such as the 'make noise' method.
• 🔄 Interfaces in Java are also used for ensuring classes implement certain methods, but they differ in that they can't contain non-static or non-final fields.
• 🔗 A class can implement multiple interfaces but can only extend one class, highlighting a key difference between abstract classes and interfaces.
• 🌐 The choice between using an abstract class or an interface depends on whether you need to share fields and closely related functionality (abstract class) or ensure unrelated classes can perform certain actions (interface).

Q & A

What is an abstract class in Java?

An abstract class in Java is a class that cannot be instantiated. It serves as a base class for other classes, providing a common definition for its subclasses.

How do you declare a class as abstract in Java?

You declare a class as abstract by using the 'abstract' keyword in the class definition, like 'public abstract class ClassName'.

Why would you create an abstract class if you can't instantiate it?

You create an abstract class to provide a common set of fields and methods that must be shared by its subclasses, but you may not want to instantiate the abstract class itself.

Can you provide an example of when to use an abstract class?

An example is the 'Animal' class, which might have fields like 'age' and 'name' and methods that all animals should have, but you wouldn't want to create an 'Animal' object without specifying what kind of animal it is.

What is an abstract method and how is it declared?

An abstract method is a method declared in an abstract class with no implementation. It is declared by providing the method signature and ending with a semicolon instead of a method body.

What is the requirement for subclasses of an abstract class that contains abstract methods?

Subclasses of an abstract class must provide an implementation for all inherited abstract methods.

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

Yes, an abstract class can have a mix of abstract methods, which must be implemented by subclasses, and concrete methods, which provide default implementations that subclasses can use or override.

What is the difference between an abstract class and an interface in Java?

An abstract class can provide both abstract and concrete methods and fields, while an interface can only have abstract methods. Additionally, a class can implement multiple interfaces but can only extend one class.

How do fields declared in an interface differ from those in an abstract class?

Fields declared in an interface are automatically static and final, meaning they have the same value for all instances of classes implementing the interface. In contrast, fields in an abstract class can have different values for each instance of its subclasses.

Can a class extend another class and implement multiple interfaces at the same time?

Yes, a class in Java can extend one class and implement multiple interfaces, combining the functionality and fields from both the superclass and the interfaces.

What is the practical use of having an abstract method in an abstract class?

The practical use of an abstract method in an abstract class is to enforce that all subclasses implement a specific method, ensuring a consistent contract for the behavior that must be defined by each subclass.

Outlines

00:00

📚 Understanding Abstract Classes in Java

This paragraph introduces the concept of abstract classes in Java, explaining that they cannot be instantiated but can be subclassed. The speaker, John, a lead Java software engineer, clarifies the abstract class's purpose using the example of an 'Animal' class that serves as a base for various animal subclasses. He demonstrates how to declare an abstract class and an abstract method, emphasizing the need for subclasses to implement these abstract methods. The explanation also touches on the inclusion of concrete methods within an abstract class, providing a broader understanding of class structure and inheritance.

05:00

🔍 Abstract Classes vs. Interfaces in Java

The second paragraph delves into the differences between abstract classes and interfaces in Java. It illustrates the limitless implementation of interfaces compared to the single inheritance restriction of classes. The speaker points out that fields declared in interfaces are automatically static and final, which contrasts with the instance-specific fields in abstract classes. The explanation highlights the use case for abstract classes when there are closely related classes sharing functionality and fields, and interfaces when implementing a specific behavior across unrelated classes. The paragraph concludes with an invitation for questions and corrections, fostering engagement with the audience.

Keywords

💡Abstract Class

An abstract class in Java is a class that cannot be instantiated directly. It serves as a blueprint for other classes, providing a way to declare fields and methods that must be shared among subclasses. In the video, the abstract class 'Animal' is used as an example to illustrate how subclasses like 'Cat' can inherit and extend its properties and behaviors, but cannot create an instance of 'Animal' directly.

💡Instantiation

Instantiation refers to the process of creating an object from a class. The video explains that while you can instantiate a regular class, such as creating a 'Cat' object from a 'Cat' class, you cannot instantiate an abstract class like 'Animal'. This concept is crucial for understanding the limitations and uses of abstract classes.

💡Inheritance

Inheritance is a fundamental concept in object-oriented programming where a class (subclass) can inherit fields and methods from another class (superclass). The video uses the 'Animal' abstract class and 'Cat' class to demonstrate how 'Cat' can inherit properties like 'age' and 'name' from 'Animal', and also how it can provide its own implementation of abstract methods like 'makeNoise'.

💡Abstract Method

An abstract method is a method declared without an implementation in an abstract class, requiring any non-abstract subclass to provide its own implementation. In the script, the 'makeNoise' method is made abstract in the 'Animal' class, which means that subclasses like 'Cat' must implement this method, ensuring that each animal type can make a noise specific to it.

💡Subclass

A subclass is a class that inherits from another class, known as its superclass. The video script discusses how subclasses like 'Cat' can extend an abstract superclass 'Animal', inheriting its properties and abstract methods, and then providing specific implementations for those methods.

💡Interface

An interface in Java is a reference type that can contain only constants, method signatures, default methods, static methods, and nested types. Unlike abstract classes, interfaces cannot have instance fields or concrete methods. The video contrasts interfaces with abstract classes, noting that a class can implement multiple interfaces but can only extend one class.

💡Method Implementation

Method implementation refers to providing the code that defines what a method does. The video explains that in abstract classes, you can have both abstract methods, which require subclasses to provide an implementation, and concrete methods, which have an implementation in the abstract class itself and can be used directly by subclasses.

💡Fields

Fields in Java are variables that are part of a class and are used to store data associated with objects of that class. The video script mentions that fields declared in an interface are static and final, whereas in abstract classes, fields like 'age' and 'name' can have individual values for each object instance.

💡Concrete Method

A concrete method is a method that has an implementation provided in its declaration. The video script explains that while abstract classes can contain abstract methods, they can also contain concrete methods, which are methods with an implementation that subclasses can use without needing to provide their own.

💡Polymorphism

Polymorphism is the ability of a single interface to represent different underlying forms (data types). The video touches on this concept when discussing abstract methods and interfaces, as they allow for a method to be defined once but implemented in various ways by different subclasses, showcasing polymorphic behavior.

💡Encapsulation

Encapsulation is the bundling of data with the methods that operate on that data. While not explicitly mentioned in the script, the concept is implied when discussing how abstract classes can contain fields and methods that are shared among subclasses, encapsulating the behavior and state of the objects.

Highlights

Abstract classes in Java are a concept that can be confusing for beginners.

An abstract class is a class that cannot be instantiated.

Making a class abstract requires only the 'abstract' keyword in the class definition.

Abstract classes are useful for creating a base class that shares fields and methods with subclasses but should not be instantiated on its own.

Subclasses of an abstract class can be instantiated even though the abstract class itself cannot.

Abstract methods are declared without a body in an abstract class and must be implemented by all subclasses.

Subclasses must provide their own implementation for abstract methods inherited from an abstract class.

Abstract classes enforce a consistent structure and behavior across subclasses.

An abstract class can contain both abstract and concrete methods.

Fields declared in an interface are automatically static and final.

Interfaces can be implemented by any class, regardless of whether it's related to the abstract class.

A class can implement multiple interfaces but can only extend one class.

Interfaces are used when you want to guarantee a certain behavior across unrelated classes.

Abstract classes are used when you have closely related classes that share functionality and fields.

The video provides a clear and understandable explanation of abstract classes and their differences with interfaces.

The presenter, John, is a lead Java software engineer who enjoys sharing knowledge in an accessible way.

A full Java course is available for those interested in a more in-depth understanding of the subject.

Transcripts

abstract classes can be a really

confusing concept when you're learning

java when i was learning they were

pretty confusing for me but in this

video we're going to go over exactly

what an abstract class is how you make

one and how you can use one with an

example and why you would want to make

one in the first place we'll also talk

about the differences between abstract

classes and interfaces my name is john

i'm a lead java software engineer and i

love sharing what i've learned in a

clear understandable way so if you

enjoyed this video please consider

subscribing so you don't miss each new

java tutorial i also have a full java

course available in a link down in the

description if you're interested so

first things first what is an abstract

class all an abstract class is is a

class that you can't instantiate you

can't create objects from abstract

classes so what does that mean well you

know if you have like a normal class

like this cat class here if you want you

can create objects of that class so you

could say cat my cat equals new cat but

with abstract classes you can't do this

so if i go back into my cat class and

make it a public abstract class if i go

back into my main method i can see that

now i get an error here cannot

instantiate the type cat and by the way

this abstract keyword here in the class

definition is the only thing you need to

make a class into an abstract class

probably the first question that comes

to mind is why the heck would i want to

do that why would i want to make a class

that i can't create objects from so

here's a situation where it makes sense

to use an abstract class let's change

this class back into a regular class

instead of just being a standalone class

let's say it extended a class called

animal and let's say the animal class

had a couple of fields like int age

string name it totally makes sense that

you could have an animal class that

would have subclasses of actual animals

like cat dog horse whatever and it makes

sense that you could create objects of

those subclasses like you could create a

cat object but what doesn't make a whole

lot of sense is creating an animal

object you know it's just kind of weird

what kind of animal is it so you might

want a parent class like animal so that

your subclasses can share some fields

like age name and maybe some methods

that you write but you might not want to

be able to create objects of this animal

class all you have to do is make this

class abstract so an abstract class is a

class you can't instantiate but you can

absolutely make subclasses of an

abstract class that can be instantiated

now what about abstract methods in any

of your abstract classes you can choose

to have abstract methods let's say we

had a method like public void make noise

of course it makes sense for an animal

to be able to make noise but each

individual animal is going to make noise

in its own way a cat's going to meow a

dog is gonna bark so because of that it

might not make a whole lot of sense to

actually implement this method make

noise here in your abstract animal class

what you can do is make this method an

abstract method when you make a method

abstract you don't specify a body for

the method all you do is declare it and

then end it with a semicolon but then in

all the child classes of your abstract

class you have to actually create an

implementation of this make noise method

so if we go back over to our cat class

now we can see that it's giving us an

error that the type cat must implement

the inherited abstract method make noise

so of course we can write it all out but

we can also just use eclipse's fix tool

here to add the unimplemented method but

now this cat class can implement this

make noise method however we want print

out meow so because this animal class

declares an abstract method make noise

any child class of this animal class has

to provide its own make noise

implementation so now back in our main

method we could take my cat and call

meow and if we run our program we can

see that it says meow now of course we

could in our animal class you know get

rid of this abstract method and in our

cat class just declare a make noise

method that says meow and if we go back

and run our program again it still says

meow so why do we need this abstract

method what do we want that for what the

abstract class does as a whole is kind

of enforce and organize exactly what

every subclass of animal has to have so

this animal class is saying hey if you

want to create a new type of animal it's

going to have an age it's going to have

a name and it has to be able to make

noise every type of animal might make

noise in completely different ways but

this makes sure that every single animal

type is able to make noise as a side

note though in your abstract class all

of the methods don't necessarily have to

be abstract you can create actual

concrete implemented methods in your

abstract classes so you could have like

public void print name just prints my

name is name so now every subclass of

animal will also have this print name

method available to it but since it's

not abstract they don't need to

implement it themselves they can just

use the implementation that's here the

question a lot of people have and what's

actually also a big interview question

is what's the difference between an

abstract class and an interface let's

say we had an interface called like

animal stuff here in an interface we

could say how about public void poop

just like every animal might make noise

in different ways every animal is

probably also going to poop in different

ways in interfaces you don't need an

abstract keyword in your methods every

method in an interface is assumed to be

abstract so you uh you don't need this

keyword as you probably know if you want

to implement an interface so all you

have to do is instead of extending

another class you just say

implements the interface which is animal

stuff and now that we implement this

animal stuff interface we have to

implement this poop method you see right

now we get an error that says type cat

must implement the inherited abstract

method poop we can just click the

suggestion here to add the unimplemented

method print out

like

sounds pretty poopy so just like this

abstract make noise method made any

subclass implement this make noise

method this interface also makes any

classes that implement the interface

implement this method they seem like

they're doing the same thing right

what's the difference the first key

difference is that you can implement as

many interfaces as you want in java

there's no limit but you can only extend

one class and you can totally do both if

you want to you can extend the animal

class and implement the animal stuff

interface another difference is that in

interfaces if you declare any fields

like if you had end age and string name

every field that's declared inside an

interface is going to be static and

final that's why i'm getting an error

here because it's final i have to

instantiate it with some kind of a value

larry because every field in an

interface is automatically static that

means the same values apply to every

object in that class if i say age equals

one that would have to apply to every

class that implements this interface so

it really doesn't make a whole lot of

sense to have fields like this inside

interfaces that's why we have abstract

classes like this so you can say hey

every animal has to have an age and a

name but i'm not going to tell you what

it is right now because that doesn't

make sense each individual cat object

dog object and horse object can all have

their own age and name values so you can

do that with an abstract class but not

with an interface so you might want to

create an abstract class if you have a

lot of closely related classes that you

want to have the same functionality and

the same types of fields available but

you might want to make an interface

instead if you have a lot of unrelated

classes that you all want to be able to

do a certain thing that makes it so you

can guarantee that other types of

classes will be able to poop even if

they aren't animals that might be the

weirdest sentence ever said in a java

tutorial if you have any questions or

clarifications let me know in the

comments or if i got something wrong

feel free to shout and rant at me

angrily thanks so much for watching and

i'll see you next time