Abstract Classes and Methods in Java Explained in 7 Minutes
Summary
TLDRIn 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
- đ Abstract classes in Java cannot be instantiated, meaning you cannot create objects directly from them.
- đ± An example given is the 'Cat' class, which if made abstract, cannot be instantiated but can be subclassed.
- đ The 'abstract' keyword is used in the class definition to make a class abstract.
- đ€ The purpose of an abstract class is to provide a base for subclasses to share fields and methods, without being instantiated themselves.
- đ Abstract classes can contain both abstract methods, which have no implementation and must be overridden by subclasses, and concrete methods, which provide default implementations.
- đŁ Abstract methods are declared without a method body and end with a semicolon, requiring subclasses to provide an implementation.
- đŸ In the example, the 'make noise' method is abstract, so each animal subclass like 'Cat' must implement its own version.
- đ Abstract classes enforce a contract for subclasses, ensuring they provide specific functionality, such as the 'make noise' method.
- đ 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.
- đ A class can implement multiple interfaces but can only extend one class, highlighting a key difference between abstract classes and interfaces.
- đ 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
đ 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.
đ 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.
Mindmap
Keywords
đĄAbstract Class
đĄInstantiation
đĄInheritance
đĄAbstract Method
đĄSubclass
đĄInterface
đĄMethod Implementation
đĄFields
đĄConcrete Method
đĄPolymorphism
đĄEncapsulation
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
5.0 / 5 (0 votes)