Inheritance in Dart
Let’s forget programming for a moment and think about real life. What do we understand when someones says the word inheritance?
Inheritance is the process of passing on properties from one generation to another. For example, a child inherits the properties of his/her parents and grandparents which continues this from generation to generation.
Likewise, in the world of programming classes pass properties to one another which is known as inheritance.
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Before we dive into inheritance in Dart, let’s first understand the keywords: extends, this and super in Dart.
| Keyword | Description |
|---|---|
| extends | The keyword extends is used to inherit properties from a parent class. |
| this | The keyword this is used to refer to the current instance of the class. |
| super | The keyword super is used to refer to the parent class. |
This keywords are very important to understand and implement inheritance in Dart.
In programming, inheritance is the process of passing on properties from one class to another.
The class that passes on the properties is called the parent class or the super class. The class that inherits the properties is called the child class or the sub class.
Inheritance is a very important concept in object oriented programming. It allows us to reuse code and also to add more features to a class without modifying it.
Example
Let’s look at an example of inheritance in Dart.
class Animal {
String name;
int age;
Animal(this.name, this.age);
void eat() {
print('$name is eating');
}
void sleep() {
print('$name is sleeping');
}
}
class Dog extends Animal {
Dog(String name, int age) : super(name, age);
void bark() {
print('$name is barking');
}
}
void main() {
var dog = Dog('Bruno', 2);
dog.eat();
dog.sleep();
dog.bark();
}
Output
Bruno is eating
Bruno is sleeping
Bruno is barking
There are different types of inheritance in Dart. Dart supports the following types of inheritance.
Single inheritance is the process of inheriting properties from a single parent class. This means, If there are class A and class B, and class B inherits from class A, then it is called single inheritance.
graph TD
A[class A]-->B[class B]
C[class Animal]-->D[class Human]
E[class Dog]-->F[class GermanShepherd]
Let’s follow the set of steps to achieve the single inheritance.
Step 1: Define a parent class
class Animal {
String name;
int age;
Animal(this.name, this.age);
void eat() {
print('$name is eating');
}
void sleep() {
print('$name is sleeping');
}
}
We have defined a parent class with the name and age as properties and eat() and sleep() as class methods.
Step 2: Delcare the child class and inherit the parent class
class Dog extends Animal {
Dog(String name, int age) : super(name, age);
void bark() {
print('$name is barking');
}
}
Here, we have defined the Dog class with its own bark() method. Since our parent class in not a zero argument constructor we are making the use of super keyword to pass the parameters to super class constructor.
Step 3: Using the inherited classes.
void main() {
var dog = Dog('Bruno', 2);
dog.eat();
dog.sleep();
dog.bark();
}
In the above block of code, we have created the object of child class Dog which inherits the Animal class. We have access to sleep() and eat() methods from the parent(Animal) class along with the bark() method of child(Dog) class.
Full Code Example
class Animal {
String name;
int age;
Animal(this.name, this.age);
void eat() {
print('$name is eating');
}
void sleep() {
print('$name is sleeping');
}
}
class Dog extends Animal {
Dog(String name, int age) : super(name, age);
void bark() {
print('$name is barking');
}
}
void main() {
var dog = Dog('Bruno', 2);
dog.eat();
dog.sleep();
dog.bark();
}
This is an example of single inheritance. The class Dog inherits from the class Animal. Let’s take the output of code.
Output
Bruno is eating
Bruno is sleeping
Bruno is barking
Hierarchical Inheritance is the example of our real world example. Our parents may have more than one chidren or our grandparents might have more than one children. All of them carry some of the other properties with each other.
Likewise, if a single class has more than a child then it is called hierarchical inheritance in dart.
This means, If there are class A, class B and class C, and class B inherits from class A and class C inherits also inheritance from class A.
graph TD
A[class A]-->B[class B]
A-->C[class C]
AA[class Animal]-->BB[class Human]
AA-->CC[class Dog]
X[class Dog]-->Y[class GoldenRetriver]
X-->Z[class Labarador]
Y-->yy[class Black]
Y-->zz[class Golden]
Let’s learn hierarchical Inheritance with the following steps:
Step 1: Let’s define the parent class. Here we are defining class name Animal with eat() and sleep() methods and name and age properties.
class Animal {
String name;
int age;
Animal(this.name, this.age);
void eat() {
print('$name is eating');
}
void sleep() {
print('$name is sleeping');
}
}
Step 2: Let’s define child class and inherit the parent classes Dog and Human with their respective methods.
class Dog extends Animal {
Dog(String name, int age) : super(name, age);
void bark() {
print('$name is barking');
}
}
class Human extends Animal {
Human(String name, int age) : super(name, age);
void speak() {
print('$name is speaking');
}
}
Step 3: Let’s use the inheritance.
void main() {
var dog = Dog('Bruno', 2);
dog.eat();
dog.sleep();
dog.bark();
var human = Human('John', 25);
human.eat();
human.sleep();
human.speak();
}
Full Code Example👇
class Animal {
String name;
int age;
Animal(this.name, this.age);
void eat() {
print('$name is eating');
}
void sleep() {
print('$name is sleeping');
}
}
class Dog extends Animal {
Dog(String name, int age) : super(name, age);
void bark() {
print('$name is barking');
}
}
class Human extends Animal {
Human(String name, int age) : super(name, age);
void speak() {
print('$name is speaking');
}
}
void main() {
var dog = Dog('Bruno', 2);
dog.eat();
dog.sleep();
dog.bark();
var human = Human('John', 25);
human.eat();
human.sleep();
human.speak();
}
This is an example of multi-level inheritance. The class Dog inherits from the class Animal and the class Human inherits from the class Animal. Let’s take the output of code.
Output
Bruno is eating
Bruno is sleeping
Bruno is barking
John is eating
John is sleeping
John is speaking
Multi-level inheritance is the process of inheriting properties from a parent class and then inheriting properties from the child class.
This means, If there are class A, class B and class C, and class B inherits from class A and class C inherits from class B, then it is called multi-level inheritance.
graph TD
A[class A]-->B[class B]-->BB[class C]
C[class Animal]-->D[class Human]-->DD[class Girl]
E[class Dog]-->F[class GermanShepherd]-->FF[class BlackGermanShepherd]
Example
Let’s follow the following steps to achieve multi-level inheritance in Dart
Step 1: Let’s define the parent class. Here we are defining class name Animal with eat() and sleep() methods and name and age properties.
class Animal {
String name;
int age;
Animal(this.name, this.age);
void eat() {
print('$name is eating');
}
void sleep() {
print('$name is sleeping');
}
}
Step 2: Let’s define child classes Dog and and extend the parent class.
Example
class Animal {
String name;
int age;
Animal(this.name, this.age);
void eat() {
print('$name is eating');
}
void sleep() {
print('$name is sleeping');
}
}
class Dog extends Animal {
Dog(String name, int age) : super(name, age);
void bark() {
print('$name is barking');
}
}
class Breed extends Dog {
String breed;
Breed(String name, int age, this.breed) : super(name, age);
void displayBreed() {
print('$name is a $breed');
}
}
void main() {
var dog = Breed('Bruno', 2, 'Labrador');
dog.eat();
dog.sleep();
dog.bark();
dog.displayBreed();
}
Output
Bruno is eating
Bruno is sleeping
Bruno is barking
Bruno is a Labrador