Java super keyword

Java super keyword

Java super keyword: In this tutorial, we will find out about the super keyword in Java with the help of examples.

The super catchphrase in Java is used in subclasses to access superclass members (attributes, constructors, and methods).

Before we find out about the super catchphrase, make a point to think about Java inheritance.

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Uses of super keyword

• To consider techniques for the superclass that is overridden in the subclass.
• To access attributes (fields) of the superclass if both superclass and subclass have attributes with a similar name.
• To expressly consider superclass no-arg (default) or defined constructor from the subclass constructor.

Let’s understand each of these uses.

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1. Access Overridden Methods of the superclass

If methods with a similar name are characterized in both superclass and subclass, the strategy in the subclass overrides the strategy in the superclass. This is called method overriding.

Example 1: Method overriding

class Animal {

  // overridden method
  public void display(){
    System.out.println("I am an animal");
  }
}

class Dog extends Animal {

  // overriding method
  @Override
  public void display(){
    System.out.println("I am a dog");
  }

  public void printMessage(){
    display();
  }
}

class Main {
  public static void main(String[] args) {
    Dog dog1 = new Dog();
    dog1.printMessage();
  }
}

Output

I am a dog

In this example, by making an object dog1 of Dog class, we can call its method printMessage() which then executes the display() statement.

Since display() is defined in both the classes, the method of subclass Dog overrides the method of superclass Animal. Hence, the display() of the subclass is called.

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What if the overridden method of the superclass has to be called?

We use super.display() if the overridden method display() of superclass Animal needs to be called.

Example 2: super to Call Superclass Method

class Animal {

  // overridden method
  public void display(){
    System.out.println("I am an animal");
  }
}

class Dog extends Animal {

  // overriding method
  @Override
  public void display(){
    System.out.println("I am a dog");
  }

  public void printMessage(){

    // this calls overriding method
    display();

    // this calls overridden method
    super.display();
  }
}

class Main {
  public static void main(String[] args) {
    Dog dog1 = new Dog();
    dog1.printMessage();
  }
}

Output

I am a dog
I am an animal

Here, how the above program works.

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2. Access Attributes of the Superclass

The superclass and subclass can have attributes with a similar name. We use the super catchphrase to access to the attribute of the superclass.

Example 3: Access superclass attribute

class Animal {
  protected String type="animal";
}

class Dog extends Animal {
  public String type="mammal";

  public void printType() {
    System.out.println("I am a " + type);
    System.out.println("I am an " + super.type);
  }
}

class Main {
  public static void main(String[] args) {
    Dog dog1 = new Dog();
    dog1.printType();
  }
}

Output

I am a mammal
I am an animal

In this example, we have defined the same instance field type in both the superclass Animal and the subclass Dog.

We then created an object dog1 of the Dog class. Then, the printType() method is called using this object.

Inside the printType() function,

type refers to the attribute of the subclass Dog.
super.type refers to the attribute of the superclass Animal.

Hence, System.out.println(“I am a ” + type); prints I am a mammal. And, System.out.println(“I am an ” + super.type); prints I am an animal.

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3. Use of super() to access to the superclass constructor

As we probably am aware, when an object of a class is made, its default constructor is consequently called.

To unequivocally call the superclass constructor from the subclass constructor, we use super(). It’s an extraordinary type of the super catchphrase.

super() can be used distinctly inside the subclass constructor and must be the first statement.

Example 4: Use of super()

class Animal {

  // default or no-arg constructor of class Animal
  Animal() {
    System.out.println("I am an animal");
  }
}

class Dog extends Animal {

  // default or no-arg constructor of class Dog
  Dog() {

    // calling default constructor of the superclass
    super();

    System.out.println("I am a dog");
  }
}

class Main {
  public static void main(String[] args) {
    Dog dog1 = new Dog();
  }
}

Output

I am an animal
I am a dog

Here, when an object dog1 of the Dog class is made, it consequently calls the default or no-arg constructor of that class.

Inside the subclass constructor, the super() statement calls the constructor of the superclass and executes the statement inside it. Consequently, we get the output I am an animal.

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The flow of the program at that point returns back to the subclass constructor and executes the remaining statements. Thus, I am a dog will be printed.

In any case, using super() isn’t compulsory. Even if super() isn’t used in the subclass constructor, the compiler verifiably calls the default constructor of the superclass.

So, why use redundant code if the compiler automatically invokes super()?

It is required if the parameterized constructor (a constructor that takes arguments) of the superclass must be called from the subclass constructor.

The defined super() should consistently be the first statement in the body of the constructor of the subclass, else, we get a compilation error.

Example 5: Call Parameterized Constructor Using super()

class Animal {

  // default or no-arg constructor
  Animal() {
    System.out.println("I am an animal");
  }

  // parameterized constructor
  Animal(String type) {
    System.out.println("Type: "+type);
  }
}

class Dog extends Animal {

  // default constructor
  Dog() {

    // calling parameterized constructor of the superclass
    super("Animal");

    System.out.println("I am a dog");
  }
}

class Main {
  public static void main(String[] args) {
    Dog dog1 = new Dog();
  }
}

Output

Type: Animal
I am a dog

The compiler can automatically call the no-arg constructor. Be that as it may, it can’t call defined constructors.

If a defined constructor must be called, we have to expressly characterize it in the subclass constructor.

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Note that in the above example, we unequivocally called the defined constructor super(“Animal”). The compiler doesn’t call the default constructor of the superclass for this case.

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Java OOP (II)