Constructors in Inheritance – Different scenarios explained

In this article, I will be covering how constructors work in inheritance.

How Constructor invocation works in Inheritance

Constructor invocation works slightly differently when inheritance is involved. When an object of the sub-class is created, it implicitly invokes the constructor in the base class. Once the code in the base class constructor is completed, control returns to the sub class constructor and the code is the sub-class constructor is executed.

There are different scenarios in which the invocation differs slightly. Let’s explore these scenarios

Scenario 1 – Base class has a constructor

When the base-class has a constructor, creating an object of the sub-class results in automatic invocation of the base class constructor. Consider the following code snippet:

public class Animal{

public Animal() {
System.out.println("In Animal constructor");
}
}

public class Cat extends Animal{

public Cat() {
System.out.println("In Cat constructor");
}
}

Here, The Animalclass has  a constructor. Cat is a sub-class of Animal and has its own constructor.

Now consider the following code:

Cat cat = new Cat();

This code creates a new Cat object. In such a scenario, Java first invokes the Animalconstructor after which it invokes the Cat constructor. So this code prints the following output:

In Animal constructor
In Cat constructor

Scenario 2 – Base class does not have a constructor

If there is no constructor in the base class, Java provides a default no arguments constructor in the base class and inserts a call to this constructor from the sub-class constructor. So consider the following code snippet:

public class Animal{
}

public class Cat extends Animal{

Cat(){
System.out.println("In cat constructor");
}

}

Here, The Animal class does not have  a constructor. Cat is a sub-class of Animal and has its own constructor.

Now consider the following code:

Cat cat = new Cat();

This code creates a new Cat object. Since there is no constructor in the Animal class, Java creates a default constructor in the Animal class and inserts a call to it. So this code prints the following output:

In cat constructor

Scenario 3 – Constructor with arguments in the sub-class

When there is a constructor with arguments in the base-class, Java is not able to add a default constructor in the base-class. So a compilation error occurs. Consider the following code:

public class Animal{

String name;

public Animal(String name) {
this.name = name;
}

}

public class Cat extends Animal{



Cat(){ //this causes compilation error

}

}

Here, the Animal class has a constructor that accepts an argument. The Cat class has a no arguments constructor. However, a compilation error occurs in the Cat class. This is because there is no default no-arguments constructor in the Animal class.  Also, Java is unable to insert a default constructor since there is a constructor with arguments in the base class. You can fix this by explicitly adding a no arguments constructor in the Animal class or by invoking the Animal constructor from the Cat constructor explicitly

Conclusion

So in this article, we saw how constructors are used in inheritance in Java. We understood the various scenarios for Java constructor inheritance. In summary, when you create an object of a sub-class, Java implicitly invokes the base class constructor. If there is no constructor in the base class, Java supplies a default no arguments constructor in the base class and inserts a call to this constructor.