Understanding the super Keyword and Constructors in Java

Understanding the super Keyword and Constructors in Java

When talking about inheritance in Java, it is important to understand that constructors are not inherited by a subclass.

A subclass can, however, invoke a constructor of its super class using the super keyword.

A few points about the super keyword

1. super is a Java keyword.

2. It can be used to access members of the super class, such as fields and methods.

3. It can be used to invoke a super class constructor.

4. When calling a super class constructor using super(...), it must be the first statement in the subclass constructor.

Example

class SUP {

    public SUP(String s) {
        System.out.println("Hi Super: " + s);
    }
}

// SUB class extending SUP class
class SUB extends SUP {

    public SUB(String p) {

        // Explicitly call the super class constructor
        // super(...) must be the first statement
        super(p);

        System.out.println("Hi SUB: " + p);
    }

    public static void main(String[] args) {
        new SUB("Rahul Sharma");
    }
}

Output

Hi Super: Rahul Sharma
Hi SUB: Rahul Sharma

When the SUB object is created, its constructor first invokes the SUP constructor using super(p). After the super class constructor completes, the remaining statements in the SUB constructor are executed.

What happens if super() is not written?

If the super class has an accessible no-argument constructor, Java automatically inserts a call to super() as the first statement of the subclass constructor, provided you don't explicitly call another super class constructor.

For example:

class SUP {

    public SUP() {
        System.out.println("Hi Super");
    }
}

class SUB extends SUP {

    public SUB() {
        // Compiler automatically inserts super();
        System.out.println("Hi SUB");
    }
}

The output is:

Hi Super
Hi SUB

However, if the super class does not have an accessible no-argument constructor, the subclass must explicitly invoke one of the available super class constructors.

Important point

Constructors are not inherited in Java. They are invoked as part of object construction.

The super keyword allows a subclass to explicitly invoke a super class constructor and access super class members.