Are constructors inheritable?

One of the questions that may occur to you at an interview, certification, or a simple test concerning the Java language is the following: Are constructors inheritable? Can a subclass call the constructor of the parent class?

We cannot inherit a constructor. What we can do is create an instance of a subclass using a constructor of the superclass. Try compiling and executing this code:

public class SuperClass {
    public SuperClass() {
        System.out.println("SuperClass()");
    }
    public void method1() {
        System.out.println("SuperClass.method1()");
    }
}

public class SubClass extends SuperClass {
    public SubClass() {
        System.out.println("SubClass()");
    }
    public void method() {
        System.out.println("SubClass.method()");
    }
    public static void main(String[] args) {
        SubClass instance = new SubClass();
        instance.method1();
    }
}

You will get the following output

SuperClass()
SubClass()
SuperClass.method1()

You will get the following output:

public class SubClass extends SuperClass {
    public SubClass() {
        System.out.println("SubClass()");
    }
    public SubClass(boolean b) {
        System.out.println("SubClass(boolean)");
    }
    public void method() {
        System.out.println("SubClass.method()");
    }
    public static void main(String[] args) {
        SubClass instance = new SubClass();
        instance.method1();
        
        SubClass instance2 = new SubClass(true);
        instance2.method1();
    }
}

You will get the following output

SuperClass()
SubClass()
SuperClass.method1()
SuperClass()
SubClass(boolean)
SuperClass.method1()

You will get the following output:

public class SubClass extends SuperClass {
    public SubClass() {
        super();
        System.out.println("SubClass()");
    }
    public SubClass(boolean b) {
        super();
        System.out.println("SubClass(boolean)");
    }
    public void method() {
        System.out.println("SubClass.method()");
    }
    public static void main(String[] args) {
        SubClass instance = new SubClass();
        instance.method1();

        SubClass instance2 = new SubClass(true);
        instance2.method1();
    }
}

which produces the same output. It doesn’t matter whether or not you insert the super() call which calls the superclass constructor, super() will be called anyway.

The existence of super() is justified by the fact that your superclass may have multiple constructors, and using super() is the only way to enforce which of them will be called.

Modify the code as follows:

public class SuperClass {
    public SuperClass() {
        System.out.println("SuperClass()");
    }
    public SuperClass(boolean b) {
        System.out.println("SuperClass(boolean)");
    }
    public void method1() {
        System.out.println("SuperClass.method1()");
    }
    public static void main(String[] args) {
        SuperClass instance = new SuperClass();
        instance.method1();
    }
}
public class SubClass extends SuperClass {
    public SubClass() {
        super();
        System.out.println("SubClass()");
    }
    public SubClass(boolean b) {
        super(b);
        System.out.println("SubClass(boolean)");
    }
    public void method() {
        System.out.println("SubClass.method()");
    }
    public static void main(String[] args) {
        SubClass instance = new SubClass();
        instance.method1();
        SubClass instance2 = new SubClass(true);
        instance2.method1();
    }
}

if you run the main method of SubClass you will get this output:

SuperClass()
SubClass()
SuperClass.method1()
SuperClass(boolean)
SubClass(boolean)
SuperClass.method1()

super() allows me to control which superclass constructor to call at runtime.

And that’s it for today. As usual, if you have questions or a topic you would like me to cover, feel free to write to me.

Are constructors inheritable?

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