Programming with Constructors in Java

In my previous articles, I already covered the concept of overloading of methods (or method overloading).  If this is a new concept for you, I suggest you refer to my previous articles.

In the previous section, I used a statement something like the following:

        MyCalc obj1 = new MyCalc(10,20);

By now you understand that the code snippet above creates an instance/object, calls the constructor which has two parameters, and finally assigns the reference to “obj1.”  Let me modify the same like this:

        MyCalc obj1 = new MyCalc();

If you execute your program with the above statement, without having a default constructor (covered in first section of this article), it generates a compilation error.  That means it could not find any constructor without parameters, and thus it cannot create an instance. 

Let me rewrite the class to something like the following:

  public class MyCalc {

    int x;
    int y;

    public void setValues(int m, int n) {
        x = m;
        y = n;
    }

    public int getSum() {
        int z;
        z = x + y;
        return z;
    }

  }

Now, the above class doesn’t have any constructor. With it, if you try to execute the following statement again, it will not result in any error.

        MyCalc obj1 = new MyCalc();

How is this possible?  I didn’t include any default constructor in the above class (nor any other constructor with parameters).  That is simply a trick.  If a class is not equipped with any constructor, JVM creates one for us (internally) automatically. 

Now, let me consider the case of the following two statements:

        MyCalc obj1 = new MyCalc();
        MyCalc obj2 = new MyCalc(10,20);

Within the above two statements, I am trying to create “obj1,” which tries to call the default constructor (or the constructor which doesn’t have any parameters).  I am also trying to create “obj2,” which tries to call the constructor having two parameters.

Can I include two constructors in the same class? Yes. You can have any number of constructors within a single class, but with a difference in parameters. This is called “constructor overloading” (or overloading of constructors).

The next section gives you the full code to demonstrate “constructor overloading.”