system
Given the following post, respond as if you're commenting on Reddit:
human
What is the role of the default constructor and why do we need more than one constructor?

I still dont understand the idea behind constructors. What does the one that doesnt take any value do? And why do some other constructors less say, one takes two parameters and the other might take two parameters of the first one plus a third one.
gpt
Say you have some default settings you want to apply for a given class. One way you can do that is create the class, then apply those settings in however many lines of code needed.  For example:

    Rectangle r = new Rectangle();
    r.setWidth(2);
    r.setHeight(4);

But what happens if you have that in multiple places? That's annoying! Better put it in the Rectangle class:

    public class Rectangle {
        // assume fields and setters are here
        public void init() {
            setWidth(2);
            setHeight(4);
        }
    }

Then you can do this everywhere, and your values are in one convenient place!

    Rectangle r = new Rectangle();
    r.init();

But wait, that's still an extra line of code that you have to remember to do. Bummer. Let's do a constructor instead!

    public class Rectangle {
        // assume fields and setters are here
        public Rectangle() {
            setWidth(2);
            setHeight(4);
        }
    }

Now you can create a rectangle with default values in one easy line.

    Rectangle r = new Rectangle();

One more step: let's say you want flexible values on creation, but you want to be absolutely sure that those values are provided. Make your class look like this instead:

    public class Rectangle {
        // assume fields and setters are here
        public Rectangle(int width, int height) {
            setWidth(width);
            setHeight(height);
        }
    }

That will work with this line of code:

    Rectangle r = new Rectangle(2, 4);

But this old line will not compile, because no constructor matches that signature (input parameter types, in this case (int, int))

    Rectangle r = new Rectangle();

If you have no constructor at all, the default is used at all times and you'll have all values set to null, 0, or 0.0 depending on the data type.

Hope that helps!