Java nested constructors

snotnose · Feb 14, 2024

My main constructor does stuff, my 'helper' (sorry, not up on OO terminology) constructors call the main constructor with various defaults:

Code:

public class Broken {
        private int foo;

        Broken() {
            Broken(0);        // Gives 'error: cannot find symbol'
            this.Broken(1); // Ditto
        }
        Broken(int bar) {
            foo = bar;
        }

        private void show(String why) {
            System.out.println(why + foo);
        }

    public static void main(String[] args) {
        Broken b = new Broken();
        Broken b2 = new Broken(42);

        b.show("broken() ");
        b2.show("broken(42) ");
    }
}

Is this forbidden, or am I missing some syntactical sugar?

Irenicus · Feb 14, 2024

If you want to call one constructor from another, you should use 'this':

Java:

public class FooBar {
    private int value;
   
    public FooBar() {
        this(123); // calls the constructor with 1 int parameter below
    }
   
    public FooBar(int value) {
        this.value = value;
    }
   
    // other code
}

snotnose · Feb 14, 2024

Bingo! Thanks.

ShuggyCoUk · Feb 23, 2024

Sounds like you might need a primer on java/OO for someone that already programs in another paradigm (procedural for you?)

snotnose · Mar 1, 2024

This program is my Java/OO primer. I'm a retired Unix/C guy (device drivers and kernel work) that wants to learn OO.

As for multiple constructors, I think they're handy as hell. This lets me instantiate my class with various defaults.

ShuggyCoUk · Mar 1, 2024

note that some OO languages support default arguments - which can reduce the need for multiple constructors a bit. But several sorts of APIs can benefit from the capability.

ShuggyCoUk · Mar 1, 2024

Also, from a discoverability point of view, sometimes it's better to have a bunch of static methods (often termed factories, but there's no need to put that in the name) with Reasonable names indicating the intent. e.g.

Java:

// this could be private if you like, but there's no need)
class Rectangle {
    public final double Height;
    public final double Length;

    public Rectangle(double height, double length) {
        this.Height = height;
        this.Length = length;
    }
    
    public static Rectangle Square(double side) {
        return new Rectangle(side, side);
    }
    
    private static final double GoldenRatio = 1.0 + Math.sqrt(5.0)) / 2;   
    
    public static Rectangle GoldenRatioFromSmallHeight(double height) {
        return new Rectangle(height, height * GoldenRatio);
    }
    
    public static Rectangle GoldenRatioFromSmallLength(double length) {
        return new Rectangle(length * GoldenRatio, length);
    }   
}

I spent 30 seconds giving thought to the names there, likely that could be improved but you get the idea.
These work well when the types of the parameters are insufficient to indicate intent well.

koala · Mar 1, 2024

I would recommend not doing that. It goes against the naming conventions and I don't think it's common enough not to cause confusion.

Lt_Storm · Mar 1, 2024

IDK, those methods seem reasonable, but they are more likely to be contained in a class called Rectangles which would also include various other static functions for things like overlap and such. Also, the method names are likely to end in of as in Rectangles.goldenRatioLongSideOf(15);

koala · Mar 2, 2024

I mean, that is the thing, you used lowerCamelCase, which indicates a class method. UpperCamelCase is a class name. new UpperCamelCase() is fine, but UpperCamelCase() is jarring in Java.

Not a huge fan of the "of" prefix, but it's a thing now through the stdlib, so that is fine.

ShuggyCoUk · Mar 3, 2024

Apologies for the casing snafu. I'm so used to c# conventions now I can't remember the java one's off the top of my head.

koala · Mar 3, 2024

Ah! That explains it. I get very confused when reading C#. I thought you were trying to make those look like constructors, which on first blush is a cute idea, but which I don't like in Java. (In Python that's quite common and I have done it on occasion.)

ShuggyCoUk · Mar 3, 2024

nope, definitely not trying to screw with language conventions - just 20 years of indoctrination!

Java nested constructors

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