Question

The way my teacher writes and organizes code is very different than what is proposed in my supplementary textbook. In a recent lab, we were required to use the wheels library to display several objects in a single window. Here is the Snowman class that was written by my teacher (code for other objects have not been included for convenience):

public class Snowman {
    private Ellipse _top;
    private Ellipse _middle;
    private Ellipse _bottom;

public Snowman() {
        _top = new Ellipse();
        _top.setColor(Color.WHITE);
        _top.setFrameColor(Color.BLACK);
        _top.setFrameThickness(1);
        _top.setSize(80, 80);
        _middle = new Ellipse();
        _middle.setColor(Color.WHITE);
        _middle.setFrameColor(Color.BLACK);
        _middle.setFrameThickness(1);
        _middle.setSize(120, 120);
        _bottom = new Ellipse();
        _bottom.setColor(Color.WHITE);
        _bottom.setFrameColor(Color.BLACK);
        _bottom.setFrameThickness(1);
        _bottom.setSize(160, 160);
    }
    public void setLocation(int x, int y) {
        _top.setLocation(x + 40, y - 170);
        _middle.setLocation(x + 20, y - 100);
        _bottom.setLocation(x, y);
    }
}    

This object, among others, is later instantiated in the SnowmanCartoon class:

public class SnowmanCartoon extends Frame{
    private Snowman _snowman;
    private Eyes _eyes;
    private Hat _hat;
    private Bubble _bubble;

    public SnowmanCartoon() {
        _snowman = new Snowman();
        _snowman.setLocation(100, 300);
        _eyes = new Eyes();
        _eyes.setLocation(165, 150);        
        _hat = new Hat();
        _hat.setLocation(152, 98);
        _bubble = new Bubble();
        _bubble.setLocation(280, 60);
    }
    public static void main(String[] args) {
        new SnowmanCartoon();
    }
}

---

Here are my concerns:

1. In both of these classes, why is there a method of the same name as the class, and what is its purpose?

2. Why is the method setLocation() a void method while the method Snowman() is not, even though Snowman() does not return anything?

3. In the SnowmanCartoon class, when it says private Snowman _snowman; and _snowman = new Snowman();, is Snowman referring to the Snowman class, or rather the Snowman() method?

4. If the instantiation of the Snowman object is referring to the Snowman() method to set all of its properties, why don’t I need to use the dot operator: Snowman.Snowman()?

5. In my textbook, instance variables and methods are declared in a one class and are instantiated in another. However, they seem to occur simultaneously in my teacher’s code.

Answer 1

The method that is named the same as a class is called “constructor” of the class. It is used to instantiate an object.

Example:

public class MyClass { // class
    public MyClass() { // constructor

    }
}

Note that constructor is just like any other method, however, it does not have a return type. It basically returns “this” object.

When you call

MyClass a = new MyClass(); 

this will actually invoke the constructor and create the object.

Note that you can have multiple constructors in a class by using different number and types of parameters.

In java, if you do not include any constructor, then any class will by default have the default parameterless constructor.

public MyClass() {
}

Tons of information available on constructors online. They are the basic concepts to start learning object oriented programming. Some more information here.

–EDIT–
answering question about how to call specific question.

Take these two constructors

public MyClass(){
// do things
}

public MyClass(String p){
// do things
}

Then when you instantiate object

MyClass c = new MyClass(); // use constructor 1
MyClass d = new MyClass("value"); // use constructor 2