FAQ: Java: Introduction to Classes - Classes: Constructor Parameters

This community-built FAQ covers the “Classes: Constructor Parameters” exercise from the lesson “Java: Introduction to Classes”.

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FAQs on the exercise Classes: Constructor Parameters

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so confusing. how am i the first person to post a reply on this discussion board?


the FAQ feature is reasonable new, many people still make new topics rather then replying to a FAQ topic.

wiki-bot made this topic, we both posted a reply in this topic. But the forum has many topics, so what do you mean by discussion board? Here is a list of the latest topics:


see? Quite a bit of activity

Anyway, do you have an actually question?

Hi. Thanks for your reply. Im just going through the lessons and then visiting the discussion board associated with each lesson to get clarification on the lessons. Several of them don’t have any comments yet. I didn’t realize others were posting questions elsewhere. I’ll Look at those other questions. :slight_smile:

Jon H

what is the difference between constructor parameter and main() method’s parameter? are they same?

the main method is called by the Java compiler/JVM. Without main, our program won’t run

while the constructor allows you to set instance fields.

I noticed that in the examples the instance field names are usually different from the instance parameters, while at the same time they are all being mapped. Is there any naming convention? I imagine in theory they could all have the same names (eg. product = product;).


why is it not necessary to write - Store variableName = new Store() but just new store() ? how does that work?

I am wondering why keyword .this is ommited in the explanation for that lesson when assigning parameter value to the field. Why is that? And what difference does it make if .this would be added?

public class Car {
  String color;

  // constructor method with a parameter
  public Car(String carColor) {
    // parameter value assigned to the field
    color = carColor;
  public static void main(String[] args) {
    // program tasks

Looking on exercises for next lessons ( INHERITANCE AND POLYMORPHISM for example) all constructors use .this:

class Noodle {
  double lengthInCentimeters;
  double widthInCentimeters;
  String shape;
  String ingredients;
  String texture = "brittle";
  Noodle(double lenInCent, double wthInCent, String shp, String ingr) {
    this.lengthInCentimeters = lenInCent;
    this.widthInCentimeters = wthInCent;
    this.shape = shp;
    this.ingredients = ingr;

Can somebode explain that please?

Hi! :wave:

I believe this is a rather basic question, but this is something I can’t figure out. I’m still struggling with the basics, I’m afraid :anguished:

Looking at the following code in the exercise:

public class Car {
String color; // constructor method with a parameter
public Car(String carColor) {
// parameter value assigned to the field
color = carColor;
public static void main(String[] args) {
// program tasks

I believe I understand what’s going on. I declare the class Car, and then I create a new instance of the class using the constructor within the class. This class accepts a string variable to define the car color. So, if later, I use the following code:

new Car("blue");
// carColor references "blue" inside constructor
new Car("yellow");
// carColor references "yellow" inside constructor

I can create an instance of Car of blue color or yellow color. So far, so good.

But, what’s the meaning of String color; right after declaring the public class Car { ? In other words, why do I need to declare a string variable color and then assign the value of color to carColor in the constructor? Why not simply declaring the String color variable inside the constructor?

This String color inside the Car class and color = carColor inside the constructor looks redundant to me.

Many thanks!

1 Like

String color right after public class Car is a member variable/field.

scope rules. Declaring a variable within a method makes it local. Then we can’t access the color in other methods

the member variable/field can be accessed in other methods of the class. So we could make a printColor method which would print something like: the color of the car is: [color], where [color] is the member variable/field

Maybe that is a good challenge? Go ahead and implement this method :slight_smile:

There is more to member variables/fields (with inheritance and so forth), but that is for later


I am totally lost as well.

If you have a specific question, you can post it here with any relevant code and someone can help you.

Otherwise, try going over some of the lessons again. Here is an article about constructors with parameters.

Hey !

Regarding this lesson,
Why can’t we use the same name for both instance field and constructor parameter?

Not sure. Java is not a language I use a lot, so not familiar with this specific aspect. Maybe @victoria_dr knows?

1 Like

Java allows you to use the same name for both, but it won’t accomplish what you intend. This will result in a semantic error. Take a look at this example.

public class Pizza {
  int diameter;  // default value of 0
  String cheeseType;  // default value of null

  public Pizza(int diameter, String cheeseType) {
    diameter = diameter;
    cheeseType = cheeseType;

  public static void main(String[] args) {
    Pizza order1 = new Pizza(12, "mozzarella");

    System.out.println(order1.diameter);  // prints 0
    System.out.println(order1.cheeseType);  // prints null

As you can see, order1's instance variables are not assigned the values of 12 and "mozzarella", as expected. Rather, they have the default values of 0 and null.

This is because the constructor doesn’t assign the parameter values to the instance variables. Rather, it sets the parameters equal to themselves.

For a much more in-depth analysis and possible alternatives, see this resource.


public double divide(int a, int b){

return a/b;

From this line of code
When divding 5/4 we get 1.0

Do we have to use double for a, b also?

Yes. When dividing two ints, the value returned will also be of type int.

There are several ways to ensure true division, rather than integer division, happens.

  • Make a a double.
  • Make b a double.
  • Make a and b doubles.
  • Use type casting (example below). More information here.

Type Casting Example
int a = 5;
int b = 4;

System.out.println((double) a / b); // prints 1.25

In this example, a is cast to the double type, which means the expressions a / b is treated as an int (b) divided by a double (a). This is similar as if a was declared as a double and b as an int.

However, note that type casting does not change the type of variable, meaning a is not changed from an int to a double. Rather, the specific occurrence of a in the expression a / b is treated as a double.

product isn’t the actual string, its a variable/parameter of type string. The actual string comes when you create a new object/class instance

guys I’m totally lost, I feel so stupid