Tutorial 5: Creating Objects of Your Classes

So in the last tutorial, we learnt about how to define your classes and how to define the packages to which they belong. In this tutorial, we take things a notch further by learning how to create objects of your classes.

So, let’s define a class called Person. The person we create will have an age, a name and an ID. These are called properties of the class. We will also define a simple function that calculates how old the Person objects will be in ten years from now.

public class Person {

            int age;

            String name;

            int id;

            int ageInTenYears () {

                        return age + 10;

            }

            public static void main (String[] args) {

            }

}

Okay. So we’ve defined the class.

In the real world, I’m a person called Brian. I am 25 years old and my ID is 11111111. You might be called Bob, with an age of 30 and an ID of 22222222.

We define the new objects within the main function i.e. within public static void main(String[] args).

public class Person {

            int age;

            String name;

            int id;

            int ageInTenYears () {

                        return age + 10;

            }

            public static void main (String[] args) {

                        Person brian = new Person();

                        Person bob = new Person();

            }

}

So as you can see above, you type the name of the class, give it an identifier or name, and then equate it to new Person();

 

What new Person(); is call a constructor that creates the new object when you run the program. We will talk about constructors in a future tutorial.

So, in the above code, we are telling Java to create a new object of the Person class with the identifier or label “brian” and another object of the Person with the identifier or label “bob.” When we were creating the Person class, we said that the Person would have an ID, a name and an age. So after creating “brian” and “bob”, we know that they have a name, ID number and age. It’s only that we have not yet defined the age and the ID.

We said that we want to tell Java that “brian” is 25 years old with an ID number 11111111 and that “bob” is 30 years old with an ID number 22222222. How do we do this?

In Java, whenever you want to define the property of an object, you use a dot (.) after the identifier or label that you gave your new object. So if I want to tell Java that the age of the Person object labeled “brian” is 25, and that the ID of the Person object labeled “brian” is 11111111, I do this:

public class Person {

            int age;

            String name;

            int id;

            int ageInTenYears () {

                        return age + 10;

            }

            public static void main (String[] args) {

                        Person brian = new Person();

                        Person bob = new Person();

                        brian.age = 25;

                        brian.id = 11111111;

            }

}

Then, to tell Java that the Person object labeled “bob” has the age of 30 and the ID number is 22222222, we do the same as we did for the Person object labeled “brian.”

public class Person {

            int age;

            String name;

            int id;

            int ageInTenYears () {

                        return age + 10;

            }

            public static void main (String[] args) {

                        Person brian = new Person();

                        Person bob = new Person();

                        brian.age = 25;

                        brian.id = 11111111;

                        bob.age = 30;

                        bob.id = 22222222;

            }

}

To test whether the objects are defined as we intended, we can output the values to the console using the System.out.println() function like this:

public class Person {

            int age;

            String name;

            int id;

            int ageInTenYears () {

                        return age + 10;

            }

            public static void main (String[] args) {

                        Person brian = new Person();

                        Person bob = new Person();

                        brian.age = 25;

                        brian.id = 11111111;

                        bob.age = 30;

                        bob.id = 22222222;

                        System.out.println(“Brian’s age is ” + brian.age + “ and his id is” +

                                    brian.id);

System.out.println(“Bob’s age is ” + bob.age + “ and his id is” +

                                    bob.id);

//Age in Ten years:

System.out.println("Brian's age in ten years will be " + brian.ageInTenYears());

System.out.println("Bob's age in ten years will be " + bob.ageInTenYears());

            }

}

Let’s test this out in Eclipse or whatever Java IDE you’re using and see if it works.

Figure 1: The figure shows the above code for the Person class. The results are on the right.

I know I promised you in the last tutorial that we would also create a Cat class and give the cat an age, a weight and a height, I would like you to try this on your own. You might encounter some difficulties, but keep at it until the program works. We learn by failing our way to understanding.

In the next tutorial, we are going to look at access specifiers. These are the keywords private, public and protected.

If you have any questions or comment, make sure you drop them in the comment section below and I will address them as soon as possible.

Until next time, stay safe.