Follow along with the examples Person.java and Dialog.java.

What are “Objects”

Conceptually, an object represents some entity, be it a physical entity or something existing only in our imaginations. Objects have attributes or states that we can use to describe them. They may also interact with one another or the “outside world.” Through these interactions, the state of an object may change.

To make things concrete, we can consider a person to be a type of object. Each person has many different attributes, such as

• a name
• an age
• species (Homo sapiens)

And how do people interact with each other? Here we list just a few ways:

• be born
• celebrate a birthday
• greet another person

Of course this set of attributes and interactions doesn’t fully specify human identity and behavior, but hopefully it gives some concrete realization of what objects, attributes, and interactions might look like.

Representing objects in Java

In Java, (almost) everything is represented as an object. In order to define a new type of object, we use the keyword class. Specifically, the following code defines a new type of object, a Person:

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public class Person {

}


This syntax is probably already familiar to you, since every program we write in Java is written inside some class.

The code above defines a Person class, but it isn’t yet a very interesting or useful class. We can implement the features described above, starting with the attributes name, and age. (We’ll talk about species a little later.) Someone interacting with a Person shouldn’t be able to change that person’s name or age, so we use the modifier private to declare variables storing these values. For example, we might have:

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public class Person {
private String name;
private int age;
}


The variables name and age are known as private instance variables. Instance variables are variables whose values are determined for each individual instance of the object. That is, each Person has their own name and age. The private modifier means that these variables are not accessible outside of the class Person itself.

Next, we should deal with the creation of a new instance of a Person. We specify how a Person is initialized and built by defining a constructor for the Person class. The constructor is a method (1) whose name coincides with the class name (for us Person), and (2) does not have a return type (not even void). The constructor may take arguments needed for initialization. For us, it might be convenient to pass the constructor a String corresponding to the person’s name. Additionally, when a person is born, their age is 0, so we can write a constructor for Person as follows.

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public class Person {
private String name;
private int age;

Person (String theName) {
name = theName;
age = 0;
}
}


Now we can, in principle, create a new person with a given name. Still, we don’t have any way of interacting with our Person object once it is created. In order to use our Person class in a program, we can write public instance methods that will allow our program to interact with the Person instances it uses. The most basic interactions we could have with a person might be to access the information stored as private instance variables. Colloquially, these such methods are referred to as getters, and they typically follow the naming convention getVariableName() (unless the variable is a boolean, in which case we use isVariableName()). For instance, here are two getter methods for a Person’s name and age:

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public class Person {
...
public String getName () {
return name;
}

public int getAge() {
return age;
}
}


If we wanted the user to be able to change a Person’s name, we could also define a setter method for this variable, such as

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public void setName(String newName) {
name = newName;
}


However, I don’t think it is prudent to allow a programmer to change a Person’s name (let alone age!) without their consent so I will not implement these functions here.

The getter methods above are public instance methods. They use the modifier public to indicate that they are accessible to anyone using/creating a Person object. We will see the distinction between public and private below.

Now, let’s add a little more functionality to our Person class. Specifically, we can write public instance methods for the following features:

• celebrateBirthday() increments a Person’s age and prints a celebratory message.
• sayGreeting() prints a greeting on the screen (along with the name of the speaker).
• greet(Person p) prints a personalized greeting for p.

Here is the code to implement these features:

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public void celebrateBirthday () {
age++;
System.out.println("Happy Birthday, " + name + "!!!");
System.out.println(name + " is now " + age + " years old.");
}

public void sayGreeting () {
System.out.println(name + ": Hello, there!");
}

public void greet (Person p) {
System.out.println(name + ": Hi, " + p.getName() + ".");
}


Now we have a complete Person object!

Creating and using instances of the Person class

One point of potential linguistic confusion is the distinction between “class” and an instance of the class. The term “class” refers to the type of object. For example the specification of the class Person above doesn’t refer to a particular individual. Rather, Person specifies traits and behaviors that all individuals exhibit: a name, an age, etc. An instance of the Person class refers to a particular individual with a specific name, age, etc.

Now that we’ve implemented the Person class, we can create new instances of the Person class and have them interact. You can declare and define new instances of Person as follows:

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Person alice = new Person("Alice");
Person bob = new Person("Bob");


This code creates two instances of the class Person. Note that new Person("Alice") on the right calls the constructor for Person, which takes a string as an argument (the Person’s name).

I’ve implemented this example in a separate file, Dialog.java. To follow along, be sure that Dialog.java and Person.java are in the same directory. To run the program, compile both files, then run Dialog:

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javac Person.java
javac Dialog.java
java Dialog


Once we’ve created our new Person instances alice and bob, we can program a brief dialog. To call an instance method for alice, you use the syntax alice.methodName(...). For example, we can first celebrate alice and bob’s birthdays with

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alice.celebrateBirthday();
bob.celebrateBirthday();


Then we can have alice and bob meet and greet each other with

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alice.sayGreeting();
bob.greet(alice);


Run Dialog.java and see how it works!

Public vs Private Methods and Variables

Now that we have created instances of the Person class, we can experiment a little with how the keywords public and private affect the behavior of the code. Consider the following modifications to the code given in Person.java and Dialog.java. Make each modification individually, the try to compile/run the program again. What happens?

1. What happens if you change the modifier of celebrateBirthday() from public to private?
2. Try changing alice’s name to Carole in Dialog.java by adding the line
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alice.name = "Carole";


after alice is declared. What happens?

3. Keep the changes in Dialog.java from step 2, but change the declaration private String name in Person.java to public String name, then compile both files. What happens now?

Class variables and methods

In our Person class, all of the variables and methods defined so far are instance variables and methods. That is, their values and functionalities apply to instances of the class individually: each Person ‘s name, age, and birthday celebrations are are particular to that individual.

However, there may be information common to all instances of a class, or functionality relevant to the class that doesn’t apply to individual instances. For example, all people belong to the same species, Homo sapiens. We don’t need to store a species name for each individual Person, yet we might want to store species information within the class Person. To indicate that we want a variable for the class Person rather than instances of the Person class, we use the keyword static. For example

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public class Person {
...
public static String SPECIES = "Homo sapiens";
...
}


makes a single String common to every Person. This is called a public class variable (to distinguish it from a public instance variable). Since class variables are not specific to instances of the class, you can access them via the following syntax: ClassName.variableName. For example

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System.out.println("A person belongs to the species " + Person.SPECIES);


will print the name of the human species. As SPECIES is declared above, we could change the species of all Persons by using, for example, Person.SPECIES = "Canis lupus". We probably do not want to allow such tomfoolery. To make it so that the SPECIES cannot be changed, we can add the keyword final to the declaration above:

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public static final String SPECIES = "Homo sapiens";


This declaration makes SPECIES a class constant. It can still be accessed via Person.SPECIES, but any attempt to modify Person.SPECIES will result in an error.

Like instance variables, class variables may be public or private. We might want to keep track of the number of Person instances we’ve created in a (private) class variable. We can declare and initialize one as follows:

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private static population = 0;


Now we can add the line population++; to the constructor for Person so that whenever a new Person is created, the population increases by one.

Since population is private, it cannot be accessed via Person.population. We can make the value of population accessible by defining a public class method that is a getter for population:

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public static int getPopulation() {
return population;
}


Now the total population can be accessed from Dialog.java using Person.getPopulation(). See Dialog.java for an application.

Concepts
• the difference between a class and an instance of a class
• the difference between a public and private variable/method
• the difference between an instance variable/method and class variable/method
Vocabulary
• class
• object
• constructor
• instance
• instance variable
• instance method
• public
• private
• getter
• setter
• class variable
• class method
• class constant
• static
• final