Introduction to object-oriented programming

We'll now begin our journey into the world of object-oriented programming. We'll start with focusing on describing concepts and data using objects. From there on, we'll learn how to add functionality, i.e., methods to our program.

Object-oriented programming is concerned with isolating concepts of a problem domain into separate entities and then using those entities to solve problems. Concepts related to a problem can only be considered once they've been identified. In other words, we can form abstractions from problems that make those problems easier to approach.

Once concepts related to a given problem have been identified, we can also begin to build constructs that represent them into programs. These constructs, and the individual instances that are formed from them, i.e., objects, are used in solving the problem. The statement "programs are built from small, clear, and cooperative objects" may not make much sense yet. However, it will appear more sensible as we progress through the course, perhaps even self-evident.

Classes and Objects

We've already used some of the classes and objects provided by Java. A class defines the attributes of objects, i.e., the information related to them (instance variables), and their commands, i.e., their methods. The values of instance (i.e., object) variables define the internal state of an individual object, whereas methods define the functionality it offers.

A Method is a piece of source code written inside a class that's been named and has the ability to be called. A method is always part of some class and is often used to modify the internal state of an object instantiated from a class.

As an example, ArrayList is a class offered by Java, and we've made use of objects instantiated from it in our programs. Below, an ArrayList object named integers is created and some integers are added to it.

// we create an object from the ArrayList class named integers
ArrayList<Integer> integers = new ArrayList<>();

// let's add the values 15, 34, 65, 111 to the integers object
integers.add(15);
integers.add(34);
integers.add(65);
integers.add(111);

// we print the size of the integers object
System.out.println(integers.size());

An object is always instantiated by calling a method that created an object, i.e., a constructor by using the new keyword.

Creating Classes

A class specifies what the objects instantiated from it are like.

• The object's variables (instance variables) specify the internal state of the object
• The object's methods specify what the object does

We'll now familiarize ourselves with creating our own classes and defining the variable that belong to them.

A class is defined to represent some meaningful entity, where a "meaningful entity" often refers to a real-world object or concept. If a computer program had to process personal information, it would perhaps be meaningful to define a seperate class Person consisting of methods and attributes related to an individual.

Let's begin. We'll assume that we have a project template that has an empty main program:

public class Main {

    public static void main(String[] args) {

    }
}

Let's create a class named Person. For this class, we create a separate file named Person.java. Our program now consists of two separate files since the main program is also in its own file. The Person.java file initially contains the class definition public class Person and the curly brackets that confine the contents of the class.

public class Person {

}

After creating a new file in NetBeans, the current state is as follows. In the image below, the class Person has been added to the SandboxExercise.

You can also draw a class diagram to depict a class. We'll become familiar with its notations as we go along. An empty person-named class looks like this:

A class defines the attributes and behaviors of objects that are created from it. Let's decide that each person object has a name and an age. It's natural to represent the name as a string, and the age as an integer. We'll go ahead and add these to our blueprint:

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

We specify above that each object created from the Person class has a name and an age. Variables defined inside a class are called instance variables, or object fields or object attributes. Other names also seem to exist.

Instance variables are written on the lines following the class definition public class Person {. Each variable is preceded by the keyword private. The keyword private means that the variables are "hidden" inside the object. This is known as encapsulation.

In the class diagram, the variables associated with the class are defined as "variableName: variableType". The minus sign before the variable name indicates that the variable is encapsulated (it has the keyword private).

We have now defined a blueprint — a class — for the person object. Each new person object has the variables name and age, which are able to hold object-specific values. The "state" of a person consists of the values assigned to their name and age.

Defining a Constructor

We want to set an initial state for an object that's created. Custom objects are created the same way as objects from pre-made Java classes, such as ArrayList, using the new keyword. It'd be convenient to pass values ​​to the variables of that object as it's being created. For example, when creating a new person object, it's useful to be able to provide it with a name:

public static void main(String[] args) {
    Person ada = new Person("Ada");
    // ...
}

This is achieved by defining the method that creates the object, i.e., its constructor. The constructor is defined after the instance variables. In the following example, a constructor is defined for the Person class, which can be used to create a new Person object. The constructor sets the age of the object being created to 0, and the string passed to the constructor as a parameter as its name:

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

    public Person(String initialName) {
        this.age = 0;
        this.name = initialName;
    }
}

The constructor's name is always the same as the class name. The class in the example above is named Person, so the constructor will also have to be named Person. The constructor is also provided, as a parameter, the name of the person object to be created. The parameter is enclosed in parentheses and follows the constructor's name. The parentheses that contain optional parameters are followed by curly brackets. In between these brackets is the source code that the program executes when the constructor is called (e.g., new Person ("Ada")).

Objects are always created using a constructor.

A few things to note: the constructor contains the expression this.age = 0. This expression sets the instance variable age of the newly created object (i.e., "this" object's age) to 0. The second expression this.name = initialName likewise assigns the string passed as a parameter to the instance variable name of the object created.

Defining Methods For an Object

We know how to create an object and initialize its variables. However, an object also needs methods to be able to do anything. As we've learned, a method is a named section of source code inside a class which can be invoked.

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

    public Person(String initialName) {
        this.age = 0;
        this.name = initialName;
    }

    public void printPerson() {
        System.out.println(this.name + ", age " + this.age + " years");
    }
}

A method is written inside of the class beneath the constructor. The method name is preceded by public void, since the method is intended to be visible to the outside world (public), and it does not return a value (void).

In addition to the class name, instance variables and constructor, the class diagram now also includes the method printPerson. Since the method comes with the public modifier, the method name is prefixed with a plus sign. No parameters are defined for the method, so nothing is put inside the method's parentheses. The method is also marked with information indicating that it does not return a value, here void.

The method printPerson contains one line of code that makes use of the instance variables name and age — the class diagram says nothing about its internal implementations. Instance variables are referred to with the prefix this. All of the object's variables are visible and available from within the method.

Let's create three persons in the main program and request them to print themselves:

public class Main {

    public static void main(String[] args) {
        Person ada = new Person("Ada");
        Person antti = new Person("Antti");
        Person martin = new Person("Martin");

        ada.printPerson();
        antti.printPerson();
        martin.printPerson();
    }
}

Prints:

This as a screencast:

Changing an Instance Variable's Value in a Method

Let's add a method to the previously created person class that increments the age of the person by a year.

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

    public Person(String initialName) {
        this.age = 0;
        this.name = initialName;
    }

    public void printPerson() {
        System.out.println(this.name + ", age " + this.age + " years");
    }

    // growOlder() method has been added
    public void growOlder() {
        this.age = this.age + 1;
    }
}

The method is written inside the Person class just as the printPerson method was. The method increments the value of the instance variable age by one.

The class diagram also gets an update.

Let's call the method and see what happens:

public class Main {

    public static void main(String[] args) {
        Person ada = new Person("Ada");
        Person antti = new Person("Antti");

        ada.printPerson();
        antti.printPerson();
        System.out.println("");

        ada.growOlder();
        ada.growOlder();

        ada.printPerson();
        antti.printPerson();
    }
}

The program's print output is as follows:

That is to say that when the two objects are "born" they're both zero-years old (this.age = 0; is executed in the constructor). The ada object's growOlder method is called twice. As the print output demonstrates, the age of Ada is 2 years after growing older. Calling the method on an object corresponding to Ada has no impact on the age of the other person object since each object instantiated from a class has its own instance variables.

The method can also contain conditional statements and loops. The growOlder method below limits aging to 30 years.

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

    public Person(String initialName) {
        this.age = 0;
        this.name = initialName;
    }

    public void printPerson() {
        System.out.println(this.name + ", age " + this.age + " years");
    }

    // no one exceeds the age of 30
    public void growOlder() {
        if (this.age < 30) {
            this.age = this.age + 1;
        }
    }
}

Returning a Value From a Method

A method can return a value. The methods we've created in our objects haven't so far returned anything. This has been marked by typing the keyword void in the method definition.

public class Door {
    public void knock() {
        // ...
    }
}

The keyword void means that the method does not return a value.

If we want the method to return a value, we need to replace the void keyword with the type of the variable to be returned. In the following example, the Teacher class has a method grade that always returns an integer-type (int) variable (in this case, the value 10). The value is always returned with the return command:

public class Teacher {
    public int grade() {
        return 10;
    }
}

The method above returns an int type variable of value 10 when called. For the return value to be used, it needs to be assigned to a variable. This happens the same way as regular value assignment, i.e., by using the equals sign:

public static void main(String[] args) {
    Teacher teacher = new Teacher();

    int grading = teacher.grade();

    System.out.println("The grade received is " + grading);
}

The method's return value is assigned to a variable of type int value just as any other int value would be. The return value could also be used to form part of an expression.

public static void main(String[] args) {
    Teacher first = new Teacher();
    Teacher second = new Teacher();
    Teacher third = new Teacher();

    double average = (first.grade() + second.grade() + third.grade()) / 3.0;

    System.out.println("Grading average " + average);
}

All the variables we've encountered so far can also be returned by a method. To sum:

• A method that returns nothing has the void modifier as the type of variable to be returned.

public void methodThatReturnsNothing() {
    // the method body
}

• A method that returns an integer variable has the int modifier as the type of variable to be returned.

public int methodThatReturnsAnInteger() {
    // the method body, requires a return statement
}

• A method that returns a string has the String modifier as the type of the variable to be returned

public String methodThatReturnsAString() {
    // the method body, requires a return statement
}

• A method that returns a double-precision number has the double modifier as the type of the variable to be returned.

public double methodThatReturnsADouble() {
    // the method body, requires a return statement
}

Let's continue with the Person class and add a returnAge method that returns the person's age.

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

    public Person(String initialName) {
        this.age = 0;
        this.name = initialName;
    }

    public void printPerson() {
        System.out.println(this.name + ", age " + this.age + " years");
    }

    public void growOlder() {
        if (this.age < 30) {
            this.age = this.age + 1;
        }
    }
    // the added method
    public int returnAge() {
        return this.age;
    }

The class in its entirety:

Let's illustrate how the method works:

public class Main {

    public static void main(String[] args) {
        Person pekka = new Person("Pekka");
        Person antti = new Person("Antti");

        pekka.growOlder();
        pekka.growOlder();

        antti.growOlder();

        System.out.println("Pekka's age: " + pekka.returnAge());
        System.out.println("Antti's age: " + antti.returnAge());
        int combined = pekka.returnAge() + antti.returnAge();

        System.out.println("Pekka's and Antti's combined age " + combined + " years");
    }
}

As we came to notice, methods can contain source code in the same way as other parts of our program. Methods can have conditionals or loops, and other methods can also be called from them.

Let's now write a method for the person that determines if the person is of legal age. The method returns a boolean - either true or false:

public class Person {
    // ...

    public boolean isOfLegalAge() {
        if (this.age < 18) {
            return false;
        }

        return true;
    }

    /*
     The method could have been written more succinctly in the following way:

    public boolean isOfLegalAge() {
        return this.age >= 18;
    }
    */
}

And let's test it out:

public static void main(String[] args) {
    Person pekka = new Person("Pekka");
    Person antti = new Person("Antti");

    int i = 0;
    while (i < 30) {
        pekka.growOlder();
        i = i + 1;
    }

    antti.growOlder();

    System.out.println("");

    if (antti.isOfLegalAge()) {
        System.out.print("of legal age: ");
        antti.printPerson();
    } else {
        System.out.print("underage: ");
        antti.printPerson();
    }

    if (pekka.isOfLegalAge()) {
        System.out.print("of legal age: ");
        pekka.printPerson();
    } else {
        System.out.print("underage: ");
        pekka.printPerson();
    }
}

Let's fine-tune the solution a bit more. In its current form, a person can only be "printed" in a way that includes both the name and the age. Situations exist, however, where we may only want to know the name of an object. Let's write a separate method for this use case:

public class Person {
    // ...

    public String getName() {
        return this.name;
    }
}

The getName method returns the instance variable name to the caller. The name of this method is somewhat strange. It is the convention in Java to name a method that returns an instance variable exactly this way, i.e., getVariableName. Such methods are often referred to as "getters".

The class as a whole:

Let's mould the main program to use the new "getter" method:

public static void main(String[] args) {
    Person pekka = new Person("Pekka");
    Person antti = new Person("Antti");

    int i = 0;
    while (i < 30) {
        pekka.growOlder();
        i = i + 1;
    }

    antti.growOlder();

    System.out.println("");

    if (antti.isOfLegalAge()) {
        System.out.println(antti.getName() + " is of legal age");
    } else {
        System.out.println(antti.getName() + " is underage");
    }

    if (pekka.isOfLegalAge()) {
        System.out.println(pekka.getName() + " is of legal age");
    } else {
        System.out.println(pekka.getName() + " is underage ");
    }
}

The print output is starting to turn out quit neat:

A string representation of an object and the toString-method

We are guilty of programming in a somewhat poor style by creating a method for printing the object, i.e., the printPerson method. A preferred way is to define a method for the object that returns a "string representation" of the object. The method returning the string representation is always toString in Java. Let's define this method for the person in the following example:

public class Person {
    // ...

    public String toString() {
        return this.name + ", age " + this.age + " years";
    }
}

The toString functions as printPerson does. However, it doesn't itself print anything but instead returns a string representation, which the calling method can execute for printing as needed.

The method is used in a somewhat surprising way:

public static void main(String[] args) {
    Person pekka = new Person("Pekka");
    Person antti = new Person("Antti");

    int i = 0;
    while (i < 30) {
        pekka.growOlder();
        i = i + 1;
    }

    antti.growOlder();

    System.out.println(antti); // same as System.out.println(antti.toString());
    System.out.println(pekka); // same as System.out.println(pekka.toString());
}

In principle, the System.out.println method requests the object's string representation and prints it. The call to the toString method returning the string representation does not have to be written explicitly, as Java adds it automatically. When a programmer writes:

System.out.println(antti);

Java extends the call at run time to the following form:

System.out.println(antti.toString());

As such, the call System.out.println(antti) calls the toString method of the antti object and prints the string returned by it.

We can remove the now obsolete printPerson method from the Person class.

The second part of the screencast:

Method parameters

Let's continue with the Person class once more. We've decided that we want to calculate people's body mass indexes. To do this, we write methods for the person to set both the height and the weight, and also a method to calculate the body mass index. The new and changed parts of the Person object are as follows:

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

    public Person(String initialName) {
        this.age = 0;
        this.weight = 0;
        this.height = 0;
        this.name = initialName;
    }

    public void setHeight(int newHeight) {
        this.height = newHeight;
    }

    public void setWeight(int newWeight) {
        this.weight = newWeight;
    }

    public double bodyMassIndex() {
        double heigthPerHundred = this.height / 100.0;
        return this.weight / (heigthPerHundred * heigthPerHundred);
    }

    // ...
}

The instance variables height and weight were added to the person. Values for these can be set using the setHeight and setWeight methods. Java's standard naming convention is used once again, that is, if the method's only purpose is to set a value to an instance variable, then it's named as setVariableName. Value-setting methods are often called "setters". The new methods are put to use in the following case:

public static void main(String[] args) {
    Person matti = new Person("Matti");
    Person juhana = new Person("Juhana");

    matti.setHeight(180);
    matti.setWeight(86);

    juhana.setHeight(175);
    juhana.setWeight(64);

    System.out.println(matti.getName() + ", body mass index is " + matti.bodyMassIndex());
    System.out.println(juhana.getName() + ", body mass index is " + juhana.bodyMassIndex());
}

Prints:

A parameter and instance variable having the same name!

In the preceding example, the setHeight method sets the value of the parameter newHeight to the instance variable height:

public void setHeight(int newHeight) {
    this.height = newHeight;
}

The parameter's name could also be the same as the instance variable's, so the following would also work:

public void setHeight(int height) {
    this.height = height;
}

In this case, height in the method refers specifically to a parameter named height and this.height to an instance variable of the same name. For example, the following example would not work as the code does not refer to the instance variable height at all. What the code does in effect is set the height variable received as a parameter to the value it already contains:

public void setHeight(int height) {
    // DON'T DO THIS!!!
    height = height;
}

public void setHeight(int height) {
    // DO THIS INSTEAD!!!
    this.height = height;
}

Calling an internal method

The object may also call its methods. For example, if we wanted the string representation returned by toString to also tell of a person's body mass index, the object's own bodyMassIndex method should be called in the toString method:

public String toString() {
    return this.name + ", age " + this.age + " years, my body mass index is " + this.bodyMassIndex();
}

So, when an object calls an internal method, the name of the method and this prefix suffice. An alternative way is to call the object's own method in the form bodyMassIndex(), whereby no emphasis is placed on the fact that the object's own bodyMassIndex method is being called:

public String toString() {
    return this.name + ", age " + this.age + " years, my body mass index is " + bodyMassIndex();
}

The screencast's third part:

Remember to check your points from the ball on the bottom-right corner of the material!