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A Guide to OOP Principles in Java

Published by Nathaniel Stickman
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Object-oriented programming gives you a set of programming principles to make your code more compartmentalized and reusable. Object-oriented programming accomplishes this by structuring programs around objects. This tutorial covers the core principles of object-oriented programming and provides examples of these concepts written in Java.

What is Object Oriented Programming (OOP)?

Object-oriented programming — often abbreviated “OOP” — is a set of programming principles centered on objects. Such a set of principles is called a programming paradigm. Objects in OOP can hold attributes and be assigned behaviors, and they allow developers to structure programs around reusable, self-contained components.

Because of its object-oriented focus, OOP shines when used for applications that need objects to be at their logical center. This is the case with user interfaces, one of the most common places to see OOP used, as well as business applications.

OOP Concepts in Java You Need to Know

Object-oriented programming tends to make use of four structures. These form the bedrock of all of the pieces a developer has to work with when building object-oriented programs.

  • Classes: These act as blueprints for objects. They define underlying properties and behaviors which can be inherited by other classes and by objects. Your OOP program’s collection of classes creates a structure off of which the rest of the program gets built.

    Often, in Java, code is constructed with one class per file. The class syntax resembles the following example:

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    public class ClassName {
    // Code related to the class.
}

  • Objects: These are derived from classes and populate the abstract of their classes’ properties with concrete values. They are the things built from the blueprints provided by classes. Objects also tend to be where the behaviors defined on classes get executed, bringing your application to life.

    Java lets you instantiate an object from a class using the new keyword. Here, a new object gets created from the class created above. This example works when the class has a constructor defined. You can see an example of a constructor definition in the Examples of Object Oriented Programming section further on.

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    ClassName objectName = new ClassName();

  • Attributes: These are fields (or properties) defined on classes and which represent the state of a particular object. A class might, for instance, define an attributeOne as a String type. An object derived from that class can then use that attribute, assigning it attributeOne = "a string", for example.

    This next example shows what it could look like to add an attribute to the ClassName class created above:

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    public class ClassName {
    public attributeOne = "a string";
}

  • Methods: These are functions defined on classes, and they provide objects with behaviors. Methods typically act on the values held by an object’s attributes, allowing each object to act in a self-contained way.

    In the following example, you can see what it looks like to add a basic method to a class, using the ClassName example started above:

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    public class ClassName {
    public attributeOne = "a string";

    public void methodOne() {
        System.out.println("The method has been called!");
    }
}

4 Basic OOP Principles

In addition to the four basic parts, object-oriented programming has four fundamental concepts. These are what primarily make OOP stand out, and developers rely on these when making the most effective and reusable OOP code.

These next four sections cover the four principles of OOP, giving you an overview of the roles they play. Then, keep reading to find a section with examples, in Java, each of which demonstrates these principles in action.

Encapsulation

This principle ensures that objects are self-contained and limits what information about their state they expose. In other words, other objects cannot directly access the state of an object. Each object manages its own state. To modify an object’s state, other objects need to use that object’s dedicated methods.

So, for instance, say you have an object called firstObject. That object has two attributes, attributeOne and attributeTwo. Encapsulation prevents another object, say secondObject, from modifying the values of the attributes on firstObject.

Now, firstObject has control of its own state. It may, for instance, define a method called setAttributeOne that outside objects can access. This way, secondObject can make changes to attributeOne on firstObject. But if firstObject does not define a similar method for attributeTwo, secondObject has no means of modifying it.

Encapsulation can make OOP applications easier to upgrade and easier for collaboration. An engineer working on one object would thus be less likely to cause breaking changes to an object someone else is working on.

Encapsulation also makes it easier to keep track of objects’ states. These states can become complicated, and more so the more outside access they allow. By ensuring that each object controls its own state, you make the code easier for yourself and other developers to follow and maintain.

Data Abstraction

This principle states that classes include only the details relevant to their context. Doing so creates abstract classes, which more specific classes and objects can extend.

Take the example of a Pet class. You can make this class to define, in the most general way, the characteristics of pets. So, the class may have name, diet, and health attributes. Now you can extend that class with more specific kinds of Pet. For instance, you may define a Dog class that extends Pet and adds a bark method. At the same time, you can also define a Cat class similarly extending on Pet.

One of the goals of abstraction is to define common characteristics. Using the example above, Dog has the unique behavior of the bark method, but otherwise it shares things like having a name in common with other pets. Abstraction makes it so that you do not need to redefine these attributes for each specific kind of pet.

Abstraction also allows you to evaluate various classes by common abstract classes. So long as you know that both Cat and Dog extend Pet, you can evaluate them based on the common attributes held in Pet.

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if (obj eitherCatOrDog instanceof Pet) {
    System.out.println("This is my pet, " + eitherCatOrDog.name + ".");
}

Inheritance

This principle declares that objects get some or all of the properties of their parents. Inheritance is the foundation of reusability in OOP. With it, you can create a class and its properties can be reused in multiple objects.

For example, you can start with a ClassName class from which you create two objects, objectOne and objectTwo. Each of these objects inherits from the parent class, ClassName, and receives all of its attributes and methods. The objects can then each individually work with those attributes and methods. But the important feature is that the ClassName class acts as a common and reusable base.

In Java, such parent classes are called super classes. Commonly, classes inheriting from super classes are called sub classes. This means that you can make additional classes that inherit from super classes, so that you can have a chain of inheritance.

Take a look at the Pet example above again. You have a Cat class and a Dog class that inherit from Pet, thus gaining its attributes. From there, you can create specific objects that inherit from the new classes:

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Pet myDog = new Dog();

The new object inherits not only properties of the Dog class — like the bark method — but also those on the Pet class, like the name attribute.

Polymorphism

This principle states that each sub class can be used in the same way as its parent class or parent classes. At the same time, each sub class may keep its own, distinct form of attributes and methods initially defined in a super class.

Polymorphism is one of the more complicated features of OOP, but it plays a useful role. To help you understand it, below is an example that reworks the Pet example elaborated in the sections above.

Say, for instance, when creating the Pet class, you include a method called makeSound:

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class Pet {
    public String name = "None";
    public String diet = "Herbivore";
    public boolean healthy = true;

    public void makeSound() {
        System.out.println("This is my pet sound.");
    }
}

Obviously, the effect of makeSound should be different for Cat and Dog, even though both, being pets, do make sounds:

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class Cat extends Pet {
    public String diet = "Carnivore";

    public void makeSound() {
        System.out.println("Meow.");
    }
}

class Dog extends Pet {
    public String diet = "Omnivore";

    public void makeSound() {
        System.out.println("Bark.");
    }
}

Following polymorphism, you can, indeed, use any property from the Pet class on any object deriving from the Cat and Dog classes. The effect may be different — you get a different sound from the makeSound method — but all of the parts are still there.

Examples of Object Oriented Programming

This section includes snippets of code that give examples of OOP concepts in Java. These are aimed to simultaneously show off some of the components of OOP as well as the four core principles discussed above. The examples also familiarize you with the elements of Java that relate to OOP.

Starting simple, this first example shows a single Java class, not counting the default Main class used to start up the program. This class covers all of the parts — class, object, attribute, and method — of OOP mentioned above.

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// Create a class.
class BookShelf {
    // Declare the class attributes.
    public int numberOfBooks;

    // Implement a constructor. This is used to create objects from the class,
    // which you can see done in the `Main` class below.
    public BookShelf(int initialNumberOfBooks) {
        numberOfBooks = initialNumberOfBooks;
    }

    // Provide a method to add more books to the shelf.
    public void addBooks(int numberToAdd) {
        numberOfBooks += numberToAdd;
    }

    // Provide a method to display the count of books on the shelf.
    public void showBookCount() {
        System.out.println("The shelf has " + numberOfBooks + " books.");
    }
}

class Main {
    public static void main(String args[]) {
        // Use the `BookShelf` class's constructor to create a BookShelf object;
        // it also lets us specify how many books the object starts with.
        BookShelf thisBookShelf = new BookShelf(5);
        thisBookShelf.addBooks(2);
        thisBookShelf.showBookCount();
    }
}
The shelf has 7 books.

Now, this next example is a little more ambitious. It has three classes — again, not counting the Main class. The first, GamingConsole, acts as a super on which other classes can extend. That is exactly what the second class, PlayStation, does — extends on the GamingConsole class. The last class, PlayStation4, does the same, but with the PlayStation class as its direct parent.

This chain of extensions lets the example demonstrate several of the concepts of OOP at once. Each extension shows the concept of abstraction in action. The PlayStation class is able to make use of both attributes and methods from its parent, demonstrating inheritance. And the PlayStation4 class illustrates polymorphism through its identification with the GamingConsole super class during construction in the Main class.

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// Create a super class, from which the other classes ultimately extend.
class GamingConsole {
    // Declare attributes.
    public String consoleType;
    public String currentGame;

    // Provide a cosntructor to set initial values.
    public GamingConsole(String initialConsoleType, String initialGame) {
        consoleType = initialConsoleType;
        currentGame = initialGame;
    }

    // Provide two methods that should be common to all gaming consoles.
    public void insertGame(String gameName) {
        currentGame = gameName;
    }

    public void playGame() {
        System.out.println("Starting up the " + consoleType + " console.");
        if (currentGame == "") {
            System.out.println("No game in the console.");
        } else {
            System.out.println("Playing " + currentGame + ".");
        }
    }
}

// Create a sub class for a specific category of gaming console.
class PlayStation extends GamingConsole {
    // Declare attributes.
    public boolean controllerConnected;

    // Provide a constructor.
    public PlayStation(String initialGame, boolean initialControllerConnected) {
        super("PlayStation", initialGame);
        controllerConnected = initialControllerConnected;
    }

    // Provide a method unique to this category of gaming consoles. (This
    // feature is not actually unique to PlayStation consoles, but just
    // pretend for the purposes of illustration.)
    public void connectController(boolean isControllerConnected) {
        controllerConnected = isControllerConnected;
    }

    // Override the default `playGame` method with a specific implementation
    // for `PlayStation` objects.
    public void playGame() {
        if (controllerConnected == false) {
            System.out.println("Connect a controller before playing.");
        } else {
            super.playGame();
        }
    }
}

// Create another sub class for an even more specific category, this being
// a type of PlayStation gaming console.
class PlayStation4 extends PlayStation {
    // Provide a constructor.
    public PlayStation4(String initialGame, boolean initialControllerConnected) {
        super(initialGame, initialControllerConnected);
        consoleType = "PlayStation 4";

        initiateWelcome();
    }

    // Provide a specific method for PlayStation 4 consoles.
    public void initiateWelcome() {
        System.out.println("Welcome to " + consoleType + ".");
    }
}



class Main {
    public static void main(String args[]) {
        // Instantiate a `PlayStation4` object. Notice the polymorphism
        // implied by the fact that we can use `GamingConsole` to identify
        // the new object's type.
        GamingConsole thisConsole = new PlayStation4("", true);

        // Use the two methods inherited from the `GamingConsole` super class.
        thisConsole.insertGame("Minecraft");
        thisConsole.playGame();
    }
}
Welcome to PlayStation 4.
Starting up the PlayStation 4 console.
Playing Minecraft.

Conclusion

In this guide you learned the fundamental principles of object-oriented programming. The concepts covered were encapsulation, abstraction, inheritance, and polymorphism. Applying these concepts helps to ensure that you are making the most of what the paradigm can do.

Throughout this tutorial, the focus has been on OOP related to Java. But keep in mind that these concepts apply anywhere that supports object-oriented programming. JavaScript, Python, and Ruby are popular examples.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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