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Object-oriented programming (OOP) organizes software around objects that combine related state and behavior. A class describes a kind of object; encapsulation controls how other code uses it; inheritance and interfaces can help share or specify behavior when the design calls for them. The phrase “the art of object-oriented programming” is also part of the title of a particular introductory book, Karel++, but it is commonly used more broadly to describe learning and designing with OOP.
What is an object-oriented program?
OOP is a way to organize a program around objects rather than treating all data and operations as unrelated pieces. In Oracle’s Java tutorial, an object is “a software bundle of related state and behavior.” In that Java framing, fields represent state and methods represent behavior. This is a useful starting model, not a rule that every object-oriented language uses identical terminology or mechanisms.
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For example, a bank account object might hold a balance as state and provide methods for depositing or withdrawing money. Callers use those operations rather than changing the balance arbitrarily. The object is useful because its data and the operations that act on it belong together.
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A class defines the structure and behavior that its objects will have; an object is an individual instance created from that class. Think of a class as a description and each object as a particular thing described by it.
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A BankAccount class could define a balance and deposit and withdrawal operations. Two accounts created from that class are separate objects, each with its own current balance. The class is not itself one of those accounts.
What does encapsulation accomplish?
Encapsulation controls how other code accesses an object’s internal data. Rather than allowing callers to change a balance directly, a class can expose methods that validate a deposit or withdrawal and maintain the account’s rules. The public behavior becomes the usable interface; implementation details can remain internal.
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Languages differ in how they enforce or express this boundary. In Java, access controls such as private and public are one way to restrict access. Do not assume that every OOP language uses Java’s exact rules or syntax.
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Inheritance shares or specializes a class
Inheritance lets a class derive from another class, reusing or specializing its state and behavior. In Java, the parent is called a superclass and the deriving class a subclass; Java classes have one direct superclass. Other languages can have different inheritance rules.
A hierarchy can be helpful when a genuine “is a kind of” relationship exists and the shared behavior makes sense for every subclass. It can also make changes harder to reason about when subclasses depend on inherited details. Prefer a hierarchy only when it clarifies the model, rather than using inheritance simply because two classes happen to share a few lines of code.
Interfaces describe a behavior contract
In Java, an interface describes behavior that a class promises to provide. Code can rely on that expected capability instead of depending on one specific class implementation. For example, a method that accepts an interface for printable items can work with different classes that fulfill that contract.
An interface is useful when different kinds of objects should be usable through the same expected behavior without forcing them into one class hierarchy. The exact meaning and capabilities of interfaces depend on the language and version.
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How do packages fit in?
In Java, a package is a namespace for organizing related classes and interfaces. Packages help structure a Java codebase and distinguish names; they are not an OOP feature shared by all languages. Other languages provide their own mechanisms for grouping and naming code.
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A practical sequence for designing with objects
- Identify the domain concept. Choose a real thing or role in the problem that needs its own data or behavior.
- Group related state. Decide which information belongs together and should be managed as one object.
- Define behavior around that state. Add operations that make sense for the object and preserve its rules.
- Expose a small usable interface. Let callers do what they need without relying on internal representation.
- Consider abstraction only when it helps. Use inheritance for a meaningful shared class relationship, or an interface when distinct implementations should meet the same behavior contract.
This is a practical learning sequence, not a universal design prescription. A design should fit its language and problem rather than force every concept into a class hierarchy.
Where can beginners learn more?
| Resource | Best fit | Language or focus | Format and currency |
|---|---|---|---|
| Karel++: A Gentle Introduction to the Art of Object-Oriented Programming | Novice programmers and introductory or second computer-science courses | Uses a robot world and an analog equivalent of C++ | Wiley lists a paperback by Joseph Bergin, Mark Stehlik, Jim Roberts, and Richard E. Pattis, ISBN 9780471138099, published October 1996. See Wiley’s listing; current price and availability are not established here. |
| Oracle Java Tutorials | Readers learning introductory OOP concepts through Java | Java-specific concepts and examples | The tutorials say they were written for JDK 8 and may not reflect later improvements or technologies no longer available. Use them for foundational concepts and check current Java documentation for version-sensitive syntax or APIs: Oracle’s concepts tutorial. |
| Designing Object-Oriented Software | Developers interested in design principles and process, rather than a beginner syntax course | Responsibilities, collaborations, hierarchies, subsystems, and protocols | InformIT describes the book by Rebecca Wirfs-Brock, Brian Wilkerson, and Lauren Wiener as a design resource: InformIT’s listing. |
The title phrase itself is associated with Wiley’s Karel++, a specific introductory text rather than the name of a single universal OOP method. Its 1996 publication date matters when choosing it as a course text; it should not be assumed to cover every modern language or current practice.
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