An object is a runtime entity in object-oriented programming that combines state and behavior: it holds data in fields and exposes operations, called methods, that read or change that data. Every object has an identity that distinguishes it from all other objects, even from one whose field values are identical.
In most mainstream languages, an object is created as an instance of a class, which acts as its blueprint. Programs are then built as networks of objects that send messages to one another by calling methods, so the object is the basic unit of design, reuse and reasoning in this paradigm.
What an object is made of
A classic definition describes an object through three properties: state, behavior and identity. State is the current set of field values, behavior is the set of methods the object responds to, and identity is what makes it a distinct thing in memory. Two objects can be equal in state and still be different objects.
The following table summarizes these properties and how they typically appear in code.
| Property | Meaning | Typical representation |
|---|---|---|
| State | Data the object currently holds | Fields, attributes, instance variables |
| Behavior | Operations the object can perform | Methods bound to the object |
| Identity | Distinctness from every other object | Memory reference or object ID |
Internally, a typical object in a managed runtime such as the JVM or the .NET CLR consists of an object header and the field data. The header usually stores a pointer to the class or type information, which enables dynamic dispatch, plus bookkeeping for garbage collection, hashing and locking. On 64-bit JVMs the header commonly occupies 12 to 16 bytes, which is why millions of tiny objects carry a noticeable memory overhead.
Object lifecycle in object-oriented programming
Every object passes through a lifecycle of creation, use and destruction. Creation allocates memory and runs a constructor that establishes valid initial state; use consists of method calls; destruction releases memory and any resources the object owns.
| Stage | What happens | Language mechanisms |
|---|---|---|
| Allocation | Memory is reserved for fields and header | new operator, factory functions |
| Initialization | Constructor sets a valid initial state | Constructors, initializers |
| Use | Methods read and modify state | Method calls, message sends |
| Unreachability | No references to the object remain | Scope exit, reassignment |
| Destruction | Memory and resources are released | Garbage collection, destructors, reference counting |
The release strategy differs sharply between languages. C++ uses deterministic destruction, so a destructor runs exactly when an object leaves scope, which supports the RAII idiom for files and locks. Java, C# and Python rely mainly on garbage collection or reference counting, so memory is reclaimed eventually, and external resources should be closed explicitly with constructs such as try-with-resources or context managers.
How an object relates to class, interface and other concepts
An object is often confused with the class that defines it. The class is a description that exists at compile time or as metadata; the object is a concrete instance at runtime with its own memory. One class can produce millions of objects, and the four pillars of the paradigm describe how these objects are structured and connected.
| Concept | Relationship to an object | Example |
|---|---|---|
| Class | Blueprint from which objects are instantiated | Invoice defines fields and methods |
| Interface | Contract that an object promises to fulfill | An object implements Comparable |
| Encapsulation | Hides the object’s state behind methods | Private balance field with a deposit method |
| Inheritance | Lets an object’s class reuse a parent class | SavingsAccount extends Account |
| Polymorphism | Lets different objects answer the same message | Each shape object computes its own area |
Not every object system uses classes. Prototype-based languages such as JavaScript create objects that delegate directly to other objects, and object literals can be built without any class at all. In languages such as Python and Ruby, everything is an object, including numbers, functions and the classes themselves.
Identity, equality and copying
Distinguishing identity from equality is one of the most practical skills in object-oriented code. Identity asks whether two references point to the same object; equality asks whether two objects represent the same value. Java uses == for identity and equals() for equality, while Python uses is and ==.
The kind of object determines which comparison makes sense, as the following table shows.
| Kind of object | Defined by | Comparison | Example |
|---|---|---|---|
| Entity object | Identity that persists over time | By identifier | Customer, order |
| Value object | Its field values only | By value, usually immutable | Money amount, date range |
| Service object | Behavior, little or no state | Rarely compared | Payment gateway client |
| Data transfer object | Plain fields for moving data | By value when needed | API request payload |
Copying follows the same logic. A shallow copy duplicates the outer object but shares the objects it references, while a deep copy duplicates the whole graph. Choosing the wrong one leads to unintended shared state, where changing one copy silently changes another.
Design guidelines and common pitfalls
Well-designed objects are small, cohesive and responsible for their own invariants. A constructor should leave the object in a valid state, and every public method should keep it valid, so that no caller can observe a half-built or inconsistent object.
- Anemic objects: classes with only getters and setters push all logic elsewhere and lose the benefit of encapsulation.
- God objects: one object that knows and does too much becomes hard to test and change.
- Leaking internal state: returning a reference to a mutable internal list lets callers bypass the object’s rules.
- Broken equality contracts: overriding equality without a matching hash function breaks hash-based collections.
- Excessive allocation: creating many short-lived objects in hot loops increases garbage collection pressure.
A useful rule of thumb is tell, do not ask: instead of reading an object’s fields and making decisions outside it, ask the object to perform the operation. This keeps behavior next to the data it depends on and reduces coupling between objects.
In everyday language
An object is a small, self-contained piece of a program that knows some facts and can do some things. A bank account object knows its balance and can accept a deposit or refuse a withdrawal that is too large.
The class is like the printed form for opening an account, and each object is one filled-in, real account. Two accounts can show the same balance and still be two different accounts, just as two people can have the same name. Programs work by having these objects ask each other to do things.
Seen in the real world
An online store models its checkout with objects. Each shopping cart object holds a list of line-item objects and offers methods such as addItem, removeItem and total. Prices are represented as immutable money value objects that combine an amount and a currency, so adding euros to dollars raises an error instead of producing a wrong number.
When 5,000 customers shop at the same time, the server holds 5,000 separate cart objects created from the same class, each with its own state. Because the cart enforces its own rules, such as a maximum quantity of 10 per item, no other part of the code can create an invalid order. Bugs stay local and easy to find, and the team can add a discount feature by changing a single class.