Object

Published Oct 6, 2021Updated Mar 7, 2023

An object is associative data that commonly takes the form of a data structure, function, method, variable, or class.

Objects provide a structured programming approach and are the core units of object-oriented-programming (OOP). Developers can easily create various identical objects and modify existing ones within the program.

Objects also provide encapsulation, meaning that the data within that object is protected from being altered or destroyed by other functions unless explicitly allowed.

Memory allocation

Declaring an object doesn’t necessarily mean that memory is allocated for it. If an object is initialized with another object, it may just get a reference to the previously created object.

To allocate memory to an object at runtime, the use of the new keyword is needed and, depending on the programming language, the memory can be allocated to a heap or stack.

Java Example

In Java, classes are the blueprints for creating objects, which is required in object-orientation/unified modeling language (OO/UML).

The following snippet is an example of a defined class being used to instantiate new objects (instances of that class):

// Employee.java
class Employee {
  // State or field
  int id;
  String firstName;
  String lastName;
  char middleInitial;
  float years;

  // Behavior or method
  void cookingBread() {
    System.out.println("Cooking some delicious bread.");
  }
}

// Creating five different objects from the Employee class
Employee tina = new Employee();
Employee louise = new Employee();
Employee linda = new Employee();
Employee bob = new Employee();
Employee gene = new Employee();

In the example above, the new keyword was used to create five object instances of the Employee class according to the class constructor.

Accessing Fields and Methods

Since objects are class instances in Java, they can use the . operator to access fields and methods in a class.

// Car.java
public class Car {
  // state or field
  int numberOfDoors;
  String color;
  String brand;
  boolean isOn;

  // Constructor declaration of class
  public Car(int numberOfDoors, String color, String brand, boolean isOn) {
    this.numberOfDoors = numberOfDoors;
    this.color = color;
    this.brand = brand;
    this.isOn = isOn;
  }

  // Behavior or method
  void turnOn() {
    isOn = true;
    System.out.println("The car has started? " + isOn);
  }
}

// Create object or class instance
Car mercedes = new Car(5, "black", "mercedes", true);

// Access state
System.out.println(mercedes.numberOfDoors)

// Access behavior
System.out.println(mercedes.turnOn());

In the snippet above, a new Car object called mercedes is created with its isOn field set to true. When the .turnOn() is invoked with the mercedes object and its numberOfDoors field is accessed, the following is printed:

5
The car has started? True

Types of Objects

Objects can be categorized based on what they do or how they work based on design patterns, which represent repeatable solutions to common tasks or problems in programming.