Object and class on OOFP 2025-26

Object

Mon, 01 Jan 0001 00:00:00 +0000

Object #

An object groups data in a meaningful way.

A simple example: JSON #

Here is a JSON object (in Javascript syntax) that describes the city of Florence:

{
  name: "Florence",
  zipCode: 50100
}

A JSON object may contain other objects, as well as arrays (of values, objects or arrays). For instance:

{
  name: "Alice",
  age: 26,
  birthPlace: {
    name: "Florence",
    zipCode: 50100
  },
  jobs: [
    {
      employer: "Kolping",
      start: 2021,
      end: 2022
    },
    {
      employer : "Eurac",
      start: 2023
    }
  ]
}

In JSON:

Class and instance

Mon, 01 Jan 0001 00:00:00 +0000

Class and instance #

A class can be viewed as a blueprint for a set of similar objects.

Terminology. An object that follows this blueprint is called an instance of the class.

Example. Consider the two following objects (in pseudocode):
{
  name: "Florence",
  zipCode: 50100,
  region: "Tuscany"
}
$\qquad$
{
  name: "Rome",
  zipCode: 00100,
  region: "Lazio"
}
Each of these objects describes a city, and they share the same keys (name, zipCode and region).

Passing by value or by reference

Mon, 01 Jan 0001 00:00:00 +0000

Passing by value or by reference #

Definition. Imperative languages differ in the way arguments are passed to methods/functions. Two common strategies are:

passing by value: the method receives as input a copy of each argument.

passing by reference: the method receives as input a reference to each argument (which lets the method modify the originals).

Example. Consider the following program (in pseudocode):
myInteger = 0
myMethod(myInteger)
print(myInteger)

myMethod(argument){
  argument += 1
}
If the argument is passed by value, then the program prints 0.

If the argument is passed by reference, then the program prints 1.

In Java #

Java (and many other programming languages, like C, Python, Javascript, etc.) passes arguments by value.