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Classes and objects

Bundle data (fields) and behavior (methods) into one blueprint, then make many objects from it.

This lesson is for Python · JavaScript · TypeScript · Java · C# · C++.
C has no classes; it does the same job with structs (struct) and functions (see the structs lesson).
Craykon has no user-defined classes and keeps such data in a Dict(), as in student["name"].

A class is a blueprint, and an object is a real thing built from it.
The example below uses a Student class with a name and a score to make several students, then sorts them by score and prints each with a grade.

class Student:    def __init__(self, name, score):        self.name = name        self.score = score     def grade(self):        if self.score >= 90:            return "A"        if self.score >= 80:            return "B"        return "C"     def describe(self):        return f"{self.name} {self.score} {self.grade()}"  n = int(input())students = []for _ in range(n):    name, score = input().split()    students.append(Student(name, int(score))) students.sort(key=lambda s: s.score, reverse=True)for s in students:    print(s.describe())
stdin
3
Mina 82
Joon 95
Sora 78
stdout
Joon 95 A
Mina 82 B
Sora 78 C

Key points

  • A class lists the data each object holds (fields) and what the object can do (methods).
  • The constructor is a special function that fills in the starting values when an object is made.
  • Inside a method, the object itself is this (self in Python).
    When a parameter has the same name, write this.name = name to tell them apart.
  • Access modifiers (public, private) decide which members outside code may use.
    Fields are usually hidden and read through methods or properties.
  • A list of objects is sorted by a field.
    You hand the sort function "what to compare".

How each language writes it

Pythonclass Student: with the constructor def __init__(self, name, score):.
Fields are made with self.name = name, and every method's first parameter is self.
Objects are made without new: Student("Mina", 82).
Sort: students.sort(key=lambda s: s.score, reverse=True)
JavaScriptclass Student { constructor(name, score) { this.name = name; } }.
Objects are made with new Student("Mina", 82); leaving out new gives TypeError: Class constructor Student cannot be invoked without 'new'.
Sort: students.sort((a, b) => b.score - a.score)
TypeScriptSame shape as JavaScript, with fields declared up front, like score: number;.
private and readonly are checked only at compile time and vanish at run time.
To really hide a field, use JavaScript's private #score
JavaThe constructor has the class's name and no return type.
Fields are hidden with private and read through getters like getScore().
With no modifier, a member is visible only inside the same package.
Sort: students.sort(Comparator.comparingInt(Student::getScore).reversed())
C#A property public int Score { get; } is read from outside like a field: s.Score.
A member with no modifier is private.
Sort: students.Sort((a, b) => b.Score.CompareTo(a.Score))
C++class defaults to private and struct to public; that is the only difference, so the example writes public: and private: itself.
The constructor fills fields with an initializer list : name_(name), score_(score), and const after a method promises it will not change the object.
Objects are values, made without new: Student s("Mina", 82);.
Sort: std::sort with the lambda [](const Student& a, const Student& b) { return a.score() > b.score(); }

Common mistakes

Forgetting self in a methodPython

class Student:
    def grade():
        return "A"

Student().grade()

→ TypeError: Student.grade() takes 0 positional arguments but 1 was given. Writing score without self. inside a method is a NameError

Make self the first parameter and always read fields as self.score.

A list [] as a default valuePython

def __init__(self, name, tags=[]):
    self.tags = tags

→ That list is made only once, when def runs. A tag added to one student shows up for the others too

Take tags=None and make a new list inside the constructor.

Comparing objects' contents with ==Java · C#

new Student("Mina", 82) == new Student("Mina", 82)

→ false — == asks whether they are the same object. Until you override equals, it behaves exactly like ==

Compare contents with equals (Equals in C#), and override that method yourself or use a record.

How languages differ
After b = a, changing b also changes a in Python, JavaScript, TypeScript, Java and C#, because both names point to the same object.
In C++ objects are values, so Student b = a; and for (Student s : students) make copies; to work on the original, use Student& (or const Student& when you only read).

Practice

  1. Café stamp cards6P0261
  2. A game character that reports its own status6P0346
  3. Bank account objects6P0078
  4. Shared ticket numbers6P0079
  5. League table6P0080
  6. A different salary for each kind of employee6P0157
  7. Hourglasses that run together6P0323
  8. Editing a floor plan with chained methods6P0276