What is the difference between class and instance methods?

The main difference between class methods and instance methods lies in how they are accessed and their association with the class or its instances.

Key Differences Between Class Methods and Instance Methods

1. Definition

• Instance Methods: Operate on a specific instance of the class. They can access both the instance (via this in Java, self in Python) and class-level data.
• Class Methods: Belong to the class as a whole rather than any specific instance. They typically operate on class-level data and are often marked with the static keyword in many programming languages.

2. Access

• Instance Methods: Require an instance of the class to be called.

  MyClass obj = new MyClass();
  obj.instanceMethod();

• Class Methods: Can be called directly on the class without creating an instance.

  MyClass.classMethod();

3. Purpose

• Instance Methods: Handle operations that depend on the state of a specific object (instance).
• Class Methods: Handle operations that are class-wide or independent of any specific instance.

4. Access to Members

• Instance Methods: Can access:
  • Instance variables and instance methods.
  • Class (static) variables and methods.
• Class Methods: Can only access:
  • Class (static) variables and methods.
  • They cannot directly access instance variables or methods because they lack a reference to a specific instance.

Examples

In Java

class MyClass {
    int instanceVar = 10; // Instance variable
    static int classVar = 20; // Class variable

    // Instance method
    void instanceMethod() {
        System.out.println("Instance variable: " + instanceVar); // Access instance data
        System.out.println("Class variable: " + classVar); // Access class data
    }

    // Class method
    static void classMethod() {
        System.out.println("Class variable: " + classVar); // Access class data
        // System.out.println("Instance variable: " + instanceVar); // Error
    }
}

public class Main {
    public static void main(String[] args) {
        // Call instance method
        MyClass obj = new MyClass();
        obj.instanceMethod();

        // Call class method
        MyClass.classMethod();
    }
}

In Python

class MyClass:
    class_var = 20  # Class variable

    def __init__(self, instance_var):
        self.instance_var = instance_var  # Instance variable

    # Instance method
    def instance_method(self):
        print(f"Instance variable: {self.instance_var}")
        print(f"Class variable: {MyClass.class_var}")

    # Class method
    @classmethod
    def class_method(cls):
        print(f"Class variable: {cls.class_var}")
        # print(f"Instance variable: {self.instance_var}") # Error

# Using instance method
obj = MyClass(10)
obj.instance_method()

# Using class method
MyClass.class_method()

Summary of Differences

Instance methods focus on individual object behavior, while class methods handle tasks that are relevant to the entire class or shared among all objects. For deeper insights into object-oriented programming principles, consider exploring Grokking Advanced Coding Patterns for Interviews on DesignGurus.io!