Classes & Objects

Easy

In plain terms

A class is a template that describes what properties and methods its instances will have. An object is one concrete instance created from that class. For example, a "Person" class might define name and age and a greet() method; you then create many Person objects (Alice, Bob) with different names and ages.

Classes promote reuse: write the structure once, create many objects. They also give you a clear place to put related logic. In many languages, "new" creates an object from a class and runs a special constructor method to set up initial state.

What you need to know

•Class = blueprint, Object = instance
•Constructor sets initial state
•Same class, many objects

Example

Code is language-agnostic in spirit; adapt the idea to your language:

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
  greet() {
    return "Hi, I'm " + this.name;
  }
}
const alice = new Person("Alice", 25);
const bob = new Person("Bob", 30);
alice.greet();  // "Hi, I'm Alice"

Why this matters

Interviewers often ask you to design a class or explain the difference between class and object. Knowing constructor, instance vs static, and when to use a class vs a plain object matters.

How it connects

Classes are where encapsulation (next) is applied: private fields and public methods. Inheritance and polymorphism (later) extend classes. SOLID principles apply to how you design classes.

Interview focus

Be ready to explain these; they come up often.

▸Class = blueprint/template; object = instance with actual data.
▸Constructor: purpose (initialize state), when it runs (on new).
▸When to use a class vs a simple object: shared behavior, multiple instances, need for constructors.

Learn more

Dive deeper with these resources:

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