# Constructor

URL: https://softwaredictionary.org/terms/constructor
Category: Programming Fundamentals
Last updated: 2026-10-05
Pronunciation: kun-STRUK-ter

In short: A constructor is a special method that runs when a new object is created from a class, setting up its starting values so it is ready to use.

## What is a constructor in programming?

A constructor is the code that runs automatically when you create a new object. Its job is to put the object into a valid starting state: storing the values passed in, setting defaults, and checking that the input makes sense. In JavaScript and TypeScript it is a method named `constructor`, in Python it is `__init__`, and in Java and C# it is a method with the same name as the class and no return type.

You rarely call a constructor by name. In JavaScript, Java and C# you write `new`, as in `new User("Ada")`, and in Python you call the class like a function, `User("Ada")`. The arguments go to the constructor's parameters. A class can offer several ways to build an object: Java and C# allow overloading with different parameter lists, while languages without overloading use default values or named factory methods such as `User.fromJson()`.

A constructor is like the setup when you open a bank account: before you can use it, the bank records your name, assigns a number and sets the balance to zero. In a subclass, the constructor usually calls the parent's constructor first, with `super()` in JavaScript, Java and Python, so the inherited part of the object is set up too. Keeping a constructor short and free of slow work, such as network calls, makes objects easy to create and to test.

## Key takeaways

- A constructor runs automatically when a new object is created from a class.
- Its job is to give the object a valid starting state.
- It is called `constructor` in JavaScript, `__init__` in Python, and takes the class's name in Java and C#.
- A subclass constructor calls the parent's constructor with `super()`.

## Example: A class with a constructor in JavaScript

```javascript
class Account {
  constructor(owner, balance = 0) {
    if (!owner) throw new Error("An account needs an owner");
    this.owner = owner;     // store what was passed in
    this.balance = balance; // or fall back to a default
  }
}

const account = new Account("Ada"); // runs the constructor
console.log(account.balance);       // 0
```

## Frequently asked questions

**What is the difference between a constructor and a method?**

A method is called on an object that already exists, as often as needed. A constructor runs once, while the object is being created, and its only job is to set it up. It doesn't return a value of its own, because the new object is the result.

**Can a constructor be private?**

Yes, in languages such as Java, C# and TypeScript. A private constructor stops other code from calling `new` directly, so a class can send callers through a factory method instead, or allow only one instance, as in the singleton pattern.

## Sources

- [MDN: constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/constructor)
- [The Java Tutorials: Providing Constructors for Your Classes](https://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html)

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