What is a constructor in OOP?
A constructor is a special method that runs automatically when an object is created. Its job is to set the starting values of the object so it begins life in a valid state. A constructor has the same name as the class and has no return type. If you do not write one, the compiler provides a default constructor.
Why constructors exist
When you create an object you often want it to start with sensible values. A constructor lets you set those values in one place, so no object is ever created half finished. It runs once, at the moment the object is made.
class Student {
String name;
int age;
Student(String n, int a) { // constructor
name = n;
age = a;
}
}
Student s = new Student("Asha", 20); // constructor runs hereTypes of constructors
- Default constructor: takes no arguments and sets default values. The compiler adds one if you write none.
- Parameterised constructor: takes arguments so you can set values while creating the object.
- Copy constructor: creates a new object by copying an existing one, common in C++.
Key rules
A constructor has the same name as the class and no return type, not even void. You can have more than one constructor in a class by giving them different parameters, which is constructor overloading.
A common follow up is the difference between a constructor and a normal method. The constructor has no return type and runs automatically on object creation, while a method has a return type and runs only when you call it. State that clearly and you look confident.
Common follow up questions
Related interview questions
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