What Does a Constructor Do?

A constructor is a special method that runs automatically when an object is instantiated. Its main purpose is to initialize the new object's instance variables, establishing its initial state.

How a Constructor Works

In object-oriented programming, a class defines the attributes and behaviours that its objects will have. When a program uses new to instantiate an object, memory is allocated for the object and its constructor is invoked.

For example, consider this Java class:

public class Student {
    private String name;
    private int score;

    public Student(String studentName, int initialScore) {
        name = studentName;
        score = initialScore;
    }
}

The statement below creates an object and passes two arguments to the constructor:

Student learner = new Student("Amira", 85);

The constructor's parameters receive "Amira" and 85. It assigns these values to the object's fields, so learner begins with name equal to "Amira" and score equal to 85.

A class may have multiple constructors with different parameter lists. This is constructor overloading. If no constructor is declared in Java, the compiler normally supplies a default no-argument constructor; however, once any constructor is declared, that automatic constructor is not supplied.

A common misconception is that the constructor itself creates or returns the object. More precisely, new initiates object creation, while the constructor initializes the object's state.

IB Exam Technique

For B3.1, define a constructor using both ideas: it is called during instantiation and initializes instance variables. When tracing code, identify the arguments, match them to constructor parameters, and state the resulting field values. Do not describe a constructor as an ordinary method or give it a return type.