Beginner 20 min read

Constructors

By the end of this lesson, you'll be able to:

Explain what a constructor is and when it runs
Write a custom constructor that accepts parameters
Distinguish the default constructor from a custom one

Prerequisites: "Creating Your First Class"

The default constructor

public class EventTicket {
    public String eventName;
    public Decimal price;
}

EventTicket ticket = new EventTicket(); // uses the invisible default constructor

Every class you write gets a free, invisible "default constructor" — a no-parameter new ClassName() that creates an empty object with every field left at its default (usually null). This is what every lesson so far has actually been using.

Writing a custom constructor

public class EventTicket {
    public String eventName;
    public Decimal price;

    public EventTicket(String eventName, Decimal price) {
        this.eventName = eventName;
        this.price = price;
    }
}

EventTicket ticket = new EventTicket('Tech Conference', 1200);

A constructor is a method with the same name as the class and no return type. this.eventName refers to the object being built; eventName (without this.) refers to the incoming parameter — this is what tells them apart when the names match.

Once you write one constructor, the default disappears

EventTicket broken = new EventTicket(); // compile error!

As soon as a class defines its own constructor, Apex stops providing the free no-argument one automatically. If you still want a no-argument option, you have to write it explicitly, alongside your custom one.

Exercise

Add a constructor to the Book class from the previous lesson that accepts title and pageCount as parameters and sets them using this.

Show hint

public Book(String title, Integer pageCount) { this.title = title; this.pageCount = pageCount; }

APEX

Constructors Quiz

1. What happens to the default no-argument constructor once you write your own custom constructor?

It disappears — you must write your own if you still need one It stays available alongside the custom one automatically Apex merges them into one Nothing changes

My Notes

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Summary

A constructor is special code that runs automatically when an object is created with new — it's the natural place to set up an object's initial data.

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