Apex Developer Guide

Apex Developer Guide
Getting Started with Apex
Writing Apex
Data Types and Variables
Control Flow Statements
Classes, Objects, and Interfaces
Classes
Apex Class Definition
Class Variables
Class Methods
Using Constructors
Access Modifiers
Static and Instance Methods, Variables, and Initialization Code
Apex Properties
Extending a Class
Extended Class Example
Interfaces
Keywords
Annotations
Classes and Casting
Differences Between Apex Classes and Java Classes
Class Definition Creation
Namespace Prefix
Apex Code Versions
Lists of Custom Types and Sorting
Using Custom Types in Map Keys and Sets
Working with Data in Apex
Running Apex
Debugging, Testing, and Deploying Apex
Apex Language Reference
Appendices
Glossary
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Using Constructors

A constructor is code that is invoked when an object is created from the class blueprint. You do not need to write a constructor for every class. If a class does not have a user-defined constructor, a default, no-argument, public constructor is used.

The syntax for a constructor is similar to a method, but it differs from a method definition in that it never has an explicit return type and it is not inherited by the object created from it.

After you write the constructor for a class, you must use the new keyword in order to instantiate an object from that class, using that constructor. For example, using the following class:
A new object of this type can be instantiated with the following code:

If you write a constructor that takes arguments, you can then use that constructor to create an object using those arguments.

If you create a constructor that takes arguments, and you still want to use a no-argument constructor, you must create your own no-argument constructor in your code. Once you create a constructor for a class, you no longer have access to the default, no-argument public constructor.

In Apex, a constructor can be overloaded, that is, there can be more than one constructor for a class, each having different parameters. The following example illustrates a class with two constructors: one with no arguments and one that takes a simple Integer argument. It also illustrates how one constructor calls another constructor using the this(...) syntax, also know as constructor chaining.
New objects of this type can be instantiated with the following code:
Every constructor that you create for a class must have a different argument list. In the following example, all of the constructors are possible:

When you define a new class, you are defining a new data type. You can use class name in any place you can use other data type names, such as String, Boolean, or Account. If you define a variable whose type is a class, any object you assign to it must be an instance of that class or subclass.