About

In Apex, you can define top-level classes (also called outer classes) as well as inner classes, that is, a class defined within another class. You can only have inner classes one level deep. For example:

public class myOuterClass 
{
    // Additional myOuterClass code here
    class myInnerClass 
    {
        // myInnerClass code here
    }
}

The following syntax is used for defining classes:

private | public | global 
[virtual | abstract | with sharing | without sharing | (none)] 
class ClassName [implements InterfaceNameList | (none)] [extends ClassName | (none)] 
{ 
    // The body of the class
}

• The private access modifier declares that this class is only known locally, that is, only by this section of code. This is the default access for inner classes—that is, if you don't specify an access modifier for an inner class, it is considered private. This keyword can only be used with inner classes.
• The public access modifier declares that this class is visible in your application or namespace.
• The global access modifier declares that this class is known by all Apex code everywhere. All classes that contain methods defined with the webService keyword must be declared as global. If a method or inner class is declared as global, the outer, top-level class must also be defined as global.
• The with sharing and without sharing keywords specify the sharing mode for this class.
• The virtual definition modifier declares that this class allows extension and overrides. You cannot override a method with the override keyword unless the class has been defined as virtual.
• The abstract definition modifier declares that this class contains abstract methods, that is, methods that only have their signature declared and no body defined.

Useful Documentation

• Apex Class Definition
• Class Variables
• Class Methods
• Access Modifiers
• Apex Properties