Apex Developer Guide

Apex Developer Guide
Getting Started with Apex
Writing Apex
Data Types and Variables
Data Types
Primitive Data Types
Collections
Lists
Sets
Maps
Parameterized Typing
Enums
Variables
Constants
Expressions and Operators
Assignment Statements
Rules of Conversion
Control Flow Statements
Classes, Objects, and Interfaces
Working with Data in Apex
Running Apex
Debugging, Testing, and Deploying Apex
Apex Language Reference
Appendices
Glossary
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Parameterized Typing

Apex, in general, is a statically-typed programming language, which means users must specify the data type for a variable before that variable can be used.
This is legal in Apex:
1Integer x = 1;
This is not legal, if x has not been defined earlier:
1x = 1;
Lists, maps and sets are parameterized in Apex: they take any data type Apex supports for them as an argument. That data type must be replaced with an actual data type upon construction of the list, map or set. For example:
1List<String> myList = new List<String>();

Subtyping with Parameterized Lists

In Apex, if type T is a subtype of U, then List<T> would be a subtype of List<U>. For example, the following is legal:
1List<String> slst = new List<String> {'foo', 'bar'};
2List<Object> olst = slst;