How can i use assertions in java?

[Ammar Salem]

Hello guys, i need to know what are Assertions and how they differ from exceptions,why do we need them? i really need help? my book doesnt help much.
Ammar

[Jilesh Lakhani]

http://java.sun.com/developer/technicalArticles/JavaLP/assertions/

[camilo lopes]

do you use assert only for test if one line of your code is error or was config for false.
Assert not can to be use in place of IF. Assert is a good programming practice.
example:

class Asert{
int number=10;
static void br(){
number = 20;
assert number==20:"the variavel number was changed for 20, this no can";
public static void main(String arg[]){
br();
}
}