APEX TRAINING -1:

This example for beginners those who wants start apex ...
public class MyHelloWorld {
public static void applyDiscount(Book__c[] books) {
for (Book__c b :books){
b.Price__c *= 0.9;
}
}
}

trigger HelloWorldTrigger on Book__c (before insert) {
Book__c[] books = Trigger.new;
MyHelloWorld.applyDiscount(books);
}

@isTest
private class HelloWorldTestClass {
static testMethod void validateHelloWorld() {
Book__c b = new Book__c(Name='Behind the Cloud', Price__c=100);
System.debug('Price before inserting new book: ' + b.Price__c);
// Insert book
insert b;
// Retrieve the new book
b = [SELECT Price__c FROM Book__c WHERE Id =:b.Id];
System.debug('Price after trigger fired: ' + b.Price__c);
// Test that the trigger correctly updated the price
System.assertEquals(90, b.Price__c);
}
}