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Docs :

Teach Yourself VISUALLY Salesforce.com by Wiley

APEX TRAINING -4:

This trigger uses boolean context variables
trigger myAccountTrigger on Account(before delete, before insert, before update,
after delete, after insert, after update) {
if (Trigger.isBefore) {
if (Trigger.isDelete) {
// In a before delete trigger, the trigger accesses the records that will be
// deleted with the Trigger.old list.
for (Account a : Trigger.old) {
if (a.name != 'okToDelete') {
a.addError('You can\'t delete this record!');
}
}
} else {
// In before insert or before update triggers, the trigger accesses the new records
// with the Trigger.new list.
for (Account a : Trigger.new) {
if (a.name == 'bad') {
a.name.addError('Bad name');
}
}
if (Trigger.isInsert) {
for (Account a : Trigger.new) {
System.assertEquals('xxx', a.accountNumber);
System.assertEquals('industry', a.industry);
System.assertEquals(100, a.numberofemployees);
System.assertEquals(100.0, a.annualrevenue);
a.accountNumber = 'yyy';
}
// If the trigger is not a before trigger, it must be an after trigger.
} else {
if (Trigger.isInsert) {
List<Contact> contacts = new List<Contact>();
for (Account a : Trigger.new) {
if(a.Name == 'makeContact') {
contacts.add(new Contact (LastName = a.Name,
AccountId = a.Id));
}
}
insert contacts;
}
}
}}}

APEX TRAINING -3:

This example shows how to perform mixed DML operations by using future method  to perform a DML operation on the user object
public class Util {
@future
public static void insertUserWithRole(
String uname, String al, String em, String lname) {
Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
// Create new user with a non-null user role ID
User u = new User(alias = al, email=em,
emailencodingkey='UTF-8', lastname=lname,
languagelocalekey='en_US',
localesidkey='en_US', profileid = p.Id, userroleid = r.Id,
timezonesidkey='America/Los_Angeles',
username=uname);
insert u;
}
}

public class MixedDMLFuture {

public static void useFutureMethod() {

// First DML operation

Account a = new Account(Name='Acme');

insert a;

// This next operation (insert a user with a role)

// can't be mixed with the previous insert unless

// it is within a future method.

// Call future method to insert a user with a role.

Util.insertUserWithRole(

'mruiz@awcomputing.com', 'mruiz',

'mruiz@awcomputing.com', 'Ruiz');

}

}

@isTest
private class MixedDML {
static testMethod void mixedDMLExample() {
User u;
Account a;
User thisUser = [SELECT Id FROM User WHERE Id = :UserInfo.getUserId()];
// Insert account as current user
System.runAs (thisUser) {
Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
u = new User(alias = 'jsmith', email='jsmith@acme.com',
emailencodingkey='UTF-8', lastname='Smith',
languagelocalekey='en_US',
localesidkey='en_US', profileid = p.Id, userroleid = r.Id,
timezonesidkey='America/Los_Angeles',
username='jsmith@acme.com');
insert u;
a = new Account(name='Acme');
insert a;
}
}
}

APEX TRAINING -2:

The following example shows how to create opportunity with a parent account.
public class ParentChildSample {
public static void InsertParentChild() {
Date dt = Date.today();
dt = dt.addDays(7);
Opportunity newOpportunity = new Opportunity(
Name='OpportunityWithAccountInsert',
StageName='Prospecting',
CloseDate=dt);
// Create the parent reference.
// Used only for foreign key reference
// and doesn't contain any other fields.
Account accountReference = new Account(
MyExtID__c='SAP111111');
newOpportunity.Account = accountReference;
// Create the Account object to insert.
// Same as above but has Name field.
// Used for the insert.
Account parentAccount = new Account(
Name='Hallie',
MyExtID__c='SAP111111');
// Create the account and the opportunity
Database.SaveResult[] results = Database.insert(new SObject[] {
parentAccount, newOpportunity });
// Check results.
for (Integer i = 0; i < results.size(); i++) {
if (results[i].isSuccess()) {
System.debug('Successfully created ID: '
+ results[i].getId());
} else {
System.debug('Error: could not create sobject '
+ 'for array element ' + i + '.');
System.debug(' The error reported was: '
+ results[i].getErrors()[0].getMessage() + '\n');
}
}
}
}

This next example uses the Database.upsert to upsert a collection of leads that are passed in.
/* This class demonstrates and tests the use of the
* partial processing DML operations */
public class DmlSamples {
/* This method accepts a collection of lead records and
creates a task for the owner(s) of any leads that were
created as new, that is, not updated as a result of the upsert
operation */
public static List<Database.upsertResult> upsertLeads(List<Lead> leads) {
/* Perform the upsert. In this case the unique identifier for the
insert or update decision is the Salesforce record ID. If the
record ID is null the row will be inserted, otherwise an update
will be attempted. */
List<Database.upsertResult> uResults = Database.upsert(leads,false);
/* This is the list for new tasks that will be inserted when new
leads are created. */
List<Task> tasks = new List<Task>();
for(Database.upsertResult result:uResults) {
if (result.isSuccess() && result.isCreated())
tasks.add(new Task(Subject = 'Follow-up', WhoId = result.getId()));
}
/* If there are tasks to be inserted, insert them */
Database.insert(tasks);
return uResults;
}

}

@isTest

private class DmlSamplesTest {

public static testMethod void testUpsertLeads() {

/* We only need to test the insert side of upsert */

List<Lead> leads = new List<Lead>();

/* Create a set of leads for testing */

for(Integer i = 0;i < 100; i++) {

leads.add(new Lead(LastName = 'testLead', Company = 'testCompany'));

}

/* Switch to the runtime limit context */

Test.startTest();

/* Exercise the method */

List<Database.upsertResult> results = DmlSamples.upsertLeads(leads);

/* Switch back to the test context for limits */

Test.stopTest();

/* ID set for asserting the tasks were created as expected */

Set<Id> ids = new Set<Id>();

/* Iterate over the results, asserting success and adding the new ID

to the set for use in the comprehensive assertion phase below. */

for(Database.upsertResult result:results) {

System.assert(result.isSuccess());

ids.add(result.getId());

}

/* Assert that exactly one task exists for each lead that was inserted. */

for(Lead l:[SELECT Id, (SELECT Subject FROM Tasks) FROM Lead WHERE Id IN :ids]) {

System.assertEquals(1,l.tasks.size());

}

}

}

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);
}
}