trigger TotalAmountReceivedforCampaign on Opportunity (after insert, after update , After delete ) 
{
 
  // gather campaigns from Opps being updated
  List<String> campIds = new List<String>();
  List<String> campIdRemoved = new List<String>();

	if(Trigger.isInsert || Trigger.isUpdate)
	{
		for (Opportunity o : trigger.new)
		{
			if (o.CampaignID != null && (trigger.isInsert || o.Amount != trigger.oldMap.get(o.id).Amount))
				campIds.add(o.CampaignId);
		}
	}
	else if (Trigger.isDelete)
	{
		for (Opportunity o : trigger.old)
		{
			if ( o.CampaignID != null )
				campIds.add(o.CampaignId);
		}
	}	
 
  if (campIds.size() > 0) 
  {
 
	  Map<id, Campaign> campaigns = new Map<id, Campaign>([SELECT id FROM Campaign WHERE id =: campIds]);
	 
	  // initialize Campaign.Total_Amount_Received__c to zero on all campaigns
	  for (Campaign c : campaigns.values())
	   c.Total_Amount_Received__c= 0;
	 
	  // add all Opp.Amount_Received__c to campaigns
		for (Opportunity o : [SELECT id, Amount, CampaignId FROM Opportunity WHERE CampaignId =: campIds])
		{
			Campaign c = campaigns.get(o.CampaignId);
			c.Total_Amount_Received__c += o.Amount;
		}
 
	// update campaigns
	update campaigns.values();
  }
  
}

@isTest
 private class TEST_TotalAmountReceived {
 
static testMethod void testTotalAmountReceived() 
{
 
	// Create a test campaign
	Campaign c = new Campaign ();
	c.Name = 'UnitTest';
	insert c;
	System.assert(c != null);
	 
	// Create a test opportunity for the campaign
	Opportunity o = new Opportunity ();
	o.Name = 'UnitTest';
	o.StageName = 'Closed Won';
	o.CloseDate = Date.Today();
	o.CampaignID = c.ID;
	o.Amount = 70.00;
	insert o;
	System.assert(o != null);
	  
	// Retrieve the Campaign
	Campaign c1 = [SELECT Total_Amount_Received__c FROM Campaign WHERE Id = :c.Id];
	System.assertEquals (70.00, c1.Total_Amount_Received__c);

	Delete o;
}
}