Test Class

Below points we should know as a developer :-
Test class must start with @isTest annotation if class class version is more than 25
Test environment support @testVisible, @testSetUp as well
To deploy to production at least 75% code coverage is required and al test case should pass .
System.debug statement are not counted as a part of apex code coverage .
@Testvisible annotaion to make visible private methods inside test classes.
@testSetup to create test records once in a method  and use in every test method in the test class .

How to write test class for both 'If 'and 'Else' statements.
public class abc
{
public integer a,b,c;
public string s;
public void maths()
{
if(s=='add')
{
  c=a+b;
}
else if(s=='sub')
{
  c=a-b;
}
}
}

@isTest
public class Testabc
{
static testMethod void testAddMethod()
{
abc obj = new abc();
obj.a = 5;
obj.b = 5;
obj.s = 'add';
obj.maths();

System.AssertEquals(obj.c,10);
}
static testMethod void testSubMethod()
{
abc obj = new abc();
obj.a = 5;
obj.b = 5;
obj.s = 'sub';
obj.maths();

System.AssertEquals(obj.c,0);
}
}

Below tips  helps us to move towards 100 % code coverage :-

1.Tips for standardcontroller

ApexPages.standradController con=ApexPages.StandradController(Needs to pass the instance of the standard/Custom Object);

2.StandardSetcontroller

ApexPages.standradSetController con=ApexPages.StandradSetController(Needs to pass the list of the standard/Custom Object);

3.For wrapper class

ClassName.WrapperclassName wrp=new ClassName.WrapperclassName();

4.Test code catch block .

We need to create a exception in test method to cover .We can do in two different ways one suppose we want an excption on update .we can do like below .

 Trick 1-

Account acc =new Account();
try{
  insert/update acc;
}catch(Exception ex){}
As mandatory fields are missing it will throw exception and it will cover the catch block .


Trick 2-We need to add two line  of code in class like below .
try{
   DML statement;
  if(Test.isRunningTest())
  Integer intTest =20/0;
 } catch(Exception qe){
 ApexPages.Message msg = new              ApexPages.Message(ApexPages.Severity.error,qe.getMessage());
  ApexPages.addMessage(msg);
 }
--------------------------------------------------------------------------------------------------------------
Good Unit Test Example:

trigger updateParent on Opportunity (after  insert) {
    // Create a Map from Account Ids to Opportunities.
    Map<Id, Opportunity> accountIdOpportunityMap = new Map<Id, Opportunity>();
   
    for(Opportunity o : Trigger.new){
        accountIdOpportunityMap.put(o.AccountId, o);
    }
   
    // Create a list of Accounts to Update.
    List<Account> accounts = new List<Account>();
   
    for(Account a : [SELECT Id, Most_Recently_Created_Opportunity_Name__c
                     FROM Account
                     WHERE Id IN :accountIdOpportunityMap.keySet()]){
                         a.Most_Recently_Created_Opportunity_Name__c =  ((Opportunity) accountIdOpportunityMap.get(a.Id)).Name;
                         accounts.add(a);
                     }
   
    update accounts;
        }

--------------------------------------------------------------------------------------------------------

Test classes:

@isTest

public class updateParent_Test {

    static testMethod void testUpdateParentAccount(){

        

        // Set up the Account record.

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

        insert a;

        

        // Verify that the initial state is as expected.

        a = [SELECT Name, Most_Recently_Created_Opportunity_Name__c 

             FROM Account 

             WHERE Id = :a.Id];

        System.assertEquals(null, a.Most_Recently_Created_Opportunity_Name__c);

        

        // Set up the Opportunity record.

        String opportunityName = 'My Opportunity';

        Opportunity o = new Opportunity(AccountId=a.Id, Name=opportunityName, 

                                        StageName='Prospecting', CloseDate=Date.today());

        

        // Cause the Trigger to execute.

        insert o;

        

        // Verify that the results are as expected.

        a = [SELECT Name, Most_Recently_Created_Opportunity_Name__c 

             FROM Account 

             WHERE Id = :a.Id];

        System.assertEquals(opportunityName, a.Most_Recently_Created_Opportunity_Name__c);

    }

    

}

   
There is a class named Test in apex which has some method which help us to write some useful test case   Test class method

Please refer below Link for  writing Test class:

http://amitsalesforce.blogspot.in/2015/06/best-practice-for-test-classes-sample.html