Salesforce Platform dev 1 Apex Testing 1

Writing test class is very essential before you deploy your code to production 
from sandbox. 
If your code coverage is not above 75 percent you won't be able to deploy 
your code.

Let's create a simple scenario where you would like to create an Apex class 
which contains a method to create account and contact and link the account to this 
newly created contact.

Create a new class

public class AccountClass {
    
    public static void CreateAccountandContact()
    {
        try
        {
           account a = new account();
           a.name = 'Demo acc Test';
           insert a;
            
           contact c = new contact();
           c.LastName = 'Demo last name';
           c.AccountId= a.id;
           insert c; 
             
            
            
        }
        catch(exception ex)
        {
            system.debug(ex.getMessage());
            
        }
        
    }
    
  
 
}
Write a test class 

@istest
public class DemoAccContTest 
{
  @istest private static void TestAccandContact()
   {
       system.test.startTest();
       AccountClass.CreateAccountandContact();
       integer contactcount = [select count() from contact];
       system.assert(contactcount==26);
       system.test.stopTest();
       
   }
}

By default (seealldata=false) which means you cant rely on production 
underlying data for your test.
if you want to use production data as part of your test case then you 
need to include (seealldata=true) as shown below
@istest(seealldata=true)
public class DemoAccContTest {}

You can see the code coverage by going to code screen as shown below