How to Write a Test Class in Salesforce? The Salesforce Developer Guide

In any development, testing is a very important step as it tells us about how good the development has been done and for that we have Test Classes in Apex. Before getting to know how to write a test class, let’s first understand what exactly a test class is.

What is a Test Class?

Test classes are the code snippets which test the functionality of your Apex code.

“@isTest” annotation is used for defining a class or a method as “Test Class or Test Method.”

Previously, a “testMethod” keyword was used to define a method as test method. But now it is deprecated by Salesforce.
You aren’t required to add any access modifier like public, private etc. For test method as it will be accessible to the whole test class.
Test Classes don’t have access to the existing data of your Org, by default. So, you need to create test data within the test class.
Test classes and their Test methods are not counted as a part of Apex code limit.
Test Class must cover at least 75% of the code. This limit is set by Salesforce, if you have not fulfilled the criteria then your code can’t be deployed.

Test Method Syntax

@isTest static void nameOfTestMethod() {   
    // code  
}

This is what a test class looks like...

@isTest 
public class FirstTestClass { 
    @isTest static void firstTestMethod() { 
        // code 
    }
}

Don't forget to check out: 10 Example Future Method with Test Class | Asynchronous Apex | Salesforce Development Training Video

How Can I Create One?

Go to Developer Console > File > New > Apex Class.
Write your test class here > Save
Click on “Run Test” button.
See if your test class ran successfully or not by clicking on the “Tests” tab at the bottom.
A “green tick” symbol indicates the successful status of the test class and a “red cross” symbol indicates failed test class.
If your test class ran successfully then go to your main class and see the code coverage by clicking on the Code Coverage button on the top left. You can also click on the arrow icon to see the code coverage method-wise.
Click on the red cross symbol to view why test class failed and try to debug.

Write your 1st Test Class

Let’s write a test class for a trigger that deletes any related account to an opportunity whenever the opportunity gets deleted.

OpportunityDeletion Trigger:

As a best practice, create only one trigger per object. So here is the trigger which executes on after delete event.

trigger OpportunityDeletion on Opportunity (after delete) { 
    if(trigger.isAfter){ 
        if(trigger.isDelete){ 
            OpportunityDeletionHandler.onAfterDelete(trigger.old);             
        } 
    } 
}

OpportunityDeletion Handler Class:

This is the handler class for the above trigger in which the main logic is written.

public class OpportunityDeletionHandler { 
    public static void onAfterDelete(List<Opportunity> oppList){ 
        Set<Id> accountToBeDeleted = new Set<Id>();          
        for(Opportunity o : oppList){            
            accountToBeDeleted.add(o.AccountId); 
        }          
        List<Account> acc = [select Id from Account where id in: accountToBeDeleted]; 
        if(!acc.isEmpty()){             
            delete acc;  
        }    
    } 
}

OpportunityDeletion Test Class:

@isTest public class OpportunityDeletionTest { 
    @isTest static void testOppDeletion(){ 
        Account acc=new Account();		//Creating test data 
        acc.Name='testAccount';			//Creating a test Account record 
        insert acc;          
        Opportunity opp= new Opportunity();	//Creating a test Opportunity record 
        opp.Name='testOpp'; 
        opp.AccountId=acc.Id; 
        opp.CloseDate = system.today(); 
        opp.StageName = 'Prospecting'; 
        insert opp;          
        Test.startTest();	// Provides new set of governor limits for testing actual code  
        delete opp;	 
        Test.stopTest();	// Return to the previous governor limits			 
    } 
}

Good job! you have written your 1st test class successfully.

Check out another amazing blog by Tanya here: What is Salesforce CPQ - Why Does Every Business Need it?

Best Practices to write an efficient Test Class:

If you have many methods in test class, use @testSetup method and create all records once within this method. Whenever you need data, just query it in the test methods and you’ll not need to create data all over again!
Don’t use hardcoded IDs anywhere in the test class, else it will break in the production org.
Take care of Governor Limits by using Test.startTest() and Test.stopTest() methods.
Always aim for 100% and cover Tests for Positive Cases, Negative Cases, Single records, Bulk Records and for Restricted Users.
Test for Exception. Create data that will throw exceptions so that if any exception occurs at Production org, error messages can be displayed for the same.
Use assert statements for the positive and negative cases in the test methods.