Best Practices

@isTest
public class Foo {
    @isTest
    static void methodATest() {
        System.assertNotEquals('123', o.StageName); // not good
        System.assertEquals('123', o.StageName, 'Opportunity stageName is wrong.'); // good
        System.assert(o.isClosed); // not good
        System.assert(o.isClosed, 'Opportunity is not closed.'); // good
    }
}

@isTest
public class Foo {
    public static testMethod void testSomething() {
        Account a = null;
        // This is better than having a NullPointerException
        // System.assertNotEquals(a, null, 'account not found');
        a.toString();
    }
}

@isTest
private class TestRunAs {
   public static testMethod void testRunAs() {
        // Setup test data
        // Create a unique UserName
        String uniqueUserName = 'standarduser' + DateTime.now().getTime() + '@testorg.com';
        // This code runs as the system user
        Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
        User u = new User(Alias = 'standt', Email='standarduser@testorg.com',
        EmailEncodingKey='UTF-8', LastName='Testing', LanguageLocaleKey='en_US',
        LocaleSidKey='en_US', ProfileId = p.Id,
        TimeZoneSidKey='America/Los_Angeles',
         UserName=uniqueUserName);

        System.runAs(u) {
              // The following code runs as user 'u'
              System.debug('Current User: ' + UserInfo.getUserName());
              System.debug('Current Profile: ' + UserInfo.getProfileId());
          }
    }
}

@isTest
private class ATest {
    @isTest
    static void methodATest() {
    }
    static void methodBTest() {
    }
    @isTest static void methodCTest() {
        System.assert(1==2);
    }
    static testmethod void methodCTest() {
        System.debug('I am a debug statement');
    }
    private void fetchData() {
    }
}

@isTest(seeAllData = true)
public class Foo {
    public static testMethod void testSomething() {
        Account a = null;
        // This is better than having a NullPointerException
        // System.assertNotEquals(a, null, 'account not found');
        a.toString();
    }
}

global class Unchangeable {
    global UndeletableType unchangable(UndeletableType param) {
        // ...
    }
}

trigger Accounts on Account (before insert, before update, before delete, after insert, after update, after delete, after undelete) {
    for(Account acc : Trigger.new) {
        if(Trigger.isInsert) {
            // ...
        }

        // ...

        if(Trigger.isDelete) {
            // ...
        }
    }
}

@isTest
public class Foo {
    @isTest
    static void bar() {
        System.debug('Hey this code executed.'); // not good
        System.debug(LoggingLevel.WARN, 'Hey, something might be wrong.'); // good
        System.debug(LoggingLevel.DEBUG, 'Hey, something happened.'); // not good when on strict mode
    }
}

// Incorrect code, does not attach a finalizer.
public class UserUpdater implements Queueable {
    public List<User> usersToUpdate;

    public UserUpdater(List<User> usersToUpdate) {
        this.usersToUpdate = usersToUpdate;
    }

    public void execute(QueueableContext context) { // no Finalizer is attached
        update usersToUpdate;
    }
}

// Proper code, attaches a finalizer.
public class UserUpdater implements Queueable, Finalizer {
    public List<User> usersToUpdate;

    public UserUpdater(List<User> usersToUpdate) {
        this.usersToUpdate = usersToUpdate;
    }

    public void execute(QueueableContext context) {
        System.attachFinalizer(this);
        update usersToUpdate;
    }

    public void execute(FinalizerContext ctx) {
        if (ctx.getResult() == ParentJobResult.SUCCESS) {
            // Handle success
        } else {
            // Handle failure
        }
    }
}

public Boolean bar(String z) {
        String x = 'some string'; // not used

        String y = 'some other string'; // used in the next line
        return z.equals(y);
    }