As the test class makes reference to the controller extension, this must be created first.

The tests successfully execute as shown in the following screenshot:

Navigating back to the Apex Classes setup page by clicking on Your Name | Setup | Develop | Apex Classes shows that the tests have achieved 100 percent coverage of the controller.

The test class contains one unit test method. As unit tests do not have access to the organization data, the first task for the test is to set up the account and contact information.

Account acc=new Account(Name='Unit Test');
insert acc;
    
List<Contact> contacts=new List<Contact>();
contacts.add(new Contact(FirstName='Unit', 
    LastName='Test', Email='Unit.Test@Unit.Test', 
    AccountId=acc.id));
contacts.add(new Contact(FirstName='Unit', 
    LastName='Test 2', Email='Unit.Test2@Unit.Test',
    AccountId=acc.id));
insert contacts;

Next, the instance of the standard controller is instantiated.

ApexPages.StandardController std=
  new ApexPages.StandardController(acc);

Note that the standard controller requires the sObject record that it is managing as a parameter to the constructor. As this is the record that will be made available to the controller extension, it must have the fields populated that the extension relies upon. In this case, the only field used by the extension is the ID of the account, and this is automatically populated when the account is inserted.

Finally, the controller extension is instantiated, taking the standard controller as a constructor parameter, and the test verifies that the extension has successfully retrieved the associated contacts.

AccountContactsExt controller=new AccountContactsExt(std);
System.assertEquals(2, controller.contacts.size());