MySecurityClass_exec.cls

// Creates an instance of the class MyUser
MyUser adminUser = new MyUser();
adminUser.username = 'admin';
adminUser.password = 'admin';

// non-static method, it needs to be called from each instance
adminUser.validateCredentials();

// Creates another instance of the class MyUser
MyUser regularUser = new MyUser();
regularUser.username = 'myusername';
regularUser.password = 'mypassword';

// non-static method, it needs to be called from each instance
regularUser.validateCredentials();

// calling the method using values that do not belong to an instance:
// You do not need a new instance of the class to use its static methods
Boolean validated = MySecurityClass.validateCredentials('test', 'test');