Function Pointer in Java

Question

I'm writing a Login page in wicket by java and want to write it as general as possible so I need to pass to class a function which famous as Function Pointer in C++. The class is:

class LoginForm extends Form
{

    public LoginForm(String id,TextField username,TextField password,WebResponse webResponse)
    {
        super(id);
    }

    @Override
    public void onSubmit()
    {
        String password = Login.this.getPassword();
        String userId = Login.this.getUserId();
        String role = authenticate(userId, password);
        if (role != null)
        {
            if (Login.this.getSave())
            {
                utilities.CreateCookie("Username", userId, false, 1209600, (WebResponse) getRequestCycle().getResponse());
                utilities.CreateCookie("Password", password, false, 1209600, (WebResponse) getRequestCycle().getResponse());
            }
            User loggedInUser = new User(userId, role);
            WiaSession session = (WiaSession) getSession();
            session.setUser(loggedInUser);
            if (!continueToOriginalDestination())
            {
                setResponsePage(UserHome.class);
            }
        }
        else
        {
            wrongUserPass.setVisible(true);
        }
    }
}

where authenticate is that function what should I do?

Answer 1

Just pass an Interface which defines the authenticate method.

void DoSomething(IAuthenticationProvider authProvider) {
    // ...
    authProvider.authenticate();
}

Answer 2

You can use inner class

Answer 3

In java there is nothing call function pointer rather it is function object. In this you can define object whose methods performs operation on the other objects. e.g.

 class MyClass 
 {
      public void myFunction(String s1, String s2)
      { 
           return s1.length() - s2.length();
      }
 }

 MyClass m = new MyClass(); 
 m.myFunction("nnnnnn", "mmmmmmmmmmm");

This is nothing but function object. If we follow code to interface rule then we can define an interface and can use generics as well.

 public interface MyInterface<T>
 {
      public void myFunction(T s1, T s2);
 }

Now the original class can implement this interface. The above example is just a simple modification of Comparator interface example. If you don't want concrete class you can use anonymous class as well.

Arrays.sort(stringArray, new Comparator <String> ()
  {
       public int compare(String s1, String s2)
             {
                   return s1.length() - s2.length();
             }
  };
  );

Hope it helps.