Change variable value within anonymous class

Question

I would like to know why whenever within an anonymous class I try to change the value of an "external" variable it does not, change remains the same. There would be something similar that I could do that would give the same result?

I used the Thread as an example, but in any anonymous class it happens. (Including on Android)

public class Teste {

    public boolean umIgualUm = false;

    public Teste() {
        fazerAlgo();
    }

    public void fazerAlgo() {

        new Thread() {
            public void run() {
                if (1 == 1) {
                    System.out.println("Entra aqui");
                    umIgualUm = true;
                }
            }
        }.start();

        System.out.println(umIgualUm); //Exibe false

    }

}

Android example

public class UsuarioDAO {

    public boolean metodoDeuCerto = false;

    public UsuarioDAO(){}

    public boolean cadastrarUsuario(Usuario usuario) {

        ParseUser parseUser = new ParseUser();
        parseUser.setUsername(usuario.getNome());
        parseUser.setEmail(usuario.getEmail());
        parseUser.setPassword(usuario.getSenha());

        parseUser.signUpInBackground(new SignUpCallback() {
            @Override
            public void done(ParseException e) {
                if(e == null) { //Quer dizer que deu certo
                    metodoDeuCerto = true;
                }else{
                    e.printStackTrace();
                    metodoDeuCerto = false;
                }

            }

        });

        return metodoDeuCerto;

    }
}

Answer 1

There are two problems here:

1. The method start may take time to actually start the new thread. With this System.out may end up running first.

2. Variables can be in thread-specific caches, and with that, different threads can see different values for the same variable at the same time. To get around this problem you can use synchronization or make the variables to be volatile (something that instructs JVM not to allow them to be kept in threadseparated caches). Another possibility is to use a AtomicBoolean.

The problem has no relation to the fact that the variable is being accessed from within the anonymous class. The problem is it’s being accessed from another thread.

If you see this problem happening when you use an anonymous class other than one Thread, request to show this code.

As for your second code, the methodsignUpInBackground creates a new thread to run what is in the anonymous class, which means that the method cadastrarUsuario will execute the return without waiting for the signUpInBackground be completed.

In fact, the method signUpInBackground() returns a Task<Void>.

What is the class Task according to the documentation?

Represents the result of an asynchronous Operation.

Translating into Portuguese:

Represents the result of an asynchronous operation.

What is an asynchronous operation?

It means that the process that starts it does not wait for its end, that will produce some result only in an uncertain future. That is, your method cadastrarUsuario is not actually registering a user, is just asking for a user to be registered and returning immediately. Note that this does not imply that the user was actually registered at any time. You would have to wait for the Task end up knowing this.

In both cases, the problem is that what appears in the body of the anonymous class, only runs long after the main method has already returned, and runs on another thread.

Answer 2

As they said before, you can’t change the value of a variable within the thread and then test it right away. Probably your test (if, println, or whatever) will be called before the thread is started.

One of the correct ways to work would be: (With your own example)

public class Teste {

    public Teste() {
        fazerAlgo();
    }

    public void fazerAlgo() {

        new Thread() {
            public void run() {                                       
                System.out.println("Entra aqui");

                if (1 == 1) {
                    metodoDeuCerto();
                } else {
                    metodoNaoDeuCerto();
                }
            }
        }.start();
    }

    public void metodoDeuCerto() {
        System.out.println("Deu certo!");
    }


    public void metodoNaoDeuCerto() {
        System.out.println("Nao deu certo!"); 
    }
}

(EDITED)

Android example:

public class UsuarioDAO {

    private SignupListener listener;

    public UsuarioDAO(){}

    public void cadastrarUsuario(Usuario usuario) {

        ParseUser parseUser = new ParseUser();
        parseUser.setUsername(usuario.getNome());
        parseUser.setEmail(usuario.getEmail());
        parseUser.setPassword(usuario.getSenha());

        parseUser.signUpInBackground(new SignUpCallback() {
            @Override
            public void done(ParseException e) {
                if(e == null) {
                   if ( listener != null ) {
                       listener.deuCerto();
                   }
                }else{
                   e.printStackTrace();
                   if ( listener != null ) {
                       listener.naoDeuCerto();
                   }
                }

            }

        });
    }

    public void setSignupListener(SignupListener signupListener) {
        this.listener = signupListener; 
    }

    public interface SignupListener {
        void deuCerto();
        void naoDeuCerto();
    }
}

// Example of Activity calling the User

public class MainActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity); 

        Usuario usuario = new Usuario();
        usuario.setUserName("usuario");
        usuario.setEmail("[email protected]");
        usuario.setPassword("senha");

        UsuarioDAO dao = new UsuarioDAO();
        dao.setSignupListener(signupListener);
        dao.cadastrarUsuario(usuario);

    }

    private UsuarioDAO.SignupListener signupListener = new UsuarioDAO.SignupListener {
        @Override
        public void deuCerto() {
           //DO SOMETHING
        }

        @Override
        public void naoDeuCerto() {
           //DO SOMETHING
        }
    }
}