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Android ndk std::to_string support

I’m using android NDK r9d and toolchain 4.8 but I’m not able to use std::to_string function, compiler throws this error:

 error: 'to_string' is not a member of 'std'

Is this function not supported on android ndk? I try APP_CPPFLAGS := -std=c++11 with no luck.

Answer

You can try LOCAL_CFLAGS := -std=c++11, but note that not all C++11 APIs are available with the NDK’s gnustl. Full C++14 support is available with libc++ (APP_STL := c++_shared).

The alternative is to implement it yourself.

#include <string>
#include <sstream>

template <typename T>
std::string to_string(T value)
{
    std::ostringstream os ;
    os << value ;
    return os.str() ;
}

int main()
{
    std::string perfect = to_string(5) ;
}
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JUnit 4.11 get test result in @After

Is there any way I can get the test result in the teardown (@After) method? I’d like to do clean up after the tests depending on the result.

Could not find much details about @After in the junit docs.

Answer

The closest thing to what you’re asking for would probably be the TestWatcher rule. That won’t give you access to a returned result or anything, but you can use it (or create your own TestRule and combined with the Description object, you could annotate your methods differently to indicate what sort of clean-up is necessary.

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Variable is already defined in method lambda

Consider the following almost compilable Java 8 code:

public static void main(String[] args) {

    LinkedList<User> users = null;
    users.add(new User(1, "User1"));
    users.add(new User(2, "User2"));
    users.add(new User(3, "User3"));

    User user = users.stream().filter((user) -> user.getId() == 1).findAny().get();
}

static class User {

    int id;
    String username;

    public User() {
    }

    public User(int id, String username) {
        this.id = id;
        this.username = username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public int getId() {
        return id;
    }
}

You’ll notice User user = users.stream().filter((user) -> user.getId() == 1).findAny().get(); throws a compiler error:

variable user is already defined in method main(String[])

My question is: Why do Lambda expressions consider the variable that is being initialized on the same line as the Lambda expression as already defined? I understand Lambdas look outside themselves for (and use) local variables, so you can’t name the variables you use inside the Lambda the same as an outside variable. But why is the variable that is being defined considered already defined?

Answer

Let’s go to the Java Language Specification on names and their scopes

The scope of a formal parameter of a method (§8.4.1), constructor (§8.8.1), or lambda expression (§15.27) is the entire body of the method, constructor, or lambda expression.

The scope of a local variable declaration in a block (§14.4) is the rest of the block in which the declaration appears, starting with its own initializer and including any further declarators to the right in the local variable declaration statement.

Then, on the subject of shadowing and obscuring

A local variable (§14.4), formal parameter (§8.4.1, §15.27.1), exception parameter (§14.20), and local class (§14.3) can only be referred to using a simple name, not a qualified name (§6.2).

Some declarations are not permitted within the scope of a local variable, formal parameter, exception parameter, or local class declaration because it would be impossible to distinguish between the declared entities using only simple names.

It is a compile-time error if the name of a local variable v is used to declare a new variable within the scope of v, unless the new variable is declared within a class whose declaration is within the scope of v.

So, in

User user = users.stream().filter((user) -> user.getId() == 1).findAny().get();

, the scope of the variable user is everything after it in that block. Now you are trying to use the name of that variable to declare a new variable within the scope, but not

within a class whose declaration is within the scope of v.

so a compile time error occurs. (It’s declared in a lambda expression, not in a class.)

Categories
discuss

Updating UI from different threads in JavaFX

I am developing an application with several TextField objects that need to be updated to reflect changes in associated back-end properties. The TextFields are not editable, only the back-end may change their content.

As I understand, the correct way about this is to run the heavy computation on a separate thread so as not to block the UI. I did this using javafx.concurrent.Task and communicated a single value back to the JavaFX thread using updateMessage(), which worked well. However, I need more than one value to be updated as the back-end does its crunching.

Since the back-end values are stored as JavaFX properties, I tried simply binding them to the textProperty of each GUI element and let the bindings do the work. This doesn’t work, however; after running for a few moments, the TextFields stop updating even though the back-end task is still running. No exceptions are raised.

I also tried using Platform.runLater() to actively update the TextFields rather than binding. The issue here is that the runLater() tasks are scheduled faster than the platform can run them, and so the GUI becomes sluggish and needs to time to “catch up” even after the back-end task is finished.

I found a few questions on here:

Logger entries translated to the UI stops being updated with time

Multithreading in JavaFX hangs the UI

but my issue persists.

In summary: I have a back-end making changes to properties, and I want those changes to appear on the GUI. The back-end is a genetic algorithm, so its operation is broken down into discrete generations. What I would like is for the TextFields to refresh at least once in between generations, even if this delays the next generation. It is more important that the GUI responds well than that the GA runs fast.

I can post a few code examples if I haven’t made the issue clear.

UPDATE

I managed to do it following James_D’s suggestion. To solve the issue of the back-end having to wait for the console to print, I implemented a buffered console of sorts. It stores the strings to print in a StringBuffer and actually appends them to the TextArea when a flush() method is called. I used an AtomicBoolean to prevent the next generation from happening until the flush is complete, as it is done by a Platform.runLater() runnable. Also note that this solution is incredibly slow.

Answer

Not sure if I completely understand, but I think this may help.

Using Platform.runLater(...) is an appropriate approach for this.

The trick to avoiding flooding the FX Application Thread is to use an Atomic variable to store the value you’re interested in. In the Platform.runLater method, retrieve it and set it to a sentinel value. From your background thread, update the Atomic variable, but only issue a new Platform.runLater if it’s been set back to its sentinel value.

I figured this out by looking at the source code for Task. Have a look at how the updateMessage method (line 1131 at the time of writing) is implemented.

Here’s an example which uses the same technique. This just has a (busy) background thread which counts as fast as it can, updating an IntegerProperty. An observer watches that property and updates an AtomicInteger with the new value. If the current value of the AtomicInteger is -1, it schedules a Platform.runLater.

In the Platform.runLater, I retrieve the value of the AtomicInteger and use it to update a Label, setting the value back to -1 in the process. This signals that I am ready for another UI update.

import java.text.NumberFormat;
import java.util.concurrent.atomic.AtomicInteger;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.AnchorPane;
import javafx.stage.Stage;

public class ConcurrentModel extends Application {

  @Override
  public void start(Stage primaryStage) {
    
    final AtomicInteger count = new AtomicInteger(-1);
    
    final AnchorPane root = new AnchorPane();
    final Label label = new Label();
    final Model model = new Model();
    final NumberFormat formatter = NumberFormat.getIntegerInstance();
    formatter.setGroupingUsed(true);
    model.intProperty().addListener(new ChangeListener<Number>() {
      @Override
      public void changed(final ObservableValue<? extends Number> observable,
          final Number oldValue, final Number newValue) {
        if (count.getAndSet(newValue.intValue()) == -1) {
          Platform.runLater(new Runnable() {
            @Override
            public void run() {
              long value = count.getAndSet(-1);
              label.setText(formatter.format(value));
            }
          });          
        }

      }
    });
    final Button startButton = new Button("Start");
    startButton.setOnAction(new EventHandler<ActionEvent>() {
      @Override
      public void handle(ActionEvent event) {
        model.start();
      }
    });

    AnchorPane.setTopAnchor(label, 10.0);
    AnchorPane.setLeftAnchor(label, 10.0);
    AnchorPane.setBottomAnchor(startButton, 10.0);
    AnchorPane.setLeftAnchor(startButton, 10.0);
    root.getChildren().addAll(label, startButton);

    Scene scene = new Scene(root, 100, 100);
    primaryStage.setScene(scene);
    primaryStage.show();
  }

  public static void main(String[] args) {
    launch(args);
  }

  public class Model extends Thread {
    private IntegerProperty intProperty;

    public Model() {
      intProperty = new SimpleIntegerProperty(this, "int", 0);
      setDaemon(true);
    }

    public int getInt() {
      return intProperty.get();
    }

    public IntegerProperty intProperty() {
      return intProperty;
    }

    @Override
    public void run() {
      while (true) {
        intProperty.set(intProperty.get() + 1);
      }
    }
  }
}

If you really want to “drive” the back end from the UI: that is throttle the speed of the backend implementation so you see all updates, consider using an AnimationTimer. An AnimationTimer has a handle(...) which is called once per frame render. So you could block the back-end implementation (for example by using a blocking queue) and release it once per invocation of the handle method. The handle(...) method is invoked on the FX Application Thread.

The handle(...) method takes a parameter which is a timestamp (in nanoseconds), so you can use that to slow the updates further, if once per frame is too fast.

For example:

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.beans.property.LongProperty;
import javafx.beans.property.SimpleLongProperty;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextArea;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;


public class Main extends Application {
    @Override
    public void start(Stage primaryStage) {
        
        final BlockingQueue<String> messageQueue = new ArrayBlockingQueue<>(1);
        
        TextArea console = new TextArea();
        
        Button startButton = new Button("Start");
        startButton.setOnAction(event -> {
            MessageProducer producer = new MessageProducer(messageQueue);
            Thread t = new Thread(producer);
            t.setDaemon(true);
            t.start();
        });
        
        final LongProperty lastUpdate = new SimpleLongProperty();
        
        final long minUpdateInterval = 0 ; // nanoseconds. Set to higher number to slow output.
        
        AnimationTimer timer = new AnimationTimer() {

            @Override
            public void handle(long now) {
                if (now - lastUpdate.get() > minUpdateInterval) {
                    final String message = messageQueue.poll();
                    if (message != null) {
                        console.appendText("n" + message);
                    }
                    lastUpdate.set(now);
                }
            }
            
        };
        
        timer.start();
        
        HBox controls = new HBox(5, startButton);
        controls.setPadding(new Insets(10));
        controls.setAlignment(Pos.CENTER);
        
        BorderPane root = new BorderPane(console, null, null, controls, null);
        Scene scene = new Scene(root,600,400);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
    
    private static class MessageProducer implements Runnable {
        private final BlockingQueue<String> messageQueue ;
        
        public MessageProducer(BlockingQueue<String> messageQueue) {
            this.messageQueue = messageQueue ;
        }
        
        @Override
        public void run() {
            long messageCount = 0 ;
            try {
                while (true) {
                    final String message = "Message " + (++messageCount);
                    messageQueue.put(message);
                }
            } catch (InterruptedException exc) {
                System.out.println("Message producer interrupted: exiting.");
            }
        }
    }
    
    public static void main(String[] args) {
        launch(args);
    }
}
Categories
discuss

how to convert UTC time to some other time zone (“CST”,”IST”)

In my Android application server will return some UTC date in following format(yyyy-MM-dd HH:mm:ss) 24hours and I need to convert those time into user’s TimeZone for example CST, IST.

I did the following code but I do know is it correct or not, please assist me to do the time zone conversion in right way.

I get UTC date as json string and converting into user’s time zone format and showing Android side

private static Date utcDate;
private static DateFormat expireFormat = new SimpleDateFormat(
        "yyyy-MM-dd HH:mm:ss");

        try {
            expireFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
            utcDate = expireFormat.parse("2014-04-01 10:32:00");
            System.out.println(TimeZone.getDefault().getID());
            expireFormat.setTimeZone(TimeZone.getTimeZone(TimeZone.getDefault().getID()));
            System.out.println(expireFormat.format(utcDate));
        } catch (ParseException e) {
            e.printStackTrace();
        }

    output of the code:
    Asia/Calcutta
    2014-04-01 16:02:00

Answer

The overall approach is OK if a re-formatted String is really what you’re trying to get.

There are some issues though

  1. SimpleDateFormat is not a threadsafe class. Setting it to a static field inside a server is a problem!

  2. Same as #1 regarding using a static field to hold the intermediate Date object.

  3. Is “CST” China Standard Time? Central Standard Time (US or Australia)? Cuba Standard Time? Three letter abbreviations TimeZone are bad news in general. Try to use an Olson Name or Alias if at all possible.

  4. Is this the server side or the android? If it’s a server, you could probably benefit from the new Java 8 API for DateTime handling.

Source: stackoverflow
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