Android String if-statement

Question

I have a if-statement in the start of my app

    if (ready.equals("yes")){
...
}

and later on my code I have

ready="yes";

but the if statement is never called, why? The ready="yes"; is called from a background thread, is that why?

    public void DownloadFromUrl(final String fileName) {  //this is the downloader method
            new Thread(new Runnable() {
                public void run() {
        try {

            URL url = new URL("https://xxxxxxx");
            File file = new File(PATH + fileName);

            long startTime = System.currentTimeMillis();
            Log.d("ImageManager", "download begining");
            Log.d("ImageManager", "download url:" + url);
            Log.d("ImageManager", "downloaded file name:" + fileName);
                    /* Open a connection to that URL. */
            URLConnection ucon = url.openConnection();

                    /*
                     * Define InputStreams to read from the URLConnection.
                     */
            InputStream is = ucon.getInputStream();
            BufferedInputStream bis = new BufferedInputStream(is);

                    /*
                     * Read bytes to the Buffer until there is nothing more to read(-1).
                     */
            ByteArrayBuffer baf = new ByteArrayBuffer(50);
            int current = 0;
            while ((current = bis.read()) != -1) {
                baf.append((byte) current);
            }

                    /* Convert the Bytes read to a String. */
            FileOutputStream fos = new FileOutputStream(file);
            fos.write(baf.toByteArray());
            fos.close();
            Log.e("Ready or not", ready);
            ready="yes";
            Log.d("ImageManager", "download ready in"
                    + ((System.currentTimeMillis() - startTime) / 1000)
                    + " sec");
            Log.e("Ready or not", ready);



        } catch (IOException e) {
            Log.d("ImageManager", "Error: " + e);
        }

    }
    }).start();

You might want to add some more context as to where in your program the ready variable is set (in onCreate, in a thread, in a onClick, etc.) — Morrison Chang
– Morrison Chang, Commented Jun 22, 2015 at 3:22
updated, the check for the if-statement is nothing special, it is just called in the begining of the onCreate(); — Emilio Gaines
– Emilio Gaines, Commented Jun 22, 2015 at 3:26

James Ko · Accepted Answer · 2015-06-22 14:40:40Z

1

Correct me if I'm wrong. If you're saying that your code looks like this:

new Thread(new Runnable() { public void run()
                            {
                                // thread code
                                if (ready.equals("yes")) {
                                    // handler code
                                }
                                // more thread code
                            }).start();

// later on...
ready = "yes";

And you're asking why ready = "yes" doesn't execute before if (ready.equals("yes")), then that's because multiple threads aren't guaranteed to execute in a certain order. If you want to wait until ready.equals("yes") before you execute the if statement, then you have to use the Object.wait() and Object.notifyAll() methods:

// this is a field
private Object waitOnThis = new Object();

new Thread(new Runnable() { public void run()
                            {
                                // thread code
                                waitOnThis.wait(); // blocks until notify / notifyAll is called on waitOnThis
                                // by this point ready.equals("yes")
                                if (ready.equals("yes")) {
                                    // handler code
                                }
                                // more thread code
                            }).start();

// later on...
ready = "yes";
waitOnThis.notifyAll(); // unblocks threads waiting on waitOnThis

Good luck!

EDIT: Be sure to wrap each of the code fragments above in a synchronized (waitOnThis) block, or else you'll get an IllegalMonitorStateException.

edited Jun 22, 2015 at 14:40

answered Jun 22, 2015 at 3:22

James Ko

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4 Comments

Emilio Gaines Over a year ago

yeah but the ready="yes" will ALWAYS be called after the if-statement since it runs in the background, seperate from the main activity and is called about 2-3 seconds AFTER the if-part is checked

Jared Burrows Over a year ago

May I suggest that anytime you use equals with a constant, put the constant on the left: "yes".equals(ready). This way you always avoid NullPointerExceptions.

Emilio Gaines Over a year ago

acually the (ready.equals("yes")) was in onCreate/a void but I tried it your way but I get unhandled.exception on waitOnThis.wait();

James Ko Over a year ago

@EmilioGaines Whoops, it looks like you need to be in a synchronized block for Object.wait() to work. Try wrapping everything in a synchronized (waitOnThis) block (and if that happens in DownloadFromUrl, add synchronized there as well).

Malvin · Accepted Answer · 2015-06-22 03:31:04Z

0

Application's OnCreate will only be called once at the beginning when the app is created.

Consider using callback if you want to notify that the download has finished.

answered Jun 22, 2015 at 3:31

Malvin

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