how to wait for lambda expression finish

Question

I have a problem with lambda expression; I used lambda expression in return type method but return isDownloaded.get() execute before lambda expression . how I can wait for lambda finish?

 public  boolean isDowloaded(int id) {

AtomicReference<Boolean> isdownload = new AtomicReference<>(false);
AtomicReference<List<Download>> downloadList = new AtomicReference<>();

MyApplication.getInstance().getFetch().getDownloads(downloads -> {

    downloadList.set(downloads);
    for (int i = 0; i < downloadList.get().size(); i++) {
        if (downloadList.get().get(i).getExtras().getString("id","").equals(String.valueOf(id)) ) {
            PlayerConstants.SONG_NUM = i;
            isdownload.set(true);
        }
    }


});

        // always return false in this case
return isdownload.get();

}

Answer 1

The lambda is not the problem there.
The getDownloads() call is asynchronous. So the isDowloaded() method that invokes that should not return a boolean since it doesn't know when the processing will be finished.
It should be a void method. One of the main Fetch api feature is downloading in the background.

So you have to implement a callback to perform a post processing.

2 possibilities :

• implement and add a FetchListener .
• perform the post processing after the return of the asynchronous call.

For example :

MyApplication.getInstance().getFetch().getDownloads(downloads -> {

    downloadList.set(downloads);
    for (int i = 0; i < downloadList.get().size(); i++) {
       //...
    }
    // ... DO your post processing here

});