async task in android

async.java

/**
 * The three types used by an asynchronous task are the following:

Params, the type of the parameters sent to the task upon execution. 
Progress, the type of the progress units published during the background computation. 
Result, the type of the result of the background computation. 
Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void:

 private class MyTask extends AsyncTask<Void, Void, Void> { ... }
 
The 4 steps
When an asynchronous task is executed, the task goes through 4 steps:

onPreExecute(), invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface. 
doInBackground, invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate step. 
onProgressUpdate, invoked on the UI thread after a call to publishProgress. The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field. 
onPostExecute, invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter. 
Cancelling a task
A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(Object[]), if possible (inside a loop for instance.)

Threading rules
There are a few threading rules that must be followed for this class to work properly:

The AsyncTask class must be loaded on the UI thread. This is done automatically as of android.os.Build.VERSION_CODES.JELLY_BEAN. 
The task instance must be created on the UI thread. 
execute must be invoked on the UI thread. 
Do not call onPreExecute(), onPostExecute, doInBackground, onProgressUpdate manually. 
The task can be executed only once (an exception will be thrown if a second execution is attempted.) 
Memory observability
AsyncTask guarantees that all callback calls are synchronized in such a way that the following operations are safe without explicit synchronizations.

Set member fields in the constructor or onPreExecute, and refer to them in doInBackground. 
Set member fields in doInBackground, and refer to them in onProgressUpdate and onPostExecute. 
Order of execution
When first introduced, AsyncTasks were executed serially on a single background thread. Starting with android.os.Build.VERSION_CODES.DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with android.os.Build.VERSION_CODES.HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.

If you truly want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.

**/

//to start a async task :
new task().execute(string1 , string2,...,stringN);

/**
 * String : pass to the asyncTask when executing as parameter; can be a series of strings
 * Void : during progression of the execution 
 * ArrayList<String> : passed to the onPostExecute method after the async execution is done 
 **/
private class task extends AsyncTask < String , Void , ArrayList<String> >{
	@Override
	protected ArrayList<String> doInBackground(String ... param){
		ArrayList<String> result ;
		//do the async task 
		return result ; //return to the onPostExecute
	}
	@Override
	protected void onPostExecute(ArrayList<String> result){
		//do stuff after the async method has finished executing.
		//this method can access and update the ui elements
		//if the class is nested under an Activity class 
	}
}