Declaring AsyncTask types

AsyncTask is a generically typed class, and exposes three type parameters:

abstract class AsyncTask<Params, Progress, Result>

When we declare an AsyncTask subclass, we'll specify types for Params, Progress, and Result; for example, if we want to pass a String parameter to doInBackground, report progress as a Float, and return a Boolean result, we would declare our AsyncTask subclass as follows:

public class MyTask extends AsyncTask<String, Float, Boolean>

If we don't need to pass any parameters, or don't want to report progress, a good type to use for those parameters is java.lang.Void, which signals our intent clearly, because Void is an uninstantiable class representing the void keyword.

Let's take a look at a first example, performing an expensive calculation in the background and reporting the result to the main thread:

public class PrimesTask
extends AsyncTask<Integer, Void, BigInteger> {
  private TextView resultView;

  public PrimesTask(TextView resultView) {
    this.resultView = resultView;
  }

  @Override
  protected BigInteger doInBackground(Integer... params) {
    int n = params[0];
    BigInteger prime = new BigInteger("2");
    for (int i=0; i<n; i++) {
      prime = prime.nextProbablePrime();
    }
    return prime;
  }

  @Override
  protected void onPostExecute(BigInteger result) {
      resultView.setText(result.toString());
  }
}

Here, PrimesTask extends AsyncTask, specifying the Params type as Integer so that we can ask for the nth prime, and the Result type as BigInteger.

We pass a TextView to the constructor so that PrimesTask has a reference to the user interface that it should update upon completion.

We've implemented doInBackground to calculate the nth prime, where n is an Integer parameter to doInBackground, and returned the result as BigInteger.

In onPostExecute, we simply display the result parameter to the view we were assigned in the constructor.

Tip

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