activity_main.xmlMainActivity.java
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    tools:context="com.codinginflow.asynctaskexample.MainActivity">

    <ProgressBar
        android:id="@+id/progress_bar"
        style="@style/Widget.AppCompat.ProgressBar.Horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:visibility="invisible"
        tools:visibility="visible" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="startAsyncTask"
        android:text="Start" />

</LinearLayout>

package com.codinginflow.asynctaskexample;

import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.Toast;

import java.lang.ref.WeakReference;

public class MainActivity extends AppCompatActivity {
    private ProgressBar progressBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        progressBar = findViewById(R.id.progress_bar);
    }

    public void startAsyncTask(View v) {
        ExampleAsyncTask task = new ExampleAsyncTask(this);
        task.execute(10);
    }

    private static class ExampleAsyncTask extends AsyncTask<Integer, Integer, String> {
        private WeakReference<MainActivity> activityWeakReference;

        ExampleAsyncTask(MainActivity activity) {
            activityWeakReference = new WeakReference<MainActivity>(activity);
        }

        @Override
        protected void onPreExecute() {
            super.onPreExecute();

            MainActivity activity = activityWeakReference.get();
            if (activity == null || activity.isFinishing()) {
                return;
            }

            activity.progressBar.setVisibility(View.VISIBLE);
        }

        @Override
        protected String doInBackground(Integer... integers) {
            for (int i = 0; i < integers[0]; i++) {
                publishProgress((i * 100) / integers[0]);
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            return "Finished!";
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);

            MainActivity activity = activityWeakReference.get();
            if (activity == null || activity.isFinishing()) {
                return;
            }

            activity.progressBar.setProgress(values[0]);
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);

            MainActivity activity = activityWeakReference.get();
            if (activity == null || activity.isFinishing()) {
                return;
            }

            Toast.makeText(activity, s, Toast.LENGTH_SHORT).show();
            activity.progressBar.setProgress(0);
            activity.progressBar.setVisibility(View.INVISIBLE);
        }
    }
}