How to call an alert, a notification for user in Android application?

For short and quick alerts, we can make use of Toasts :

A toast allows you to display a short, quick message to the user (used only with text).

See the example of an application:

In my application package br.com.toasts , the file MainActivity.java :

package br.com.toasts;

import android.app.Activity;
import android.os.Bundle;
import android.view.Gravity;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);
    }

    public void notify1(View v) {
        Toast t = Toast.makeText(this, "Aviso!", Toast.LENGTH_SHORT);
        t.show();
    }

    public void notify2(View v) {
        Toast t = Toast.makeText(this, "Aviso!", Toast.LENGTH_SHORT);
        t.setGravity(Gravity.CENTER, 0, 0);
        t.show();
    }

    public void notify3(View v) {
        Toast t = new Toast(this);
        t.setGravity(Gravity.CENTER,0, 0);

        View view = getLayoutInflater().inflate(R.layout.toast, null);
        TextView textView = (TextView) view.findViewById(R.id.txt_toast);

        textView.setText("Aviso!");

        t.setView(view);
        t.show();
    }
}

In the layout > file activity_main.xml :

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:orientation="horizontal"
    style="?android:attr/buttonBarStyle"
    tools:context=".MainActivity">

    <Button
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:onClick="notify1"
        android:text="@string/btn_toast1"
        style="?android:attr/buttonBarButtonStyle"/>

    <Button
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:onClick="notify2"
        android:text="@string/btn_toast2"
        style="?android:attr/buttonBarButtonStyle"/>

    <Button
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:onClick="notify3"
        android:text="@string/btn_toast3"
        style="?android:attr/buttonBarButtonStyle"/>

</LinearLayout>

In the layout > file toast.xml :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/root"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal"
    android:background="#FFFF00"
    android:padding="10dp" >

    <ImageView
        android:id="@+id/img"
        android:layout_width="wrap_content"
        android:layout_height="50dp"
        android:src="@android:drawable/ic_dialog_alert"
        tools:ignore="ContentDescription" />

    <TextView
        android:id="@+id/txt_toast"
        android:layout_width="wrap_content"
        android:layout_height="50dp"
        android:gravity="center_vertical"
        android:layout_marginStart="10dp"
        android:textStyle="bold"
        style="?android:attr/textAppearanceMedium" />

</LinearLayout>

In the values > file strings.xml :

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">Toast</string>
    <string name="btn_toast1">Toast 1</string>
    <string name="btn_toast2">Toast 2</string>
    <string name="btn_toast3">Toast 3</string>
</resources>

In this example application, there are three types of alert, as in the image below: