Introduction
Most applications have a need to work with files and, in this article, we will cover the basics of file management and have a simple demo.
Basics
Like other modern operating systems, Android has a disk-based file system. There are two storage areas: internal and external. Internal storage refers to the built-in, non-volatile memory. Devices also can come with removable memory (like a SD card), which is referred to as external storage.
The internal storage is also occasionally referred to as permanent storage. Files saved in internal storage are accessible only to your app by default.
External storage can be removed at any time and files saved here can be accessible to everyone.
Applications can allow themselves to be installed on the external storage by specifying the android:installLocation attribute. (The default is internal storage.)
When the user uninstalls the application, if the app is installed on internal storage, all files are removed. On the other hand, if the application was installed on external storage, the app files are moved only if we save them in the directory that is obtained from calling API getExternalFilesDir().
To grant everyone permission to your files, you need to declare the following in your manifest file.
<manifest ...>
<uses-permission android_name=
"android.permission.READ_EXTERNAL_STORAGE" />
...
</manifest>
If we declare READ_EXTERNAL_STORAGE, global read permissions are granted implicitly.
To save files on internal storage, we need to acquire a directory as a file by calling either the getFilesDir() API (which represents an internal directory for your app), or by calling getCacheDir(), which represents your app’s temp cache files.
A new file can be created by specifying the file and the name of the directory where the file needs to be created by using the File constructor as follows.
File file = new File(context.getFilesDir(), fileName);
We also can create a file by calling openFileOutput() to get a FileOutputStream.
To create temporary files meant for caching purposes, use the createTempFile() API.
Hands On
Let us create a simple application that demonstrates working with files.
Fire up Android Studio and Start a new Android Studio Project.
Figure 1: Starting a new Android Studio Project
Provide FileDemo as the Application Name and Click Next.
Figure 2: Configuring the new project
On the next screen, leave the default values and click Next.
Figure 3: Leaving the default values in place
On the “Add an activity to Mobile” page, choose “Blank Activity”. This creates an application with a single activity.
Figure 4: Adding a new, blank activity
We are then prompted to customize the activity. We will leave the default values unchanged.
Figure 5: Here is where you would customize the activity (but don’t do so)
Click Finish to creating the project files.
Next, Open the AndroidManifest.xml file.
Figure 6: The AndroidManifest.xml file
We need to declare permissions to write to files here.
<?xmlversion="1.0"encoding="utf-8"?>
<manifestxmlns:android="http://schemas.android.com/apk/
res/android"
package="com.example.vipul.filedemo">
<application
android_allowBackup="true"
android_icon="@mipmap/ic_launcher"
android_label="@string/app_name"
android_theme="@style/AppTheme">
<activity
android_name=".MainActivity"
android_label="@string/app_name">
<intent-filter>
<actionandroid:name=
"android.intent.action.MAIN"/>
<categoryandroid:name=
"android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
</application>
<uses-permissionandroid:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
</manifest>
We are only doing the preceding declaration to demo how to declare what is needed to write to external storage. We now will switch to working with a file on internal storage.
Switch to the design view and add two buttons as shown in the following code.
<RelativeLayoutxmlns:android="http://schemas.android.com/
apk/res/android"
xmlns_tools="http://schemas.android.com/tools
"android:layout_width="match_parent"
android_paddingRight="@dimen/activity_horizontal_margin"
android_paddingTop="@dimen/activity_vertical_margin"
android_paddingBottom="@dimen/activity_vertical_margin
"tools:context=".MainActivity">
<TextViewandroid:text="@string/hello_world"
android_layout_width="wrap_content"
android_layout_height="wrap_content"
android_id="@+id/textView"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="CreateFile"
android:id="@+id/buttonCreateFile"
android:layout_below="@+id/textView"
android:layout_alignLeft="@+id/buttonCreateTempFile"
android:layout_alignStart="@+id/buttonCreateTempFile"
android:clickable="true"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="CreateTempFile"
android:id="@+id/buttonCreateTempFile"
android:layout_below="@+id/buttonCreateFile"
android:layout_toRightOf="@+id/textView"
android:layout_toEndOf="@+id/textView"
android:layout_marginTop="41dp"/>
</RelativeLayout>
Next, we will wire up the click event handlers to create files.
Create two method stubs in MainActivity.java
publicvoidsendFileMessage(Viewview){
//Dosomethinginresponsetobuttonclick
}
publicvoidsendCacheFileMessage(Viewview){
//Dosomethinginresponsetobuttonclick
}
Next, we will wire up the click events in the layout file.
<Button android_layout_width="wrap_content" android_layout_height="wrap_content" android_text="CreateFile" android_id="@+id/buttonCreateFile" android_layout_below="@+id/textView" android_layout_alignLeft="@+id/buttonCreateTempFile" android_layout_alignStart="@+id/buttonCreateTempFile" android_clickable="true" android:onClick="sendFileMessage"/> <Button android_layout_width="wrap_content" android_layout_height="wrap_content" android_text="CreateTempFile" android_id="@+id/buttonCreateTempFile" android_layout_below="@+id/buttonCreateFile" android_layout_toRightOf="@+id/textView" android_layout_toEndOf="@+id/textView" android_layout_marginTop="41dp" android:onClick="sendCacheFileMessage"/>
Now, we implement the method stubs to create the files.
public void sendCacheFileMessage(View view) {
// Do something in response to button click
File file = new File(view.getContext().getCacheDir(),
"cachefileName");
tView = (TextView) findViewById(R.id.textViewStatus);
tView.setText("cache file create");
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the
// action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
When we run our application, we will see the home screen as shown in Figure 7.
Figure 7: The home screen, showing the two new buttons
When we click on the “CREATE FILE” button, we will see that a file is created and the Status text changes.
Figure 8: The file has been created
Similarly, when we click on the “CREATE TEMP FILE”, a file is created in the cache directory.
Figure 9: The cache file has been created
We now have created a simple application that works with files.
Summary
In this article, we learned the basics about file and made a simple application that creates files. I hope you have found this information useful.
About the Author
Vipul Patel is a technology geek based in Seattle. He can be reached at vipul.patel@hotmail.com. You can visit his LinkedIn profile at https://www.linkedin.com/pub/vipul-patel/6/675/508.
