There are several ways to store data in Android app development, and one of them is to utilize the file system. In this tutorial, we will explain in detail how to save data to a file in an Android app using Java. The main contents include creating files, writing and reading data, as well as covering key concepts related to file I/O.
1. Overview of Android File System
Android provides several methods to store data in files. The commonly used file storage methods can be divided into two categories: internal storage and external storage.
- Internal Storage: Stores data in a dedicated space within the device where the application is installed. This data cannot be accessed by other apps, and it will be deleted along with the app when the app is uninstalled.
- External Storage: Stores data in external storage devices like SD cards. Users can access it directly, and data can be shared with other apps. External storage can further be divided into public and private storage.
2. Saving Files in Internal Storage
The process of saving files in internal storage is as follows.
2.1 Creating and Writing Files
First, we will open the ‘MainActivity.java’ file and write code to save data in internal storage.
public class MainActivity extends AppCompatActivity {
private static final String FILENAME = "example_file.txt";
private Button writeButton;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
writeButton = findViewById(R.id.write_button);
writeButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
writeFileToInternalStorage("Hello, World!");
}
});
}
private void writeFileToInternalStorage(String data) {
FileOutputStream fos;
try {
fos = openFileOutput(FILENAME, Context.MODE_PRIVATE);
fos.write(data.getBytes());
fos.close();
Toast.makeText(this, "File saved successfully!", Toast.LENGTH_SHORT).show();
} catch (IOException e) {
e.printStackTrace();
Toast.makeText(this, "File write failed!", Toast.LENGTH_SHORT).show();
}
}
}The above code is an example that saves the text “Hello, World!” in a file named ‘example_file.txt’ in internal storage when a simple button is clicked.
2.2 Reading Files
To read the saved file, add the following code.
private void readFileFromInternalStorage() {
FileInputStream fis;
try {
fis = openFileInput(FILENAME);
InputStreamReader isr = new InputStreamReader(fis);
BufferedReader reader = new BufferedReader(isr);
StringBuilder sb = new StringBuilder();
String line;
while ((line = reader.readLine()) != null) {
sb.append(line);
}
fis.close();
Toast.makeText(this, "Read from file: " + sb.toString(), Toast.LENGTH_SHORT).show();
} catch (IOException e) {
e.printStackTrace();
Toast.makeText(this, "File read failed!", Toast.LENGTH_SHORT).show();
}
}Now it is possible to read from the file after writing to it. When the user clicks the button, the saved content is read and displayed as a Toast message on the screen.
3. Saving Files in External Storage
Saving files in external storage is slightly different from internal storage, and users can access the files. To save data in external storage, you must first request permissions.
3.1 Requesting Permissions
Add the following code to the AndroidManifest.xml file to grant access permissions for external storage.
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>Now, add the following code to ‘MainActivity.java’ for runtime permission requests.
private static final int REQUEST_EXTERNAL_STORAGE = 1;
private String[] permissions = {Manifest.permission.WRITE_EXTERNAL_STORAGE, Manifest.permission.READ_EXTERNAL_STORAGE};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
ActivityCompat.requestPermissions(this, permissions, REQUEST_EXTERNAL_STORAGE);
}
writeButton = findViewById(R.id.write_button);
writeButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
writeFileToExternalStorage("Hello from External Storage!");
}
});
}3.2 Creating and Writing Files
Now you can create files and write data to external storage. The code to write files to external storage is as follows.
private void writeFileToExternalStorage(String data) {
File file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS), "example_external_file.txt");
FileOutputStream fos;
try {
fos = new FileOutputStream(file);
fos.write(data.getBytes());
fos.close();
Toast.makeText(this, "External file saved successfully!", Toast.LENGTH_SHORT).show();
} catch (IOException e) {
e.printStackTrace();
Toast.makeText(this, "External file write failed!", Toast.LENGTH_SHORT).show();
}
}The above code creates a file named ‘example_external_file.txt’ in the documents directory of external storage and fills it with the data “Hello from External Storage!”.
3.3 Reading Files
The code to read the saved file is as follows.
private void readFileFromExternalStorage() {
File file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS), "example_external_file.txt");
FileInputStream fis;
try {
fis = new FileInputStream(file);
InputStreamReader isr = new InputStreamReader(fis);
BufferedReader reader = new BufferedReader(isr);
StringBuilder sb = new StringBuilder();
String line;
while ((line = reader.readLine()) != null) {
sb.append(line);
}
fis.close();
Toast.makeText(this, "Read from external file: " + sb.toString(), Toast.LENGTH_SHORT).show();
} catch (IOException e) {
e.printStackTrace();
Toast.makeText(this, "External file read failed!", Toast.LENGTH_SHORT).show();
}
}4. Additional File Handling Features
When dealing with file I/O, there are a few additional functions that can be implemented beyond simple reading and writing.
4.1 Checking File Existence
The code to check if a file exists is as follows.
private boolean fileExists(String fileName) {
File file = new File(getFilesDir(), fileName);
return file.exists();
}4.2 Deleting Files
If you want to delete a file, use the code below.
private void deleteFileFromInternalStorage(String fileName) {
File file = new File(getFilesDir(), fileName);
if (file.delete()) {
Toast.makeText(this, "File deleted successfully!", Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(this, "File deletion failed!", Toast.LENGTH_SHORT).show();
}
}5. Conclusion
In this tutorial, we have learned in detail how to save data to a file in an Android app using Java. We broadly covered methods using internal and external storage, file creation and reading, and the aspects of permission requests.
The file system continues to play an important role in various applications that require data storage. I hope this tutorial enhances your understanding of file I/O and enables you to apply it in real application development.
In future tutorials, we will also cover more complex data storage methods, such as databases. I hope you develop the ability to learn and appropriately utilize various data storage methods along with the file system.
Thank you!