Logging functionality

Using Log in Android

1. Android Log Utility Class: Log

Android SDK provides a dedicated logging utility class android.util.Log, which offers 5 levels of logging methods, arranged from lowest to highest priority:

Method	Level	Description
`Log.v()`	Verbose	Lowest level, used for the most trivial log messages
`Log.d()`	Debug	Debug information, most commonly used during development
`Log.i()`	Info	General informational messages
`Log.w()`	Warn	Warning messages, indicating potential risks
`Log.e()`	Error	Error messages, such as exceptions caught in catch blocks

2. Basic Usage

Add logging in the onCreate() method of MainActivity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    
    // Output Info level log
    Log.i("foxdevelop", "xxxxxx");
}Code language: JavaScript (javascript)

Method Parameter Description:

Parameter	Description
First parameter (tag)	Log tag, usually a short string identifying the source for easy filtering
Second parameter (msg)	Log content, i.e., the message you want to output

After running the project, open the Logcat window to view the output logs.

3. Logcat Window Tips

Feature	Operation
View logs	After running the app, the Logcat panel at the bottom displays automatically
Keyword search	Enter a keyword (e.g., `foxdevelop`) in the Logcat search box to filter
Filter by level	Click the dropdown menu to select Show only selected application / No Filters / Edit Filter Configuration
Filter by tag	Enter `tag:foxdevelop` in the search box

4. Why System.out.println() Is Not Recommended

Comparison	System.out.println	Log
Controllability	Uncontrollable	Can be controlled via levels and filters
Timestamp	No timestamp	Each log includes a precise timestamp
Filtering	Cannot filter by tag	Supports multi-dimensional filtering by tag, level, process, etc.
Log levels	No level distinction	5 levels for easy troubleshooting
Production environment	Cannot be disabled	Can be configured to disable or output only specific levels

// Not recommended
System.out.println("xxxxxxx");

// Recommended
Log.i(TAG, "Foxdevelop");Code language: JavaScript (javascript)

5. Android Studio Log Shortcuts

AS provides powerful Live Template shortcuts. Type and press Tab to auto-complete:

Shortcut (type and press Tab)	Generated Code	Description
`logd`	`Log.d(TAG, "onCreate: ");`	Generate Debug log
`logi`	`Log.i(TAG, "onCreate: ");`	Generate Info log
`logw`	`Log.w(TAG, "onCreate: ");`	Generate Warn log
`loge`	`Log.e(TAG, "onCreate: ");`	Generate Error log
`logv`	`Log.v(TAG, "onCreate: ");`	Generate Verbose log
`logt`	`private static final String TAG = "ClassName";`	Use in class body to auto-generate current class name as TAG

6. Complete Example

public class MainActivity extends AppCompatActivity {

    // Use logt + Tab in class body to auto-generate
    private static final String TAG = "MainActivity";

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

        // Output log using TAG constant
        Log.i(TAG, "foxdevelop");
        Log.e(TAG, "An exception occurred here");

        // Not recommended
        System.out.println("xxxxxxx");
    }
}Code language: PHP (php)

Logcat output after running:

I/MainActivity: foxdevelop
E/MainActivity: An exception occurred hereCode language: HTTP (http)