Six Ways to Print Text

While you have the previous project open in Android Studio, edit the MainActivity.java file.

1. Pop up a piece of Toast.

This is the most self-contained way to print debugging output, since only one file has to be edited. Insert the following statements into the onCreate method of the MainActivity immediately after the call to setContentView. 

		Toast toast = Toast.makeText(this, "MainActivity onCreate", Toast.LENGTH_LONG);
		toast.show();
We said this because the first parameter of Toast.makeText must be an Activity object. (Actually, it must be a Context object, which is a superclass of Activity.) "MainActivity onCreate" is the message to be displayed. LENGTH_LONG is a public static final field of class Toast. makeText is a static method of class Toast; show is a non-static method. Try Toast.LENGTH_SHORT (2 seconds vs. 3½).

Combine the above two statements into one statement by letting the variable that refers to the Toast be an anonymous temporary variable: 

		Toast.makeText(this, "MainActivity onCreate", Toast.LENGTH_LONG).show();

To avoid complains about the word Toast, you will need the following import at the top of the MainActivity.java file. We configured Android Studio to write the import automatically when we installed it. 

import android.widget.Toast;	//Write this with the other imports at the top of MainActivity.java.

Toast bibliography:

  1. Snackbars & toasts in the Material Design spec.
  2. Toasts in the User Interface guide.
  3. The Java class android.widget.Toast
    1. Documentation
    2. platforms_frameworks_base/core/java/android/widget/Toast.java source code on GitHub

2. Pop up a dialog box.

A dialog is like a piece of toast, but it’s bigger and lasts longer. The dialog stays on the screen until you press its OK button. Insert the following statements into the onCreate method of the MainActivity immediately after the call to setContentView. We must import android.app.AlertDialog. 

		//The builder will build the dialog.
		AlertDialog.Builder builder = new AlertDialog.Builder(this);

		//Tell the builder about the dialog you want it to create.
		builder.setTitle("The Title");
		builder.setMessage("The message");
		//null because we want the positive button to do nothing
		//except dismisss the dialog.
		builder.setPositiveButton("OK", null);

		//Now that we've finished describing the dialog,
		//tell the builder to create it.
		AlertDialog alertDialog = builder.create();
		alertDialog.show();

The last two statements could be replaced by the following. 

		builder.show();

Instead of hardcoding the title "The Title" into the Java code, it should have been a string resource in strings.xml. Ditto for the other strings. 

	<string name="dialog_title">The Title</string>

The above name attribute adds a static field named dialog_title to the Java class R.string in the R.java file. We can now use the integer variable R.string.dialog_title in our Java code. 

public final class R {
	public static final class string {
		public static final int dialog_title=some number;
	}
}

		builder.setTitle(R.string.dialog_title);

Dialog bibliography:

  1. Dialogs in the Material Design spec.
  2. Dialogs in the User Interface guide.
  3. The Java class android.app.AlertDialog
    1. Documentation
    2. platforms_frameworks_base/core/java/android/app/AlertDialog.java source code on GitHub

3. Display text in a TextView.

Our res/layout/activity_main.xml already contains the following TextView object. We created the string resource greeting in the exercises in Hello. Let’s give it an ID number. The plus sign creates a new ID number. Without the plus, we would merely be referring to an existing ID number. 

	<TextView
		android:id="@+id/textView"
		android:layout_width="wrap_content"
		android:layout_height="wrap_content"
		android:text="@string/greeting"/>

The above android:id attribute adds a static field named textView to the Java class R.id in the R.java file. 

public final class R {
	public static final class id {
		public static final int textView=0x7f05003d;
	}
}

We can now use the integer variable R.id.textView in our Java code. Insert the following statements after the call to setContentView in the onCreateView method in MainActivity.java. findViewById returns an object of class View, which is the superclass of all the view classes. This plain vanilla return value must be converted to the data type TextView before we can call setText. The (TextView) in parentheses is a conversion operator called a cast. In this case it’s a downcast, a conversion from a superclass to a subclass. The Java instanceof operator returns true if the downcast will succeed, i.e., if the view actually is a TextView. In addition to setText, there’s also append. 

		View view = findViewById(R.id.textView);

		if (view == null) {
			Toast.makeText(this, "could not find R.id.textView", Toast.LENGTH_LONG).show();
		} else if (!(view instanceof TextView)) {
			Toast.makeText(this, "view belongs to class " + view.getClass(), Toast.LENGTH_LONG).show();
		} else {
			TextView textView = (TextView)view;
			textView.setText("Debuging output in a TextView.");
		}

Most people omit the error checking: 

		TextView textView = (TextView)findViewById(R.id.textView);
		textView.setText("Debuging output in a TextView.");

We could even do it all in one statement, with an extra pair of parentheses to execute the cast before the dot whose right operand is setText. 

		((TextView)findViewById(R.id.textView)).setText("Debuging output in a TextView.");

The .java file will also need the following imports. 

import android.widget.TextView;
import android.widget.Toast;

TextView bibliography:

  1. The Java class android.widget.TextView
    1. Documentation
    2. platforms_frameworks_base/core/java/android/widget/TextView.java source code on GitHub

4. Log.d into the LogCat window

Call the static method Log.d in the onCreate method of the MainActivity. See Write and View Logs with LogCat. The six levels of priority (in increasing order) are Verbose, Debug, Info, Warning, Error, Assert: Very Dark Icebergs With Eskimos? 

		Log.d("myTag", "my message");

Import android.util.Log, not the homonymous org.apache.commons.logging.Log. 

import android.util.Log;

Click on the LogCat tab at the bottom of Android Studio. (If you don’t see the tab, click on 6: Android Monitor.) For the log level, select the “Debug” level of verbosity from the drop-down menu. Next to it, where the magnifying glass is, write myTag. The 8825’s are the Process ID and Thread ID. 

07-27 08:46:16.392 8825-8825/edu.nyu.sps.hello D/myTag: my message

Better yet, create a new filter by pulling the rightmost menu (“Show only selected application”) down to Edit Filter Configuration. That way, you won’t have to see any lines other than the ones that have myTag.

Create New Logcat Filter
Specify one or several filtering parameters:
Filter Name: MyFilter
Log Tag: myTag (Check regex if you know about regular expressions.)
Log Level: Debug
OK

To clear the log, click on the blue garbage can with the gray top. If you don’t see the can, click on the >>.

5. Log.d to the standard output of adb logcat

Call Log.d as above. Type the following command in the Terminal window of a Mac or the Command Prompt window of a PC. It will output every line with the tag myTag printed by Log.d or by the higher methods Log.i (info), Log.w (warning), or Log.e (error). Lines with other tags are silenced with an uppercase S. The 'single quotes' are necessary on platforms where the * is a shell wildcard. The -s option stands for “serial number”. 

adb devices
List of devices attached
192.168.56.101:5555
emulator-5554
ca1784a34445a8d0308

adb devices -l
List of devices attached
192.168.56.101:5555    device product:vbox86p model:Samsung_Galaxy_S5___4_4_4___API_19___1080x1920 device:vbox86p
emulator-5554          device product:sdk_google_phone_x86_64 model:Android_SDK_built_for_x86_64 device:generic_x86_64
ca1784a34445a8d0308    device usb:14100000 product:polaris_inet model:A727 device:polaris-inet

adb -s emulator-5554 logcat -v brief myTag:D '*:S'
--------- beginning of main
--------- beginning of system
D/myTag   ( 8825): my message

Or you can direct the standard output into a file: 

adb -s emulator-5554 logcat myTag:D '*:S' > outfile.txt

Then launch the app. After the output has been produced, you can kill adb with a control-C.

6. System.out.println

The standard way of producing output in Java is with the method(s) System.out.println. 

		//Create a variable named i and put 10 into it.
		int i = 10;

		System.out.println("standard output: i = " + i);
		System.err.println("standard error output: i = " + i);

The documentation Viewing stdout and stderr says that System.out.println writes lines with the tag stdout with priority I. The tags are actually System.out with priority I and System.err with priority W. You can see the output in the LogCat window or with adb logcat. 

adb -s emulator-5554 logcat System.out:I '*:S'
--------- beginning of main
--------- beginning of system
07-27 07:05:21.103  1884  2266 I System.out: gmsNlpServiceThread This NLP should run continuously. intent is Intent { act=com.google.android.location.internal.GMS_NLP pkg=com.google.android.gms }
07-27 09:14:23.005  8825  8825 I System.out: standard output: i = 10

Source code in Hello.zip

The app in this .zip file is pictured in the following screenshots. It’s exactly the same as the Hello app, except for the following additions.

  1. MainActivity.java. I added extra statements to onCreate. Import java.util.Date and java.text.DateFormat.
  2. R.java
  3. activity_main.xml. I added the properties android:id and android:scrollbars="vertical" to the TextView. The TextView also needed a call to setMovementMethod.
  4. strings.xml
  5. AndroidManifest.xml. I added the ACCESS_NETWORK_STATE and INTERNET permission needed to see if device is connected to Internet and to list the network interfaces.
  6. build.gradle (Module: app)

The two screenshots were taken on the Nexus 5X API 23 emulator. The change of orientation must have triggered another call to the onCreate method of the MainActivity, because the text changed (ORIENTATION_LANDSCAPE). But onCreate is called only when an Activity object is created. Therefore a second Activity object must have been created.

In fact, what actually happens is far more drastic. A change of orientation destroys the original Activity object. Then it creates a new Activity object and calls its onCreate method. We can verify this by writing Toast messages in the onCreate method of the Activity.

The primary purpose of an Activity object is to create the user interface. If the orientation changes, we will probably have to destroy and re-create the interafce, so Android destroys and re-creates the Activity.

Is this too extreme for you? We can prevent a change of orientation from destroying and re-creating the Activity object by adding the following attribute to the <activity> element in the AndroidManifest.xml file. 

	android:configChanges="orientation|screenSize|keyboardHidden"
See Handling the Configuration Change Yourself. Also see Orientation and Configuration.

On my Amazon Fire HD 6:

Things to try

Run the above app. The onCreate method of the MainActivity in MainActivity.java calls setText to write all the information into the TextView in activity_main.xml.

  1. Append a Log statement to the onCreate method of class MainActivity. 
        @Override
    protected void onCreateBundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //etc.
        Log.d("myTag", "onCreate");
    }
    Add the five other lifecycle methods to MainActivity. To get Android Studio to type them for you, click on the whitespace after the } that ends the onCreate method and pull down
    Code → Override Methods…
    Select Methods to Override/Implement
    Press the ↓a–z button for alphabetical order. Select the five methods onDestroy, onStart and onStop, onResume and onPause, and press OK. Insert a Log statement to each one. The Log.d in onDestroy comes before the super.onDestroy because I was afraid that the super.onDestroy would leave us with a destroyed Activity object that was unable to call Log.d. 
        @Override
    protected void onStart() {
        super.onStart();
        Log.d("myTag", "onStart");
    }

    @Override
    protected void onResume() {
        super.onResume();
        Log.d("myTag", "onResume");
    }

    @Override
    protected void onPause() {
        Log.d("myTag", "onPause");
        super.onPause();
    }

    @Override
    protected void onStop() {
        Log.d("myTag", "onStop");
        super.onStop();
    }

    @Override
    protected void onDestroy() {
        Log.d("myTag", "onDestroy");
        super.onDestroy();
    }
    1. Press the green Run button on Android Studio. It will create an Activity object. 
      07-28 08:50:40.026 6414-6414/edu.nyu.sps.hello D/myTag: onCreate
07-28 08:50:40.030 6414-6414/edu.nyu.sps.hello D/myTag: onStart
07-28 08:50:40.030 6414-6414/edu.nyu.sps.hello D/myTag: onResume
    2. Press the green Run button again. After the app appears on the screen, press the Android Home button on your device or emulator. Then revive the app by going to the launcher page and tapping on the app’s icon. 
      07-28 08:52:25.126 8378-8378/edu.nyu.sps.hello D/myTag: onCreate
07-28 08:52:25.163 8378-8378/edu.nyu.sps.hello D/myTag: onStart
07-28 08:52:25.163 8378-8378/edu.nyu.sps.hello D/myTag: onResume
(The home button is pressed at this point.)
07-28 08:52:41.938 8378-8378/edu.nyu.sps.hello D/myTag: onPause
07-28 08:52:42.343 8378-8378/edu.nyu.sps.hello D/myTag: onStop
(The icon is tapped at this point.)
07-28 08:52:54.184 8378-8378/edu.nyu.sps.hello D/myTag: onStart
07-28 08:52:54.184 8378-8378/edu.nyu.sps.hello D/myTag: onResume
    3. Press the green Run button again. After the app appears on the screen, press the Android back button on your device or emulator. The back button will remove the Activity object from the screen and destroy it. For the time being, assume that the back button always destroys the Activity from which you are going back. See Tasks and Back Stack. 
      07-28 08:54:39.732 10114-10114/edu.nyu.sps.hello D/myTag: onCreate
07-28 08:54:39.771 10114-10114/edu.nyu.sps.hello D/myTag: onStart
07-28 08:54:39.771 10114-10114/edu.nyu.sps.hello D/myTag: onResume
(The back button is pressed at this point.)
07-28 08:54:42.460 10114-10114/edu.nyu.sps.hello D/myTag: onPause
07-28 08:54:42.840 10114-10114/edu.nyu.sps.hello D/myTag: onStop
07-28 08:54:42.840 10114-10114/edu.nyu.sps.hello D/myTag: onDestroy
    4. Press the green Run button again. After the app appears on the screen, change the orientation of the device (e.g., from portrait to landscape). The Activity object is destroyed and re-created. See Handling Runtime Changes. 
      07-28 08:55:45.768 11009-11009/edu.nyu.sps.hello D/myTag: onCreate
07-28 08:55:45.804 11009-11009/edu.nyu.sps.hello D/myTag: onStart
07-28 08:55:45.804 11009-11009/edu.nyu.sps.hello D/myTag: onResume
(The orientation is changed at this point.)
07-28 08:56:11.117 11009-11009/edu.nyu.sps.hello D/myTag: onPause
07-28 08:56:11.119 11009-11009/edu.nyu.sps.hello D/myTag: onStop
07-28 08:56:11.119 11009-11009/edu.nyu.sps.hello D/myTag: onDestroy
07-28 08:56:11.154 11009-11009/edu.nyu.sps.hello D/myTag: onCreate
07-28 08:56:11.162 11009-11009/edu.nyu.sps.hello D/myTag: onStart
07-28 08:56:11.162 11009-11009/edu.nyu.sps.hello D/myTag: onResume

  2. See if the PID and UID numbers agree with the output of the Unix “process status” command ps on the emulator and the device. 
    adb devices
List of devices attached
emulator-5554	device
0088080744830RTG	device

adb -s emulator-5554 shell ps
USER      PID   PPID  VSIZE   RSS      WCHAN             PC   NAME
u0_a64    10059 1233  1292556 51988    ep_poll 7ff2555796ca S edu.nyu.sps.hello

  3. Would the app become simpler if we replaced the big if/else statement with a Java Map?

  4. To do: Height of S5 screen should be 1920 pixels. getPackageName. Two versions of thread id and priority. Print name of current directory, name of user. Print date first, followed by version numbers. Print parent of pid.
    Locale.getDefault().toString()
    System.getProperty("java.version")
    java.specification.version 1.6 java.version 1.6.0_23 java.vm.version 19.0-b09 java.runtime.version unknown