- Starting a New Project
- Coding the Interface
- Coding App Behavior
- Summary
- Exercises
This chapter is from the book
This chapter is from the book
This chapter is from the book
Coding App Behavior
Code to give behavior to the layout is written and stored in the Java class file associated with the layout. Open the HelloWorldActivity.java file by double-clicking it. If it is already open, click its tab in the editor. You should see the basic code structure (Listing 3.4).
Listing 3.4 Initial Activity Code
//1packagecom.example.helloworld; //2importandroid.os.Bundle;importandroid.app.Activity;importandroid.view.Menu; //3public classHelloWorldActivityextendsActivity { @Override //4protected voidonCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState); setContentView(R.layout.activity_hello_world); } @Override //5public booleanonCreateOptionsMenu(Menu menu) {// Inflate the menu; this adds items to the action bar if it is present.getMenuInflater().inflate(R.menu.main, menu);return true; } }
This code was generated by Eclipse when you created the activity at the start of the HelloWorld project. It is important to understand what this code does to properly code an activity.
- At the top of the file is the keyword “package” followed by com.example.helloworld. This identifies this class as belonging to the Hello World package. All source Java files (in src folder) will have this entry as the first code in the file.
After the package line and before any other code are the imports. Click the plus (+) sign in front of the import android.os.Bundle; line of code. You should now see three import lines. This code is used to get the source code needed for your activity. The Activity class provides the functionality required for any class that uses or interacts with other Activities used in this class. The Menu class provides the functionality for the menu that is displayed when the user presses the device’s Menu button. The Bundle import requires a bit more explanation.
A Bundle is an object for passing data between activities. In this way we can have an application that can perform some activity based on what another activity has done or the data it has used. You will use this functionality later in the book. However, Bundle also performs another very important function. It passes data back to the activity itself. When the user rotates the device, the displayed activity is destroyed and re-created in the new orientation. So that the user doesn’t have to start over if this happens, the activity stores its current state just before it is destroyed in a bundle and passes that data to itself when it re-creates the activity in the new orientation.
- The public class line of code begins the Activity class and declares that this class is referred to as HelloWorldActivity and that it is a subclass of the SDK-provided Activity class. Within the class are two methods, onCreate and onCreateOptionsMenu. Before each method declaration is @Overide. This annotation tells the compiler that the following method is to be used in place of the super class’s method of the same name.
- The onCreate method is the first method executed by the Activity when it is started. The method has a parameter that is of type Bundle named savedInstanceState. This is the object that contains information on the state of the Activity if it was destroyed in an orientation change as explained earlier. The next line super.onCreate calls the super class’s onCreate method. Because this method is overriding the Activity class’s inherited onCreate method, it must call that method explicitly to use that functionality to create the Activity. It is passed the savedInstanceState bundle. The final line of code is setContentView(R.layout.activity_hello_world). This code tells the activity to use the activity_hello_world.xml file as the layout to be displayed when the activity is running. It is very important to understand the parameter R.layout.activity_hello_world. The R parameter tells the compiler that we want to use a resource from the layout folder named activity_hello_world. Whenever we want to access or manipulate a resource, it has to be referred to in this manner. However, this does not refer directly to the res folders; instead it refers to a file generated by the compiler that is named R.java. To see this file, double-click into the gen folder in the Package Explorer until you see it. You should not edit this file because it is automatically generated by the compiler. The onCreate method will be modified with our code to add further functionality to the activity.
- The onCreateOptionsMenu(Menu menu) method is called when the user clicks the device’s Menu button. It returns a Boolean (true or false) value indicating whether the menu was successfully created. The first line of code (getMenuInflator()) gets an object that can create a menu from the running activity. It then tells it to inflate (create) a visual representation of the menu based on the main.xml file in the menu resource folder and refer to it with the name “menu”.
Adding Code
Our app has only one function, to display the name entered into the EditText when the Display button is pressed. Enter the code in Listing 3.5 before the last curly bracket in the activity Java file:
Listing 3.5 Display Button Code
//1private voidinitDisplayButton() { Button displayButton = (Button) findViewById(R.id.buttonDisplay); //2 displayButton.setOnClickListener(newOnClickListener () { //3 @Overridepublic voidonClick(View arg0) { EditText editName = (EditText) findViewById(R.id.editTextName); //4 TextView textDisplay = (TextView) findViewById(R.id.textViewDisplay); //5 String nameToDisplay = editName.getText().toString(); //6 textDisplay.setText("Hello "+ nameToDisplay); //7 } }); }
This code does the work and illustrates a number of important concepts in Android development.
- This line declares a new method in the HelloWorldActivity class. The method is only useable by this class (private) and does not return any value (void). The method signature is initDisplayButton(). The signature, or name, of the method is completely up to you. However, you should name it to give some idea what it does.
- Associate the code with the button on the layout. This line of code declares a variable of type Button that can hold a reference to a button and then gets the button reference using the command findViewById. All widgets on a layout are subclasses of the View class. The method findViewById is used to get a reference to a widget on a layout so it can be used by the code. The method can return any View object, so you have to use (Button) before it to cast the returned View to a Button type before it can be used as a Button by the code. Button is underlined in red after you type it in. This is because the code for the button class is not automatically available in the class. You have to import it. Fortunately, this is easy. Hover your cursor over the underlined word and a menu will pop up. Select Import Button... Do this for any other items underlined in red.
- Set the button’s listener. There are a number of different listeners for widgets, which gives great flexibility when coding app behavior. For this button we use an onClickListener. The code creates a new instance of the listener and then adds a method (public void onClick(View arg0)) to be executed when the button is clicked.
- 4–5. The code for when the button is clicked gets references to the EditText where the name was entered and the TextView where the message will be displayed.
- The name entered by the user is retrieved from the EditText and stored in a String variable named nameToDisplay.
- The text attribute of the TextView is changed to the value of the String variable.
Notice that initDisplayButton() is underlined in yellow. This is because the method is never called by the code. To call it and get the behavior associated with the button to execute, you have to call the method in the onCreate method. After the setContentView line of code enter
initDisplayButton();
The yellow underline goes away and your code is done! Run the app in the emulator using Run > Run Configurations, and click the Run button to test your first app. You could also run your app using Run > Run or by pressing Ctrl+F11.