Your first Android project

6. Your first Android project

6.1. Create Project

This app is also available on the Android Marketplace. Search for "vogella" for find this example.

Select File → New → Other → Android → Android Project and create the Android project "de.vogella.android.temperature". Enter the following.

Press "Finish". This should create the following directory structure.

While "res" contains structured values which are known to the Android platform the directory "assets" can be used to store any kind of data. In Java you can access this data via the AssetsManager and the method getAssets().

6.2. Two faces of things

The Android SDK allows to define certain artifacts, e.g. strings and UI's, in two ways, via a rich editor and directly via XML. The following description tries to use the rich UI but for validation lists also the XML. You can switch between both things by clicking on the tab on the lower part of the screen. For example in the Package Explorer select "res/layout/main.xml".

6.3. Create attributes

Android allows you to create attributes for resources, e.g. for strings and / or colors. These attributes can be used in your UI definition via XML or in your Java source code.

Select the file "res/values/string.xml" and press "Add". Select "Color" and enter "myColor" as the name and "#3399CC" as the value.

Add also the following "String" attributes. String attributes allow to translate the application at a later point.

Table 1. String Attributes

Name	Value
celsius	to Celsius
fahrenheit	to Fahrenheit
calc	Calculate

Switch to the XML representation and validate the values.

        Hello World, Convert!  Temperature Converter  #3399CC  myClickHandler  to Celsius  to Fahrenheit  Calculate     

6.4. Add UI Elements

Select "res/layout/main.xml" and open the Android editor via a double-click. This editor allows you to create the UI via drag and drop or via the XML source code. You can switch between both representations via the tabs at the bottom of the editor. For changing the position and grouping elements you can use the outline view.

The following shows a screenshot of the Palette view from which you can drag and drop new UI elements into your layout. Please note that the "Palette" view changes frequently so your view might be a bit different.

Right-click on the text object “Hello World, Hello!” in the layout. Select Delete on the popup menu to remove the text object. Then, from the “Palette” view, select Text Fields and locate “Plain Text”. Drag this onto the layout to create a text input field. All object types in the section "Text Fields” derive from the class "EditText", they just specify via an additional attribute which text type can be used.

Now select the Palette section “Form Widgets” and drag a “RadioGroup” object onto the layout. The number of radio buttons added to the radio button group depends on your version of Eclipse. Make sure there are two radio buttons by deleting or adding radio buttons to the group.

From the Palette section Form Widgets, drag a Button object onto the layout.

The result should look like the following.

Switch to "main.xml" and verify that your XML looks like the following.

6.5. Edit UI properties

If you select a UI element you can change its properties via the properties view. Most of the properties can be changed via the right mouse menu. You can also edit properties of fields directy in XML. Typically you change properties directly in the XML file as this is much faster. But the right mouse functionality is nice if you are searching for a certain property.

Open your file "main.xml" We will delete the initial text for the EditText field in XML. Switch to the XML tab called "main.xml" and delete the android:text="EditText" property from the EditText part. Switch back to the "Graphical Layout" tab and check that the text is removed.

Use the right mouse click on the first radio button to assign the "celsius" string attribute to its "text" property. Assign the and "fahrenheit" string attribute to the second radio button.

From now on I assume you are able to use the properties menu on the UI elements. You can either edit the XML file or modify the properties via right mouse click.

Set the property "Checked" to true for the first RadioButton. Assign "calc" to the text property of your button and assign "myClickHandler" to the "onClick" property. Set the "Input type" property to "numberSigned" and "numberDecimal" on your EditText.

All your other UI controls are contained in a LinearLayout. We want to assign a background color to this LinearLayout. Right-click on an empty space in Graphical Layout mode, then select Other Properties → All by Name → Background. Select “Color” and then “myColor” in the list.

Switch to the "main.xml" tab and verify that the XML is correctly maintained.

6.6. Code your application

During the generation of your new Android project you specified that an Activity called CovertActivity should get created. The project wizard also created the correspondig Java classs.

Change your code in ConvertActivity.java to the following. Note that the myClickHandler will be called based on the OnClick property of your button.

     package de.vogella.android.temperature;  import android.app.Activity; import android.os.Bundle; import android.view.View; import android.widget.EditText; import android.widget.RadioButton; import android.widget.Toast;  public class ConvertActivity extends Activity {  private EditText text;   @Override  public void onCreate(Bundle savedInstanceState) {   super.onCreate(savedInstanceState);   setContentView(R.layout.main);   text = (EditText) findViewById(R.id.editText1);   }   // This method is called at button click because we assigned the name to the  // "On Click property" of the button  public void myClickHandler(View view) {   switch (view.getId()) {   case R.id.button1:    RadioButton celsiusButton = (RadioButton) findViewById(R.id.radio0);    RadioButton fahrenheitButton = (RadioButton) findViewById(R.id.radio1);    if (text.getText().length() == 0) {     Toast.makeText(this, "Please enter a valid number",       Toast.LENGTH_LONG).show();     return;    }     float inputValue = Float.parseFloat(text.getText().toString());    if (celsiusButton.isChecked()) {     text.setText(String       .valueOf(convertFahrenheitToCelsius(inputValue)));     celsiusButton.setChecked(false);     fahrenheitButton.setChecked(true);    } else {     text.setText(String       .valueOf(convertCelsiusToFahrenheit(inputValue)));     fahrenheitButton.setChecked(false);     celsiusButton.setChecked(true);    }    break;   }  }   // Converts to celsius  private float convertFahrenheitToCelsius(float fahrenheit) {   return ((fahrenheit - 32) * 5 / 9);  }   // Converts to fahrenheit  private float convertCelsiusToFahrenheit(float celsius) {   return ((celsius * 9) / 5) + 32;  } }    

6.7. Start Project

To start the Android Application, select your project, right click on it, Run-As-> Android Application Be patient, the emulator starts up very slow. You should get the following result.

Type in a number, select your conversion and press the button. The result should be displayed and the other option should get selected.