Your first Android project

Install the demo application

This application is also available on the Android Marketplace under Android Temperature converter .
Alternatively you can also scan the following barcode with your Android smartphone to install it via the Google Play application.

Create Project

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.

Modifying resources

As described in the Android Development Tools (ADT) chapter, ADT provides specialized editors for resources files, e.g. layout files. These editors allow to switch between the XML representation of the file and a richer user interface via tabs on the bottom of the editor.
The following description uses the rich user interface to build layout files. For validation purposes, the resulting XML is also included in the description.

Create attributes

Android allows you to create static attributes, e.g. Strings or colors. These attributes can for example be used in your XML layout files or referred to via Java source code.
Select the file "res/values/string.xml" and press the Add button. Select "Color" and enter "myColor" as the name and "#3399CC" as the value.

Add the following "String" attributes. String attributes allow the developer 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 that the values are correct.

<?xml version="1.0" encoding="utf-8"?>
<resources>
 <string name="hello">Hello World, Convert!</string>
 <string name="app_name">Temperature Converter</string>
 <color name="myColor">#3399CC</color>
 <string name="myClickHandler">myClickHandler</string>
 <string name="celsius">to Celsius</string>
 <string name="fahrenheit">to Fahrenheit</string>
 <string name="calc">Calculate</string>
</resources> 

Add Views

Select "res/layout/main.xml" and open the Android editor via a double-click. This editor allows you to create the layout 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 Eclipse "Outline" view.
The following shows a screenshot of the "Palette" view from which you can drag and drop new user interface components into your layout. Please note that the "Palette" view changes frequently so your view might be a bit different.

You will now create your new layout.
Right-click on the existing text object “Hello World, Hello!” in the layout. Select "Delete" from 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.
Afterwards 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.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="EditText" >
    </EditText>

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <RadioButton
            android:id="@+id/radio0"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:checked="true"
            android:text="RadioButton" >
        </RadioButton>

        <RadioButton
            android:id="@+id/radio1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="RadioButton" >
        </RadioButton>
    </RadioGroup>

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button" >
    </Button>

</LinearLayout> 

Edit View properties

If you select a user interface component (an instance of View), you can change its properties via the Eclipse "Properties" view. Most of the properties can be changed via the right mouse menu. You can also edit properties of fields directly in XML. Changing properties in the XML file is much faster, if you know what you want to change. But the right mouse functionality is nice, if you are searching for a certain property.
Open your main.xml layout file . The EditText control shows currently a default text. We want to delete this initial text in the XML code. 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 "fahrenheit" string attribute to the second radio button.

From now on, I assume you are able to use the properties menu on user interface components. You can always 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 user interface components 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 select "myColor" "in the list which is displayed.

Switch to the main.xml tab and verify that the XML is correct.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="@color/myColor"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="numberDecimal|numberSigned" >
    </EditText>

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <RadioButton
            android:id="@+id/radio0"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:checked="true"
            android:text="@string/celsius" >
        </RadioButton>

        <RadioButton
            android:id="@+id/radio1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/fahrenheit" >
        </RadioButton>
    </RadioGroup>

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="myClickHandler"
        android:text="@string/calc" >
    </Button>

</LinearLayout> 

Change the Activity source code

During the generation of your new Android project you specified that an Activity called ConvertActivity should be created. The project wizard created the corresponding Java class.
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;
  }
} 

Start Project

To start the Android Application, select your project, right click on it, and select Run-As → Android Application. If an emulator is not yet running, it will be started. Be patient, the emulator starts up very slowly.
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.

Starting an installed application

After you run your application on the virtual device, you can start it again on the device. If you press the "Home" button you can select your application.

(vogella.com)