In my great expectations of Google Android coming to Canada on June 2nd, I’ve started experimenting with developing some apps for the Android platform. My first app is called “The Taxman” and will calculate the amount of tax you owe per year in your province/state – well only Canada for now.

I had trouble adjusting to what an “Activity” was and how to handle it. Here is a quick and dirty way to create an Activity, and to switch to another Activity (think of it as another screen) on the click of a button.

1. Create a new Android project – or you might already have one created.

2. Add a new Class that extends android.app.Activity. You need a total of two classes that extend Activity. You will switch from one Activity to another.

3. Now, we’ll create two XML files to store the layout of each Activity. Under the res/layouts directory create a copy of main.xml

4. Each XML file will contain 1 button. On the click of the button, the Activities will switch.

main.xml will contain:

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

    <TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textColor="#000000"
    android:text="This is Activity 1" />

       <Button android:text="Next"
        android:id="@+id/Button01"
        android:layout_width="250px"
            android:textSize="18px"
        android:layout_height="55px">
    </Button>    

</LinearLayout>

main2.xml will contain:

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

    <TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textColor="#000000"
    android:text="This is Activity 2" />

       <Button android:text="Previous"
        android:id="@+id/Button02"
        android:layout_width="250px"
            android:textSize="18px"
        android:layout_height="55px">
    </Button>    

</LinearLayout>

So each Activity will have a text that says “This is Activity x” and a button to switch the Activity.

5. Add the second Activity to the main manifest file. Open AndroidManifest.xml and add:

        <activity android:name=".Activity2"></activity>

The final result will look similar to this:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.warriorpoint.taxman2"
      android:versionCode="1"
      android:versionName="1.0">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".Activity1"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".Activity2"></activity>
    </application>
    <uses-sdk android:minSdkVersion="3" />
</manifest>

If you forget to do this, then the you will get a Null Pointer exception because “Activity2” will not be found at runtime. It took me some time to find out how to find what Exception was getting thrown as well. I will include how to debug and look at Exceptions in another future post.

5. Open Activity1.java and enter the following code:

package com.warriorpoint.taxman2;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class Activity1 extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        Button next = (Button) findViewById(R.id.Button01);
        next.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                Intent myIntent = new Intent(view.getContext(), Activity2.class);
                startActivityForResult(myIntent, 0);
            }

        });
    }
}

Here’s a quick explanation of what this does:

- setContentView(R.layout.main) makes sure that main.xml is used as the layout for this Activity.

- Gets a reference to the button with ID Button01 on the layout using (Button) findViewById(R.id.Button01).

- Create san OnClick listener for the button – a quick and dirty way.

- And the most important part, creates an “Intent” to start another Activity. The intent needs two parameters: a context and the name of the Activity that we want to start (Activity2.class)

- Finally, the Activity is started with a code of “0”. The “0” is your own code for whatever you want it to mean. Activity2 will get a chance to read this code and use it. startActivityForResult means that Activity1 can expect info back from Activity2. The result from Activity2 will be gathered in a separate method which I will not include here.

6. Open Activity2.java and enter the code below:

package com.warriorpoint.taxman2;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class Activity2 extends Activity {

    /** Called when the activity is first created. */
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main2);

        Button next = (Button) findViewById(R.id.Button02);
        next.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                Intent intent = new Intent();
                setResult(RESULT_OK, intent);
                finish();
            }

        });
    }

This code does the following:

- Sets main2 as the layout for this Activity

- Gets a reference to Button02 and creates an OnClick listener

- In the OnClick listener, the Activity finishes with finish(). setResult() returns information back to Activity 1. In this example, it returns no information; and Activity1 doesn’t even have the listener to receive this information anyway.

That’s it! Run it!

The app will load in Activity 1:

When you click the button you will see Activity 2. There are no animations, no tweens, etc, so the screen will just “change”. I’ll talk about animations in future posts.

And clicking on the button “Previous” here will go back to Activity1.

Still to come:

1. How to create animations when switching screens.

2. How to switch using a dragging motion of your finger.

3. How to see a log of the exceptions that your app throws.