Question

I’m beginner in Android (and java), and I’m just playing around and want to create new View after clicking a button. Here is my code so far:

Main class

package myTests.homeSpace;

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

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

        Button button1 = (Button) findViewById(R.id.button1);
        button1.setOnClickListener( new OnClickListener()
        {
            public void onClick(View v)
            {
                Intent myIntent = new Intent(TestsActivity.this, screen1.class);
                startActivity(myIntent);
            }
        });
    }
}

Class to run after clicking button

package myTests.homeSpace;

import android.app.Activity;
import android.os.Bundle;

public class screen1 extends Activity {
    @Override
    public void onCreate(Bundle SavedBundleInstance)
    {
        super.onCreate(SavedBundleInstance);
        setContentView(R.layout.screen1);
    }
}

The problem is, after hitting button I got “Unforunately, Tests has stopped” error. (“Tests” is application name). I know (or guess) the problem is in this line Intent myIntent = new Intent(TestsActivity.this, screen1.class);
I guess my reference to class screen1 is wrong somehow, but I have no idea why. There are no compilation errors nor warnings, layout .xmls shouldn’t be wrong.

Could any of you please advice me any solution?

EDIT

MANIFEST

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="myTests.homeSpace"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:name=".TestsActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

CONSOLE OUTPUT

[2012-06-20 19:56:21 - ddms] null
java.lang.NullPointerException
    at com.android.ddmlib.JdwpPacket.writeAndConsume(JdwpPacket.java:213)
    at com.android.ddmlib.Client.sendAndConsume(Client.java:575)
    at com.android.ddmlib.HandleHello.sendHELO(HandleHello.java:142)
    at com.android.ddmlib.HandleHello.sendHelloCommands(HandleHello.java:65)
    at com.android.ddmlib.Client.getJdwpPacket(Client.java:672)
    at com.android.ddmlib.MonitorThread.processClientActivity(MonitorThread.java:317)
    at com.android.ddmlib.MonitorThread.run(MonitorThread.java:263)

Answer 1

I agree, not declaring the activity in your Manifest file (AndroidManifest.xml) is most likely your problem. See the android developer page on activities for more information. Here is the relevant description:

Declares an activity (an Activity subclass) that implements part of
the application’s visual user interface. All activities must be
represented by elements in the manifest file. Any that are
not declared there will not be seen by the system and will never be
run.

I just posted this so you would have more space to research on your own.