Your null pointer exception seems to be on this line:

String url = intent.getExtras().getString("userurl");

because intent.getExtras() returns null when the intent doesn't have any extras.

You have to realize that this piece of code:

Intent Main = new Intent(this, ToClass.class);Main.putExtra("userurl", url);startActivity(Main);

doesn't start the activity you wrote in Main.java, it will attempt to start an activity called ToClass and if that doesn't exist, your app crashes.

Also, there is no such thing as "android.intent.action.start" so the manifest should look more like:

<activity android:name=".start" 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= ".Main"></activity>

I hope this fixes some of the issues you are encountering but I strongly suggest you check out some "getting started" tutorials for android development and build up from there.

The question is answered already, but I want add more information about the causes.

Android app unable to start activity componentinfo

This error often comes with appropriate logs. You can read logs and can solve this issue easily.

Here is a sample log. In which you can see clearly ClassCastException. So this issue came because TextView cannot be cast to EditText.

Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText

11-04 01:24:10.403: D/AndroidRuntime(1050): Shutting down VM11-04 01:24:10.403: W/dalvikvm(1050): threadid=1: thread exiting with uncaught exception (group=0x41465700)11-04 01:24:10.543: E/AndroidRuntime(1050): FATAL EXCEPTION: main11-04 01:24:10.543: E/AndroidRuntime(1050): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.troysantry.tipcalculator/com.troysantry.tipcalculator.TipCalc}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.access$600(ActivityThread.java:141)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.os.Handler.dispatchMessage(Handler.java:99)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.os.Looper.loop(Looper.java:137)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.main(ActivityThread.java:5103)11-04 01:24:10.543: E/AndroidRuntime(1050):     at java.lang.reflect.Method.invokeNative(Native Method)11-04 01:24:10.543: E/AndroidRuntime(1050):     at java.lang.reflect.Method.invoke(Method.java:525)11-04 01:24:10.543: E/AndroidRuntime(1050):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)11-04 01:24:10.543: E/AndroidRuntime(1050):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)11-04 01:24:10.543: E/AndroidRuntime(1050):     at dalvik.system.NativeStart.main(Native Method)11-04 01:24:10.543: E/AndroidRuntime(1050): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText11-04 01:24:10.543: E/AndroidRuntime(1050):     at com.troysantry.tipcalculator.TipCalc.onCreate(TipCalc.java:45)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.Activity.performCreate(Activity.java:5133)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)11-04 01:24:10.543: E/AndroidRuntime(1050):     ... 11 more11-04 01:29:11.177: I/Process(1050): Sending signal. PID: 1050 SIG: 911-04 01:31:32.080: D/AndroidRuntime(1109): Shutting down VM11-04 01:31:32.080: W/dalvikvm(1109): threadid=1: thread exiting with uncaught exception (group=0x41465700)11-04 01:31:32.194: E/AndroidRuntime(1109): FATAL EXCEPTION: main11-04 01:31:32.194: E/AndroidRuntime(1109): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.troysantry.tipcalculator/com.troysantry.tipcalculator.TipCalc}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.access$600(ActivityThread.java:141)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.os.Handler.dispatchMessage(Handler.java:99)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.os.Looper.loop(Looper.java:137)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.main(ActivityThread.java:5103)11-04 01:31:32.194: E/AndroidRuntime(1109):     at java.lang.reflect.Method.invokeNative(Native Method)11-04 01:31:32.194: E/AndroidRuntime(1109):     at java.lang.reflect.Method.invoke(Method.java:525)11-04 01:31:32.194: E/AndroidRuntime(1109):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)11-04 01:31:32.194: E/AndroidRuntime(1109):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)11-04 01:31:32.194: E/AndroidRuntime(1109):     at dalvik.system.NativeStart.main(Native Method)11-04 01:31:32.194: E/AndroidRuntime(1109): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText11-04 01:31:32.194: E/AndroidRuntime(1109):     at com.troysantry.tipcalculator.TipCalc.onCreate(TipCalc.java:44)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.Activity.performCreate(Activity.java:5133)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)11-04 01:31:32.194: E/AndroidRuntime(1109):     ... 11 more11-04 01:36:33.195: I/Process(1109): Sending signal. PID: 1109 SIG: 911-04 02:11:09.684: D/AndroidRuntime(1167): Shutting down VM11-04 02:11:09.684: W/dalvikvm(1167): threadid=1: thread exiting with uncaught exception (group=0x41465700)11-04 02:11:09.855: E/AndroidRuntime(1167): FATAL EXCEPTION: main11-04 02:11:09.855: E/AndroidRuntime(1167): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.troysantry.tipcalculator/com.troysantry.tipcalculator.TipCalc}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.access$600(ActivityThread.java:141)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.os.Handler.dispatchMessage(Handler.java:99)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.os.Looper.loop(Looper.java:137)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.main(ActivityThread.java:5103)11-04 02:11:09.855: E/AndroidRuntime(1167):     at java.lang.reflect.Method.invokeNative(Native Method)11-04 02:11:09.855: E/AndroidRuntime(1167):     at java.lang.reflect.Method.invoke(Method.java:525)11-04 02:11:09.855: E/AndroidRuntime(1167):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)11-04 02:11:09.855: E/AndroidRuntime(1167):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)11-04 02:11:09.855: E/AndroidRuntime(1167):     at dalvik.system.NativeStart.main(Native Method)11-04 02:11:09.855: E/AndroidRuntime(1167): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText11-04 02:11:09.855: E/AndroidRuntime(1167):     at com.troysantry.tipcalculator.TipCalc.onCreate(TipCalc.java:44)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.Activity.performCreate(Activity.java:5133)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)11-04 02:11:09.855: E/AndroidRuntime(1167):     ... 11 more

Some Common Mistakes.

1.findViewById() of non existing view

Like when you use findViewById(R.id.button) when button id does not exist in layout XML.

2. Wrong cast.

If you wrong cast some class, then you get this error. Like you cast RelativeLayout to LinearLayout or EditText to TextView.

3. Activity not registered in manifest.xml

If you did not register Activity in manifest.xml then this error comes.

4. findViewById() with declaration at top level

Below code is incorrect. This will create error. Because you should do findViewById() after calling setContentView(). Because an View can be there after it is created.

public class MainActivity extends Activity {  ImageView mainImage = (ImageView) findViewById(R.id.imageViewMain); //incorrect way  @Override  protected void onCreate(Bundle savedInstanceState){    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    mainImage = (ImageView) findViewById(R.id.imageViewMain); //correct way    //...  }}

5. Starting abstract Activity class.

When you try to start an Activity which is abstract, you will will get this error. So just remove abstract keyword before activity class name.

6. Using kotlin but kotlin not configured.

If your activity is written in Kotlin and you have not setup kotlin in your app. then you will get error. You can follow simple steps as written in Android Link or Kotlin Link. You can check this answer too.

More information

Read about Downcast and Upcast

Read about findViewById() method of Activity class

Super keyword in Java

How to register activity in manifest