Question

I have a normal LinearLayout for my main.xml, and my second activity is a ListView. I have a button on main.xml that should take me to the ListView. I get an Unexpected Error message and have to force close when I press the button.

Main.xml:

<?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:weightSum="1"
    android:background="@drawable/forzabg">

        <ImageView
            android:id="@+id/mainlogo"
            android:layout_width="210dp"
            android:layout_height="119dp"
            android:layout_gravity="center"
            android:layout_weight="0.00"
            android:scaleType="fitXY"
            android:src="@drawable/forzalogo" >

 </ImageView>

        <ImageView
            android:id="@+id/menuButton1"
            android:layout_width="185dp"
            android:layout_height="56dp"
            android:layout_gravity="center"
            android:layout_weight="0.14"
            android:src="@drawable/carslist" />

 />

</LinearLayout>

Java for main.xml:

    package com.forza;

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

public class Forza2Activity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        ImageView carbutton = (ImageView) findViewById(R.id.menuButton1);

        carbutton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                Intent myIntent = new Intent(view.getContext(), CarList.class);
                startActivity(myIntent);
            }
        });

    }
}

carlist.xml (ListView) activity:

<?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">

    <ListView
        android:id="@+id/mylist"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:entries="@array/Brands" >
    </ListView>

</LinearLayout>

Java for ListView activity:

package com.forza;

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


    public class CarList extends Activity {



        ListView carList = (ListView) findViewById(R.id.mylist);
            protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.carlist);

            ImageView carbutton = (ImageView) findViewById(R.id.menuButton1);

            carbutton.setOnClickListener(new View.OnClickListener() {
                public void onClick(View view) {
                    Intent intent = new Intent();
                    setResult(RESULT_OK, intent);
                    finish();
                };
          });       
       }
    }

LogCat:

02-26 16:35:50.158: W/dalvikvm(341): threadid=3: thread exiting with uncaught exception (group=0x4001b188)
02-26 16:35:50.158: E/AndroidRuntime(341): Uncaught handler: thread main exiting due to uncaught exception
02-26 16:35:50.168: E/AndroidRuntime(341): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.forza/com.forza.CarList}: java.lang.NullPointerException
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2417)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.ActivityThread.access$2200(ActivityThread.java:119)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.os.Looper.loop(Looper.java:123)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.ActivityThread.main(ActivityThread.java:4363)
02-26 16:35:50.168: E/AndroidRuntime(341):  at java.lang.reflect.Method.invokeNative(Native Method)
02-26 16:35:50.168: E/AndroidRuntime(341):  at java.lang.reflect.Method.invoke(Method.java:521)
02-26 16:35:50.168: E/AndroidRuntime(341):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
02-26 16:35:50.168: E/AndroidRuntime(341):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
02-26 16:35:50.168: E/AndroidRuntime(341):  at dalvik.system.NativeStart.main(Native Method)
02-26 16:35:50.168: E/AndroidRuntime(341): Caused by: java.lang.NullPointerException
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.Activity.findViewById(Activity.java:1612)
02-26 16:35:50.168: E/AndroidRuntime(341):  at com.forza.CarList.<init>(CarList.java:15)
02-26 16:35:50.168: E/AndroidRuntime(341):  at java.lang.Class.newInstanceImpl(Native Method)
02-26 16:35:50.168: E/AndroidRuntime(341):  at java.lang.Class.newInstance(Class.java:1479)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2409)
02-26 16:35:50.168: E/AndroidRuntime(341):  ... 11 more
02-26 16:35:50.178: I/dalvikvm(341): threadid=7: reacting to signal 3
02-26 16:35:50.178: E/dalvikvm(341): Unable to open stack trace file '/data/anr/traces.txt': Permission denied
02-26 16:35:53.198: I/Process(341): Sending signal. PID: 341 SIG: 9
02-26 16:35:53.608: D/dalvikvm(348): GC freed 611 objects / 48680 bytes in 58ms
02-26 16:35:54.258: D/dalvikvm(348): GC freed 59 objects / 2336 bytes in 50ms

Is it even possible to go from a regular view to a ListView?

Answer 1

The key line in the log is this :

02-26 16:35:50.168: E/AndroidRuntime(341): Caused by: java.lang.NullPointerException
02-26 16:35:50.168: E/AndroidRuntime(341):  at android.app.Activity.findViewById(Activity.java:1612)

You are getting a null pointer exception trying to find some view.

I suspect this is your line 1612:

  ListView carList = (ListView) findViewById(R.id.mylist);

This should be inside your onCreate() because at that point (where it is now) it wonT have been initilized yet.

---

Is it even possible to go from a regular view to a ListView?

It sure is. But in case you donT know about it already, I also suggest you should check the ListActivity which would allow easy handling of the entire list data and events on the items of your CarList.

---

EDIT Missing button in the carlist.xml

As you set your ContentView as the carlist.xml with the line

    setContentView(R.layout.carlist);

You need to have a matching ImageView called menuButton1 in the carlist.xml, otherwise you ll get an error in the following call:

 ImageView carbutton = (ImageView) findViewById(R.id.menuButton1);

To avoid the error, just add a new ImageView button in the carlist.xml (this time call it something else, not menuButton1)