Weird behaviour Android

Question

Do you know why does it crash?

I will show you my testing java file. Android.mk, c++ file and Application.mk are not important.

This is java file what works only when there isnt BUTTON part. When I run it as you can see with BUTTON part it crashes. Do you know why? What affect it?

package com.example.hellojnicpp;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
public class Hellojnicpp extends Activity {
 public native String stringFromJNI();
 static {
 System.loadLibrary("algo-rithm");
 }
 Button button1;
 TextView tv1;
 @Override
 public void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 TextView tv = new TextView(this);
 tv.setText(stringFromJNI());
 setContentView(tv); 
//////////////////////////////BUTTON////////////////////////////////////// 
 button1 = (Button)findViewById(R.id.button1);
 button1.setOnClickListener(new OnClickListener(){
 public void onClick(View view){ 
 tv1 = (TextView)findViewById(R.id.textView1);
 tv1.setText("Hello");
 }
 });
///////////////////////////////BUTTON/////////////////////////////////// 
 }
}

This is LogCat output:

05-01 18:34:34.030: D/dalvikvm(18786): Trying to load lib /data/data/com.example.hellojnicpp/lib/libalgo-rithm.so 0x409e3998
05-01 18:34:34.030: D/dalvikvm(18786): Added shared lib /data/data/com.example.hellojnicpp/lib/libalgo-rithm.so 0x409e3998
05-01 18:34:34.030: D/dalvikvm(18786): No JNI_OnLoad found in /data/data/com.example.hellojnicpp/lib/libalgo-rithm.so 0x409e3998, skipping init
05-01 18:34:34.040: D/AndroidRuntime(18786): Shutting down VM
05-01 18:34:34.040: W/dalvikvm(18786): threadid=1: thread exiting with uncaught exception (group=0x40185760)
05-01 18:34:34.050: E/AndroidRuntime(18786): FATAL EXCEPTION: main
05-01 18:34:34.050: E/AndroidRuntime(18786): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.hellojnicpp/com.example.hellojnicpp.Hellojnicpp}: java.lang.NullPointerException
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1815)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.ActivityThread.access500ドル(ActivityThread.java:122)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.os.Handler.dispatchMessage(Handler.java:99)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.os.Looper.loop(Looper.java:132)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.ActivityThread.main(ActivityThread.java:4123)
05-01 18:34:34.050: E/AndroidRuntime(18786): at java.lang.reflect.Method.invokeNative(Native Method)
05-01 18:34:34.050: E/AndroidRuntime(18786): at java.lang.reflect.Method.invoke(Method.java:491)
05-01 18:34:34.050: E/AndroidRuntime(18786): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:864)
05-01 18:34:34.050: E/AndroidRuntime(18786): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:622)
05-01 18:34:34.050: E/AndroidRuntime(18786): at dalvik.system.NativeStart.main(Native Method)
05-01 18:34:34.050: E/AndroidRuntime(18786): Caused by: java.lang.NullPointerException
05-01 18:34:34.050: E/AndroidRuntime(18786): at com.example.hellojnicpp.Hellojnicpp.onCreate(Hellojnicpp.java:29)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.Activity.performCreate(Activity.java:4397)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1048)
05-01 18:34:34.050: E/AndroidRuntime(18786): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1779)
05-01 18:34:34.050: E/AndroidRuntime(18786): ... 11 more
05-01 18:40:55.340: D/dalvikvm(19183): Trying to load lib /data/data/com.example.hellojnicpp/lib/libalgo-rithm.so 0x409e4fa0
05-01 18:40:55.340: D/dalvikvm(19183): Added shared lib /data/data/com.example.hellojnicpp/lib/libalgo-rithm.so 0x409e4fa0
05-01 18:40:55.340: D/dalvikvm(19183): No JNI_OnLoad found in /data/data/com.example.hellojnicpp/lib/libalgo-rithm.so 0x409e4fa0, skipping init

I am very curious why it is happening...

Thank you in advance.

• Alex Cohn: The log complains about Hellojnicpp.java line 29. Which line is that?
• User: this one button1.setOnClickListener(new OnClickListener(){

Answer 1

This is java file what works only when there isnt BUTTON part.

 TextView tv = new TextView(this);
 tv.setText(stringFromJNI());
 setContentView(tv); 
 button1 = (Button)findViewById(R.id.button1);

Your set your ContentView to be just one solitary TextView, rather than an XML layout or a ViewGroup of any sort.

Then you try to find something called R.id.button1 inside the ContentView, which is just your TextView which cannot contain a Button. That means your button1 variable is null, so of course this:

button1.setOnClickListener(new OnClickListener()...

Triggers an exception

Answer 2

Unfortunately, button1 is null. The method findViewById(R.id.button1) looks for this button in current content view, which is a dynamically generated TextView, not some layout loaded from XML resources. It is quite natural that this view does not have a child with android.id="@+id/button1"

• User: Oh, you are completely right... beginner's mistake. Thank you very much gentlemens!