Question

I’m writing an app to display the values of the accelerometer in Android in the X, Y, and Z directions. I am an Android noob so please bear with me. Here is my code, it is quite simple but I receive a null pointer exception and I’m not sure what’s wrong with my code. Thanks!

package com.*****.accelo;

import android.app.Activity;

import android.content.Context;

import android.hardware.Sensor;

import android.hardware.SensorEvent;

import android.hardware.SensorEventListener;

import android.hardware.SensorManager;

import android.os.Bundle;

import android.widget.TextView;



public class AcceloActivity extends Activity implements SensorEventListener{



TextView textViewX, textViewY, textViewZ, textViewError;

StringBuilder builderx = new StringBuilder();
StringBuilder buildery = new StringBuilder();
StringBuilder builderz = new StringBuilder();




public void onCreate(Bundle savedInstanceState){

    super.onCreate(savedInstanceState);

    textViewError = (TextView) findViewById(R.id.textViewError);
    textViewX = (TextView) findViewById(R.id.textViewX);
    textViewY = (TextView) findViewById(R.id.textViewY);
    textViewX = (TextView) findViewById(R.id.textViewZ);




    SensorManager manager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);

    Sensor accelerometer = manager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);

        if(!manager.registerListener(this, accelerometer,  
SensorManager.SENSOR_DELAY_UI)){
        setContentView(textViewError);

        }
}

public void onAccuracyChanged(Sensor sensor, int accuracy) {



}



public void onSensorChanged(SensorEvent event) {

    builderx.setLength(0);
    buildery.setLength(0);
    builderz.setLength(0);

    builderx.append("X " + event.values[0]);
    buildery.append("Y"+  event.values[1] );
    builderz.append("Z " + event.values[2]);

    textViewX.setText(builderx.toString());
    textViewY.setText(buildery.toString());
    textViewZ.setText(builderz.toString());

}



}

From logcat:

10-24 19:52:43.574: ERROR/AndroidRuntime(711): java.lang.RuntimeException: Unable to   
start activity ComponentInfo{com.amrit.accelo/com.amrit.accelo.AcceloActivity}: 
java.lang.NullPointerException
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at 
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at  
android.app.ActivityThread.access$1500(ActivityThread.java:117)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at     
android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at     
android.os.Handler.dispatchMessage(Handler.java:99)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at 
android.os.Looper.loop(Looper.java:123)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at   
android.app.ActivityThread.main(ActivityThread.java:3683)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at      
java.lang.reflect.Method.invokeNative(Native Method)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at    
java.lang.reflect.Method.invoke(Method.java:507)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at   
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at   
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at   
dalvik.system.NativeStart.main(Native Method)
10-24 19:52:43.574: ERROR/AndroidRuntime(711): Caused by: java.lang.NullPointerException      
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at   
com.amrit.accelo.AcceloActivity.onCreate(AcceloActivity.java:50)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at     
android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
10-24 19:52:43.574: ERROR/AndroidRuntime(711):     at   
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)

Thanks!

Answer 1

You are getting this error from line 50:

textViewError.setText("Error, could not register sensor listener");

so your textViewError variable is null here. You will need to create an XML layout, use setContentView, and then add a TextView to your XML layout and then assign your TextView to it with findViewById. This is probably a good place to start.

EDIT:

Just to be clear, you need to set content view before you call findViewById.

public void onCreate(Bundle savedInstanceState){

    super.onCreate(savedInstanceState);

    setContentView(R.layout.yourlayout);

    textViewError = (TextView) findViewById(R.id.textViewError);
    textViewX = (TextView) findViewById(R.id.textViewX);
    textViewY = (TextView) findViewById(R.id.textViewY);
    textViewX = (TextView) findViewById(R.id.textViewZ);
    ...

Important here is that you must have an xml layout file called yourlayout.xml under res/layout/ which contains the views that you are loading.