APK is created but doesnt run on a device

Question

I successfully compiled an android app and created it's apk. I also signed the .apk file with my keystore. But it gives me a NullPointerException for some reason when running in USB debugging mode.

Here is the exception:

06-16 15:23:11.639  30531-30531/com.adi.play E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.adi.play/com.adi.play.PlayActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2277)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2403)
            at android.app.ActivityThread.access$600(ActivityThread.java:165)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1373)
            at android.os.Handler.dispatchMessage(Handler.java:107)
            at android.os.Looper.loop(Looper.java:194)
            at android.app.ActivityThread.main(ActivityThread.java:5391)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.content.ContextWrapper.getResources(ContextWrapper.java:89)
            at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:78)
            at com.adi.play.PlayActivity.<init>(PlayActivity.java:12)
            at java.lang.Class.newInstanceImpl(Native Method)
            at java.lang.Class.newInstance(Class.java:1319)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1123)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2268)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2403)
            at android.app.ActivityThread.access$600(ActivityThread.java:165)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1373)
            at android.os.Handler.dispatchMessage(Handler.java:107)
            at android.os.Looper.loop(Looper.java:194)
            at android.app.ActivityThread.main(ActivityThread.java:5391)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
            at dalvik.system.NativeStart.main(Native Method)

My AndroidManifest.xml file:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.adi.play"
      android:versionCode="1"
      android:versionName="1.0">
    <application android:label="@string/app_name" 
     android:icon="@drawable/me">
        <activity android:name="PlayActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

My com.adi.play.PlayActivity .java file:

package com.adi.play;

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

public class PlayActivity extends Activity {

 public final String RESULT = getResources().getString(R.string.result);
 
 private int total = 0;
 private TextView displayResult;
 private Button incrementButton;
 private Button decrementButton;

 @Override
 public void onCreate(Bundle bundle) {
 
  super.onCreate(bundle);
  setContentView(R.layout.activity_play);
  
  displayResult = (TextView)findViewById(R.id.display_result);
  incrementButton = (Button)findViewById(R.id.increment_button);
  decrementButton = (Button)findViewById(R.id.decrement_button);
  
  displayResult.setText(RESULT+total);
  incrementButton.setOnClickListener(new View.OnClickListener() {
  
   @Override
   public void onClick(View view) {
   
    total++;
    displayResult.setText(RESULT+total);
   }
  });
  
  decrementButton.setOnClickListener(new View.OnClickListener() {
  
   @Override
   public void onClick(View view) {
   
    total--;
    displayResult.setText(RESULT+total);
   }
  });
 }
}

Please help. This thing is annoying me.

Answer 1

This is your problem, I dont know how your IDE does not complain about the same:

public final String RESULT = getResources().getString(R.string.result);

Try to assign value to RESULT in onCreate();

Here you can find the official docs for resource usage in Android and how to access resources.

Answer 2

This line caused it

public final String RESULT = getResources().getString(R.string.result);

the getResources() method needs context available during call, there are no available context when you run it.

Solution:

Initialize RESULT on your onCreate instead :

public class PlayActivity extends Activity {

private String result; //this line

private int total = 0;
private TextView displayResult;
private Button incrementButton;
private Button decrementButton;

@Override
public void onCreate(Bundle bundle) {

    super.onCreate(bundle);
    setContentView(R.layout.activity_play);

    result = getResources().getString(R.string.result); //this line

Answer 3

getResources() requires Context which will be created after onCreate() gets executed.

Try this code.

package com.adi.play;

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

public class PlayActivity extends Activity {

    public String RESULT;

    private int total = 0;
    private TextView displayResult;
    private Button incrementButton;
    private Button decrementButton;

    @Override
    public void onCreate(Bundle bundle) {

        super.onCreate(bundle);
        setContentView(R.layout.activity_play);

        RESULT = getResources().getString(R.string.result);

        displayResult = (TextView)findViewById(R.id.display_result);
        incrementButton = (Button)findViewById(R.id.increment_button);
        decrementButton = (Button)findViewById(R.id.decrement_button);

        displayResult.setText(RESULT+total);
        incrementButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {

                total++;
                displayResult.setText(RESULT+total);
            }
        });

        decrementButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {

                total--;
                displayResult.setText(RESULT+total);
            }
        });
    }
}