Android app crashes on startup when I reference ArrayList.add method

Question

Here is the complete code for my MainActivity. As you can see I added just 2 lines:

ArrayList<String> y = null; 
y.add(new String("buggy")); // When I remove this line it works

When I comment out y.add() the Application does not crash.

My question is: Why does this use of y.add() cause the program to fail?

The mobile device is a Samsung Galaxy Mini S5570, running Android 2.3.4

Here is the complete code, followed by the LogCat:

package com.testbug;
import javax.*;
import java.util.*;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu; 
import android.view.MenuItem;
import android.support.v4.app.NavUtils;

public class MainActivity extends Activity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //
    // Only these 2 lines added by me. The rest is automatically generated in Eclipse.
    ArrayList<String> y = null; 
    y.add(new String("buggy")); // When I remove this line it works
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}
}

Stacktrace:

07-05 21:01:37.919: D/AndroidRuntime(32067): Shutting down VM
07-05 21:01:37.919: W/dalvikvm(32067): threadid=1: thread exiting with uncaught exception (group=0x40018578)
07-05 21:01:37.929: E/AndroidRuntime(32067): FATAL EXCEPTION: main
07-05 21:01:37.929: E/AndroidRuntime(32067): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.testbug/com.testbug.MainActivity}: java.lang.NullPointerException
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.os.Looper.loop(Looper.java:130)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.app.ActivityThread.main(ActivityThread.java:3687)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at java.lang.reflect.Method.invokeNative(Native Method)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at java.lang.reflect.Method.invoke(Method.java:507)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at dalvik.system.NativeStart.main(Native Method)
07-05 21:01:37.929: E/AndroidRuntime(32067): Caused by: java.lang.NullPointerException
07-05 21:01:37.929: E/AndroidRuntime(32067):    at com.testbug.MainActivity.onCreate(MainActivity.java:20)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
07-05 21:01:37.929: E/AndroidRuntime(32067):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
07-05 21:01:37.929: E/AndroidRuntime(32067):    ... 11 more

I should add that I explicitly added the rt.jar file in the Java Build Path, and I also tried adding it as a System library. Neither of these changes made any impact on the App. — John
– John, Commented Jul 5, 2012 at 21:35
I would suggest you take a short break from your android project and brush up on your Java syntax, there are many free learning materials available online. You will have a much easier time with Android programming if you have your java fundamentals down a bit better. — FoamyGuy
– FoamyGuy, Commented Jul 5, 2012 at 21:36

FoamyGuy · Accepted Answer · 2012-07-05 21:33:55Z

6

you are setting it to null and then calling a method on it... This will result in null pointer

Initialize it like this before calling add():

ArrayList<String> y = new ArrayList<String>();

answered Jul 5, 2012 at 21:33

FoamyGuy

46.9k19 gold badges132 silver badges160 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

John Over a year ago

Thank you so much. You got it. I think Android is less forgiving than "normal" Java. I'm sure I've done this and gotten away with it.

yugidroid · Accepted Answer · 2012-07-05 21:45:19Z

0

You have to initialize a declared array. If you try to add items to a null array it will cause a java.lang.NullPointerException (exactly what you have).

You have to do something like:

//Declaring and Initializing
ArrayList<String> y = new ArrayList<String>();
    y.add(new String("buggy"));

answered Jul 5, 2012 at 21:45

yugidroid

6,6902 gold badges32 silver badges45 bronze badges

Collectives™ on Stack Overflow

Android app crashes on startup when I reference ArrayList.add method

Stacktrace:

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

Stacktrace:

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related