Null pointer exception on button click

Question

I have 4 images as buttons and when the correct button is selected, an arrow button appears which is working fine.

My problem is that I'm trying to change the background resource of each button to change when this arrow is clicked but I'm getting a null pointer exception at this line :

happybutton.setBackgroundResource(R.drawable.mum1);

I have declared the nextArrow button in my java onCreate -

nextArrow = (Button) findViewById(R.id.nextArrow);
nextArrow.setOnClickListener(onClickListener);

class:

protected void onCreate (Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.expressions);

        Button happybutton = (Button) findViewById(R.id.happybutton);
        happybutton.setOnClickListener(onClickListener);
        Button sadbutton = (Button)findViewById(R.id.sadbutton);
        sadbutton.setOnClickListener(onClickListener);
        Button worriedbutton = (Button) findViewById(R.id.worriedbutton);
        worriedbutton.setOnClickListener(onClickListener);
        Button excitedbutton = (Button) findViewById(R.id.excitedbutton);
        excitedbutton.setOnClickListener(onClickListener);
        //Button nextArrow = (Button) findViewById(R.id.nextArrow);
        //nextArrow.setOnClickListener(onClickListener);

        tick = (ImageView) findViewById(R.id.tick);
        cross = (ImageView)findViewById(R.id.cross);
        cross2 = (ImageView) findViewById(R.id.cross2);
        cross3 = (ImageView) findViewById(R.id.cross3);

        nextArrow = (Button) findViewById(R.id.nextArrow);
        nextArrow.setOnClickListener(onClickListener);


        tryagain = MediaPlayer.create(Expressions.this,R.raw.tryagain);
        correct = MediaPlayer.create(Expressions.this,R.raw.correct);

    }

private OnClickListener onClickListener = new OnClickListener(){

    public void onClick(final View view){
        switch(view.getId()){

        case R.id.happybutton:

            tick.setVisibility(View.VISIBLE);
            nextArrow.setVisibility(View.VISIBLE);
            correct.start();
            break;

        case R.id.sadbutton:

            cross.setVisibility(View.VISIBLE);
            tryagain.start();
            break;

        case R.id.worriedbutton:

            cross3.setVisibility(View.VISIBLE);
            tryagain.start();
            break;

        case R.id.excitedbutton:

            cross2.setVisibility(View.VISIBLE);
            tryagain.start();
            break;


        case R.id.nextArrow:

            happybutton.setBackgroundResource(R.drawable.mum1);
            worriedbutton.setBackgroundResource(R.drawable.mum2);
            sadbutton.setBackgroundResource(R.drawable.mum3);
            excitedbutton.setBackgroundResource(R.drawable.mum4);
            break;
        }   
    }   

Logcat:

 07-31 16:17:11.312: E/AndroidRuntime(7818): FATAL EXCEPTION: main
    07-31 16:17:11.312: E/AndroidRuntime(7818): java.lang.NullPointerException
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at 
com.example.Autism_App.Expressions$1.onClick(Expressions.java:84)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at android.view.View.performClick(View.java:4383)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at android.view.View$PerformClick.run(View.java:18097)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at android.os.Handler.handleCallback(Handler.java:725)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at android.os.Handler.dispatchMessage(Handler.java:92)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at android.os.Looper.loop(Looper.java:176)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at android.app.ActivityThread.main(ActivityThread.java:5279)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at java.lang.reflect.Method.invokeNative(Native Method)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at java.lang.reflect.Method.invoke(Method.java:511)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
    07-31 16:17:11.312: E/AndroidRuntime(7818):     at dalvik.system.NativeStart.main(Native Method)

xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <TextView
        android:id="@+id/question"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="29dp"
        android:text="Which face is the happy face?"
        android:textSize="80sp" />

    <Button
        android:id="@+id/happybutton"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_alignLeft="@+id/excitedbutton"
        android:layout_alignParentBottom="true"
        android:layout_marginBottom="43dp"
        android:background="@drawable/happyface"/>

    <Button
        android:id="@+id/nextArrow"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:layout_marginRight="65dp"
        android:visibility="invisible"
        android:background="@drawable/nextarrow" />

    <Button
        android:id="@+id/worriedbutton"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_alignBaseline="@+id/happybutton"
        android:layout_alignBottom="@+id/happybutton"
        android:layout_alignLeft="@+id/sadbutton"
        android:background="@drawable/worriedface" />

    <Button
        android:id="@+id/sadbutton"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_below="@+id/question"
        android:layout_marginRight="113dp"
        android:layout_marginTop="29dp"
        android:layout_toLeftOf="@+id/nextArrow"
        android:background="@drawable/sadface" />

    <Button
        android:id="@+id/excitedbutton"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_alignBaseline="@+id/sadbutton"
        android:layout_alignBottom="@+id/sadbutton"
        android:layout_alignLeft="@+id/question"
        android:layout_marginLeft="116dp"
        android:background="@drawable/excitedface" />

    <ImageView
        android:id="@+id/cross2"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_alignLeft="@+id/excitedbutton"
        android:layout_alignTop="@+id/excitedbutton"
        android:src="@drawable/cross"
        android:visibility="invisible" />

    <ImageView
        android:id="@+id/cross"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_alignLeft="@+id/sadbutton"
        android:layout_alignTop="@+id/sadbutton"
        android:src="@drawable/cross"
        android:visibility="invisible" />

    <ImageView
        android:id="@+id/tick"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_alignLeft="@+id/excitedbutton"
        android:layout_alignTop="@+id/worriedbutton"
        android:src="@drawable/tick"
        android:visibility="invisible" />

    <ImageView
        android:id="@+id/cross3"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_alignLeft="@+id/worriedbutton"
        android:layout_alignTop="@+id/worriedbutton"
        android:src="@drawable/cross"
        android:visibility="invisible" />

</RelativeLayout>

Am I missing something really obvious?

Answer 1

You are initializing a local variable not the instance field

protected void onCreate (Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.expressions);

    Button happybutton = (Button) findViewById(R.id.happybutton);
    happybutton.setOnClickListener(onClickListener);

When you say Button happybutton you are creating a new variable whose scope is confined to the onCreate() method, and will supersede the instance field of that name for the duration of this method. You initialize this variable, so calling its setOnClickListener method works fine. But the variable disappears at the end of the onCreate() method, and other methods see only the never-initialized instance field of the same name.

Instead, simply initialize the instance field which you your presumably already declared outside of onCreate()

Button happybutton;

protected void onCreate (Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.expressions);

    happybutton = (Button) findViewById(R.id.happybutton);
    happybutton.setOnClickListener(onClickListener);

This can be a fairly subtle mistake, since superceding an instance field with a local variable is entirely valid code which will not trigger any compiler complaints.

Answer 2

The buttons should be class variables like so:

public class MyClass {
    Button happybutton;
    Button sadbutton;
    Button worriedbutton;
    Button excitedbutton;

    protected void onCreate (Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.expressions);

        happybutton = (Button) findViewById(R.id.happybutton);
        happybutton.setOnClickListener(onClickListener);
        sadbutton = (Button)findViewById(R.id.sadbutton);
        sadbutton.setOnClickListener(onClickListener);
        worriedbutton = (Button) findViewById(R.id.worriedbutton);
        worriedbutton.setOnClickListener(onClickListener);
        excitedbutton = (Button) findViewById(R.id.excitedbutton);
        excitedbutton.setOnClickListener(onClickListener);
...

Answer 3

I believe, that class memeber happybutton is not initialized.