java.lang.NullPointerException in xml

Question

this code correctly work in MainActivity, but i am want to put some animation methods in different class, and call it from there, but i have got an error i am trying to understand, why it shows me an error any ideas?

public class AnimationClass extends MainActivity{
    private static ImageButton heart_icon,bee_icon;
    private static int kill_flag = 0;
    private static Animation anim_heart, anim_bee;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        heart_icon=(ImageButton)findViewById(R.id.heart_icon);
        bee_icon = (ImageButton)findViewById(R.id.bee_icon);
    }

    public static void loadAnimations(Activity activity){
        anim_heart = AnimationUtils.loadAnimation(activity, R.anim.heart_anim);
        heart_icon.startAnimation(anim_heart);

        anim_bee = AnimationUtils.loadAnimation(activity, R.anim.bee_anim);
        bee_icon.startAnimation(anim_bee);
    }
}

Animation:

<set xmlns:android="http://schemas.android.com/apk/res/android"
    android:interpolator="@android:anim/accelerate_decelerate_interpolator"
    android:shareInterpolator="true">
    <scale
        android:fillAfter="true"
        android:repeatCount="infinite"
        android:repeatMode="reverse"
        android:duration="1000"
        android:fromXScale="1.0"
        android:toXScale="1.2"
        android:fromYScale="1.0"
        android:toYScale="1.2"
        android:pivotX="50%"
        android:pivotY="50%"/>
    <rotate
        android:fillAfter="true"
        android:repeatCount="infinite"
        android:repeatMode="reverse"
        android:duration="1200"
        android:fromDegrees="-20"
        android:toDegrees="15"
        android:pivotX="80%"
        android:pivotY="20%"/>
    <translate
        android:fillAfter="true"
        android:repeatCount="infinite"
        android:repeatMode="reverse"
        android:duration="800"
        android:toXDelta="5"
        android:toYDelta="-5"/>
    <alpha
        android:fromAlpha="0"
        android:toAlpha="1"
        android:duration="1500"/>

</set>

the error in:anim_heart = AnimationUtils.loadAnimation(activity, R.anim.heart_anim);

Answer 1

You seem to misunderstand how static variables and methods work.

When you create an object of AnimationClass:

new AnimationClass()

onCreate is called and the two ImageButtons are set properly.

However when you call the static method loadAnimations, the ImageButton variables might be still null, which in return will throw the NPE.

Different solutions:

1. Check for null variables:

   public static void loadAnimations(Activity activity){
       anim_heart = AnimationUtils.loadAnimation(activity, R.anim.heart_anim);
       if (heart_icon != null) {
           heart_icon.startAnimation(anim_heart);
       }
   
       anim_bee = AnimationUtils.loadAnimation(activity, R.anim.bee_anim);
       if (bee_icon != null) {
           bee_icon.startAnimation(anim_bee);
       }
   }
   

2. If the Activity you pass as an argument to loadAnimations contains the views in question, you may want to find them on the spot:

   public static void loadAnimations(Activity activity){
       heart_icon = (ImageButton) activity.findViewById(R.id.heart_icon);
       bee_icon = (ImageButton) activity.findViewById(R.id.bee_icon);
   
       anim_heart = AnimationUtils.loadAnimation(activity, R.anim.heart_anim);
       heart_icon.startAnimation(anim_heart);
   
       anim_bee = AnimationUtils.loadAnimation(activity, R.anim.bee_anim);
       bee_icon.startAnimation(anim_bee);
   }