使用setBackgroundResource()时发生NullPointerException [英] NullPointerException on using setBackgroundResource()
本文介绍了使用setBackgroundResource()时发生NullPointerException的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我正在开发一个Android应用程序以在网格中显示图像.我正在按照 youtube教程中列出的步骤进行操作.在运行应用程序时，getview方法遇到NullpointerException. 我无法弄清楚此异常的原因.您能帮我找出问题所在吗?例外是
I am developing an android application to show images in a grid. I am following the steps listed in the youtube tutorial. On running the app, the getview method encounters a NullpointerException. I am unable to figure out the reason of this exception. Can you help me figure out what is wrong? The exception is
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setBackgroundResource(int)' on a null object reference
activity_main.xml
activity_main.xml
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" tools:context=".MainActivity" android:id="@+id/container">
<GridView
android:id="@+id/grid_view"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:columnWidth="80dp"
android:horizontalSpacing="10dp"
android:numColumns="auto_fit"
android:stretchMode="columnWidth"
android:verticalSpacing="10dp">
</GridView>
<ImageView
android:id="@+id/expanded_image"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:contentDescription="@string/app_name"
android:visibility="invisible"
/>
</FrameLayout>
single_grid_item.xml
single_grid_item.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent"
android:padding="5dp">
<ImageView
android:id="@+id/thumb"
android:layout_width="72dp"
android:layout_height="72dp"
android:layout_gravity="center_horizontal"
android:contentDescription="@string/app_name"/>
</LinearLayout>
MainActivity.Java
MainActivity.Java
public class MainActivity extends Activity {
private GridView gv;
private Animator mCurrentAnimator;
private int mShortAnimationDuration;
private int j=0;
private GestureDetector detector;
private static final int SWIPE_MIN_DISTANCE=120;
private static final int SWIPE_THRESHOLD_VELOCITY=200;
private int thumb[] = {
R.drawable.chrysanthemum,R.drawable.desert, R.drawable.hydrangeas, R.drawable.jellyfish, R.drawable.koala, R.drawable.lighthouse, R.drawable.penguins, R.drawable.tulips
};
private ImageView expandedImageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
detector = new GestureDetector(this, new SwipeGestureDetector());
gv = (GridView) findViewById(R.id.grid_view);
gv.setAdapter(new ImageAdapter(this));
gv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View v, int pos, long id) {
j = pos;
zoomImageFromThumb(v, thumb[pos]);
}
});
mShortAnimationDuration = getResources().getInteger(android.R.integer.config_shortAnimTime);
}
class ImageAdapter extends BaseAdapter {
private LayoutInflater layoutInflater;
public ImageAdapter(MainActivity activity){
layoutInflater = (LayoutInflater) activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
@Override
public int getCount() {
return thumb.length;
}
@Override
public Object getItem(int pos) {
return pos;
}
@Override
public long getItemId(int pos) {
return pos;
}
@Override
public View getView(int pos, View convertView, ViewGroup parent) {
View listItem = convertView;
int p = pos;
if(listItem == null) {
Log.d("Debug","1");
listItem = layoutInflater.inflate(R.layout.single_grid_item, null);
}
Log.d("Debug","2");
ImageView iv = (ImageView) findViewById(R.id.thumb);
Log.d("Debug","3");
iv.setBackgroundResource(thumb[p]);
Log.d("Debug", "4");
return listItem;
}
}
private void zoomImageFromThumb(final View thumbView, int imageResId) {
if(mCurrentAnimator != null){
mCurrentAnimator.cancel();
}
expandedImageView = (ImageView) findViewById(R.id.expanded_image);
expandedImageView.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
if (detector.onTouchEvent(event)) {
return true;
} else {
return false;
}
}
});
expandedImageView.setImageResource(imageResId);
final Rect startBounds = new Rect();
final Rect finalBounds = new Rect();
final Point globalOffSet = new Point();
thumbView.getGlobalVisibleRect(startBounds);
findViewById(R.id.container).getGlobalVisibleRect(finalBounds, globalOffSet);
startBounds.offset(-globalOffSet.x, globalOffSet.y);
finalBounds.offset(-globalOffSet.x, globalOffSet.y);
float startScale;
if((float) finalBounds.width()/finalBounds.height() > startBounds.width()/startBounds.height()) {
startScale = (float) startBounds.height() / finalBounds.height();
float startWidth = startScale * finalBounds.width();
float deltaWidth = (startWidth - startBounds.width()) / 2;
startBounds.left -= deltaWidth;
startBounds.right += deltaWidth;
}else {
startScale = (float) startBounds.width() / finalBounds.width();
float startHeight = startScale * finalBounds.height();
float deltaHeight = (startHeight - startBounds.height())/2;
startBounds.top -= deltaHeight;
startBounds.bottom += deltaHeight;
}
thumbView.setAlpha(0f);
expandedImageView.setVisibility(View.VISIBLE);
expandedImageView.setPivotX(0f);
expandedImageView.setPivotY(0f);
AnimatorSet set = new AnimatorSet();
set.play(ObjectAnimator.ofFloat(expandedImageView, View.X, startBounds.left, finalBounds.left)).with(ObjectAnimator.ofFloat(expandedImageView, View.Y, startBounds.top, finalBounds.top)).with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_X, startScale, 1f)).with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_Y, startScale, 1f));
set.setDuration(mShortAnimationDuration);
set.setInterpolator(new DecelerateInterpolator());
set.addListener(new AnimatorListenerAdapter() {
@Override
public void onAnimationEnd(Animator animation) {
mCurrentAnimator = null;
}
@Override
public void onAnimationCancel(Animator animation) {
mCurrentAnimator = null;
}
});
set.start();
mCurrentAnimator = set;
final float startScaleFinal = startScale;
expandedImageView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if(mCurrentAnimator != null){
mCurrentAnimator.cancel();
}
AnimatorSet set = new AnimatorSet();
set.play(ObjectAnimator.ofFloat(expandedImageView, View.X, startBounds.left))
.with(ObjectAnimator.ofFloat(expandedImageView, View.Y, startBounds.top))
.with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_X, startScaleFinal))
.with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_Y, startScaleFinal));
set.setDuration(mShortAnimationDuration);
set.setInterpolator(new DecelerateInterpolator());
set.addListener(new AnimatorListenerAdapter() {
@Override
public void onAnimationEnd(Animator animation) {
thumbView.setAlpha(1f);
expandedImageView.setVisibility(View.GONE);
mCurrentAnimator = null;
}
@Override
public void onAnimationCancel(Animator animation) {
thumbView.setAlpha(1f);
expandedImageView.setVisibility(View.GONE);
mCurrentAnimator = null;
}
});
set.start();
mCurrentAnimator = set;
}
});
}
private class SwipeGestureDetector extends GestureDetector.SimpleOnGestureListener {
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
try{
if(e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY){
if(thumb.length > j) {
j++;
if(j < thumb.length) {
expandedImageView.setImageResource(thumb[j]);
return true;
} else {
j = 0;
expandedImageView.setImageResource(thumb[j]);
return true;
}
}
} else if(e2.getX() - e1.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY){
if( j >0 ) {
j--;
expandedImageView.setImageResource(thumb[j]);
return true;
} else {
j= thumb.length-1;
expandedImageView.setImageResource(thumb[j]);
return true;
}
}
}catch (Exception e){
e.printStackTrace();
}
return false;
}
}
}
推荐答案
替换
ImageView iv = (ImageView)findViewById(R.id.thumb);
Replace
ImageView iv = (ImageView)findViewById(R.id.thumb);
使用
ImageView iv = (ImageView)listItem.findViewById(R.id.thumb);
因为您的listItem视图包含布局single_grid_item的引用
because your listItem view contains reference of your layout single_grid_item
这篇关于使用setBackgroundResource()时发生NullPointerException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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