Android RuntimeException和NullPointerException

Question

从图库中抓取图像并将该图像设置为RelativeLayout的背景时，我遇到了这些错误。因此，此代码将在OnClick（）中打开Gallery，从库中抓取图像，并将其设置为RelativeLayout的背景。

logcat的： Gist

SettingsActivity：

MainActivity mainActivity;
RelativeLayout backgroundView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_settings);

    mainActivity = new MainActivity();
    backgroundView = (RelativeLayout) findViewById(R.id.wallpaperView);

    // Show the Up button in the action bar.
    setupActionBar();
}

/**
 * Set up the {@link android.app.ActionBar}, if the API is available.
 */
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
private void setupActionBar() {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        getActionBar().setDisplayHomeAsUpEnabled(true);
    }
}

public void setBackgroundClick(View v)
{
    Intent intent = new Intent(Intent.ACTION_PICK,
            android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
    startActivityForResult(intent, 0);
}


@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (resultCode == RESULT_OK){
        Uri targetUri = data.getData();
        //textTargetUri.setText(targetUri.toString());
        Bitmap bitmap;
        try {
            bitmap = BitmapFactory.decodeStream(getContentResolver().openInputStream(targetUri));
            //imageView.setImageBitmap(bitmap);
            backgroundView.setBackgroundDrawable(new BitmapDrawable(bitmap));
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            Toast.makeText(this, "Error", Toast.LENGTH_LONG).show();
            e.printStackTrace();
        }
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            // This ID represents the Home or Up button. In the case of this
            // activity, the Up button is shown. Use NavUtils to allow users
            // to navigate up one level in the application structure. For
            // more details, see the Navigation pattern on Android Design:
            //
            // http://developer.android.com/design/patterns/navigation.html#up-vs-back
            //
            //NavUtils.navigateUpFromSameTask(this);
            finish();
            return true;
    }
    return super.onOptionsItemSelected(item);
}

由于

Answer 1

从日志中可以看出backgroundView为null。你能检查findViewById（R.id.wallpaperView）是否找到了一个视图吗？

（看起来日志属于imageView没有评论，因为日志说：

Attempt to invoke virtual method 'void android.widget.ImageView.setImageBitmap(android.graphics.Bitmap)' on a null object reference

所以我认为位图检索很好，但在你的视图中设置是问题。