致命异常：主要无法启动活动 ComponentInfo 由 java.lang.NullPointerException 引起

Question

在将代码从我的主要活动移动到适配器的过程中，我开始遇到一个问题：The method findViewById(int) is undefined for the type VideosAdapter 和 The method getResources() is undefined for the type new View.OnClickListener(){}

我找到了这个解决方案：

The method findViewById(int) is undefined for the type

但是当我尝试改变这个时：

fav_up_btn1 = (Button) findViewById(R.id.fav_up_btn1);

到这里：

fav_up_btn1 = (Button) v.findViewById(R.id.fav_up_btn1);

我最终得到了这个：

v cannot be resolved

我不确定在我的情况下我应该如何声明 v。

JAVA：

public class VideosAdapter extends BaseAdapter {
// The list of videos to display
List<Video> videos;
// An inflator to use when creating rows
private LayoutInflater mInflater;
Button fav_up_btn1;
Button fav_dwn_btn1;

/**
 * @param context this is the context that the list will be shown in - used to create new list rows
 * @param videos this is a list of videos to display
 */
public VideosAdapter(Context context, List<Video> videos) {
    this.videos = videos;
    this.mInflater = LayoutInflater.from(context);
}

@Override
public int getCount() {
    return videos.size();
}

@Override
public Object getItem(int position) {
    return videos.get(position);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    // If convertView wasn't null it means we have already set it to our list_item_user_video so no need to do it again
    if(convertView == null){
        // This is the layout we are using for each row in our list
        // anything you declare in this layout can then be referenced below
        convertView = mInflater.inflate(R.layout.list_item_user_video, parent, false);
    }
    // We are using a custom imageview so that we can load images using urls
    // For further explanation see: http://blog.blundell-apps.com/imageview-with-loading-spinner/
    UrlImageView thumb = (UrlImageView) convertView.findViewById(R.id.userVideoThumbImageView);

    TextView title = (TextView) convertView.findViewById(R.id.userVideoTitleTextView); 
        fav_up_btn1 = (Button) findViewById(R.id.fav_up_btn1);

    fav_up_btn1.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            boolean favIsUp = fav_up_btn1
                    .getBackground()
                    .getConstantState()
                    .equals(getResources().getDrawable(
                            R.drawable.fav_up_btn1).getConstantState());
            // set the background
            fav_up_btn1
            .setBackgroundResource(favIsUp ? R.drawable.fav_dwn_btn1
                    : R.drawable.fav_up_btn1);
        }
    });

    // Get a single video from our list
    final Video video = videos.get(position);
    // Set the image for the list item
    thumb.setImageDrawable(video.getThumbUrl());
    // Set the title for the list item
    title.setText(video.getTitle());

    return convertView;
}

}

Answer 1

你应该使用convertView 而不是v：

fav_up_btn1 = (Button) convertView.findViewById(R.id.fav_up_btn1);

Answer 2

你需要：

fav_up_btn1 = (Button) convertView.findViewById(R.id.fav_up_btn1);

在 getView 中，它是您正在使用的视图“convertView”中的元素。在这种情况下，该布局是 list_item_user_video.xml

您说您在使用 getResources() 时也遇到了问题。该方法需要一个上下文（当您在 Activity 中使用它时，该上下文是隐式的，因为 Activity 是一个上下文）。但是，在您的适配器中并非如此。

您已经在适配器构造函数中接受了上下文，因此请保留对它的引用。有一个类级别的变量：

Context my_context;

在构造函数中设置：

public VideosAdapter(Context context, List<Video> videos) {
    this.videos = videos;
    this.mInflater = LayoutInflater.from(context);
    my_context = context;
}

然后你可以在你的适配器中调用 getResources 使用

my_context.getResources()