Changing Image Colour [duplicate]

问题:

This question already has an answer here:

• What is a NullPointerException, and how do I fix it? 12 answers
• Using context in a fragment 26 answers

I am currently trying to change the colour of an imageview. I am trying to do this from a fragment, and it works fine until I switch activities. When I return to the fragment that I use to change the colour of the image I get this error,

Attempt to invoke virtual method 'int android.content.Context.getColor(int)' on a null object reference

I use the following code to change the colour of my image,

imgUpvote.setColorFilter(ContextCompat.getColor(getContext(), R.color.lGrey));

Anyone know a fix? This error only occurs when I switch activities, thanks for any answers.

EDIT:here is where code is located,

firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<postsGetInfo, postsGetInfoViewHolder>(postsGetInfo.class, R.layout.posts_layout,postsGetInfoViewHolder.class,postRef) {
                @Override
                protected void populateViewHolder(final postsGetInfoViewHolder viewHolder, postsGetInfo model, int position) {
                    final String postKey = getRef(position).getKey();
                    UpdateTheDisplayVotes(postKey); //Displays the votes at the start of creation
                    postRef.removeEventListener(VoteListener);

                    defaultVote = VotesRef.addValueEventListener(new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {
                            if (!dataSnapshot.hasChild(postKey)) {
                                VotesRef.child(postKey).child(current_user_id).child("votes").setValue("none");
                            }
                            String voteStatus = dataSnapshot.child(postKey).child(current_user_id).child("votes").getValue().toString();
                           if (voteStatus.equals("upvoted")){
                                ImageView btnUpvote = viewHolder.myView.findViewById(R.id.imgUpvote);
                                ImageView btnDownvote = viewHolder.myView.findViewById(R.id.imgDownvote);
                               btnUpvote.setColorFilter(ContextCompat.getColor(getActivity(), R.color.lBlue));
                                btnDownvote.setColorFilter(ContextCompat.getColor(getActivity(), R.color.lGrey));
                            }if (voteStatus.equals("downvoted")){
                                ImageView btnUpvote = viewHolder.myView.findViewById(R.id.imgUpvote);
                                ImageView btnDownvote = viewHolder.myView.findViewById(R.id.imgDownvote);
                                btnUpvote.setColorFilter(ContextCompat.getColor(getActivity(), R.color.lGrey));
                                btnDownvote.setColorFilter(ContextCompat.getColor(getActivity(), R.color.Black));
                            }if (voteStatus.equals("none")){
                                ImageView btnUpvote = viewHolder.myView.findViewById(R.id.imgUpvote);
                                ImageView btnDownvote = viewHolder.myView.findViewById(R.id.imgDownvote);
                                btnUpvote.setColorFilter(ContextCompat.getColor(getActivity(), R.color.lGrey));
                                btnDownvote.setColorFilter(ContextCompat.getColor(getActivity(), R.color.lGrey));
                            }
                        }

回答1:

Try this get Context like this in your fragment

public class BlankFragment extends Fragment {


    public BlankFragment() {

    }

    Context mContext;

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        mContext = context;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment


        return inflater.inflate(R.layout.fragment_blank, container, false);
    }

}

than use like this

kimgUpvote.setColorFilter(ContextCompat.getColor(mContext, R.color.lGrey));