android - 获取空指针异常 - 使用接口(interface)将数据从适配器传递到子 fragment

Question

我有一个 Adapter 和一个 Sub-Fragment。我正在使用一个接口(interface)将数据从 Adapter 传递到 Sub-Fragment。我遇到的问题是我不断收到 Null Pointer Exception。我已经阅读了此处与该问题相关的各种帖子，但无法弄清楚我哪里出错了。我在“passAdapterVariable.passAdapterVariable(mname)”这一行得到了 NPE。根据我所阅读的内容，我怀疑这可能是因为我没有正确初始化 passAdapterVariable。我已经尝试根据其他示例以几种不同的方式初始化它，但我一直收到 NPE。

这是适配器

public class MatchAdapter extends RecyclerView.Adapter<MatchAdapter.MatchViewHolder> {

    //declaration of variables
    private Fragment fragment;
    private FragmentManager fragmentManager;
    private DiscoverPage discoverPage;
    private Context context;
    private int size;
    private int mposition;
    private TextView txt_matchname;
    private ImageView img_matchpic;
    List<String> maImg = new ArrayList<>();
    private String mname;
    PassAdapterVariable passAdapterVariable;


    public interface PassAdapterVariable {

        void passAdapterVariable(String mname);
    }


    //the constructor
    public MatchAdapter(List<String> maImg, int size, Context context, DiscoverPage discoverPage){//, PassAdapterVariable passAdapterVariable) {

        this.maImg = maImg;
        this.context = context;
        this.discoverPage = discoverPage;
        this.size = size;
        //this.passAdapterVariable = (PassAdapterVariable)context;
    }

    public MatchAdapter(String mname, Context context) {
        this.context = context;
        this.passAdapterVariable  = (PassAdapterVariable)context;
    }


    //PassAdapterVariable passAdapterVariable = (PassAdapterVariable) context;


    @Override
    public MatchAdapter.MatchViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.match_items, parent, false);
        MatchViewHolder matchViewHolder = new MatchViewHolder(view, maImg, discoverPage);


        return matchViewHolder;
    }


    @Override
    public void onBindViewHolder(MatchViewHolder holder, int position) {

        Picasso.with(context).load(maImg.get(position)).into(holder.img_match);
        holder.setIsRecyclable(false);

    }


    @Override
    public int getItemCount() {
        return maImg.size();
    }


    //viewholder class
    public class MatchViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {

        private static final String TAG = "error";
        //declare variables
        private DiscoverPage discoverPage;
        private ImageView img_match;


        //ViewHolder constructor
        public MatchViewHolder(View itemView, final List<String> maImg, final DiscoverPage discoverPage) {
            super(itemView);

            //initialize variables inside the viewholder constructor
            this.discoverPage = discoverPage;
            img_match = (ImageView) itemView.findViewById(R.id.img_match);
            txt_matchname = (TextView) itemView.findViewById(R.id.txt_matchname);
            img_matchpic = (ImageView) itemView.findViewById(R.id.img_matchpic);


            //set click listener for the img_match
            img_match.setOnClickListener(this);

        }


        @Override
        public void onClick(View view) {
            if (view == img_match) {
                //discoverPage.isHidden();
                Fragment currentFragment;
                fragment = new ClickedMatch();
                fragmentManager = ((AppCompatActivity) context).getSupportFragmentManager();
                FragmentTransaction transaction = fragmentManager.beginTransaction();
                transaction.replace(android.R.id.content, fragment);
                transaction.addToBackStack("DiscoverPage");

                if ((currentFragment = ((AppCompatActivity) context).getSupportFragmentManager().findFragmentById(R.id.main_container)) != null) {
                    transaction.hide(currentFragment);
                }
                else {
                    transaction.commit();
                }


                mname = maImg.get(getAdapterPosition());
                mposition = getAdapterPosition();
                mname = maImg.get(mposition);
                passAdapterVariable.passAdapterVariable(mname);

            }

        }
    }
}

这是子 fragment

public class ClickedMatch extends Fragment implements MatchAdapter.PassAdapterVariable{

    //declare variables
    private Toolbar toolbar;
    private TextView txt_matchname;
    private TextView txt_matchprice;
    private ImageView img_matchpic;
    private String mname;
    private String imgmatch;
    List<String> maImg = new ArrayList<>();
    int size;
    Context context;
    DiscoverPage discoverPage;
    private String pname;
    private int i;
    MatchAdapter.PassAdapterVariable passAdapterVariable;



    public ClickedMatch() {
        // Required empty public constructor

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_clicked_match, container, false);

        //initialize variables
        toolbar = (Toolbar) view.findViewById(R.id.toolbar);
        ((AppCompatActivity)getActivity()).setSupportActionBar(toolbar);//set toolbar as action bar
        txt_matchname = (TextView)view.findViewById(R.id.txt_matchname);
        img_matchpic = (ImageView)view.findViewById(R.id.img_matchpic);

        //setHasOptionsMenu(true);

        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getActivity().onBackPressed();
            }
        });

        if(((AppCompatActivity)getActivity()).getSupportActionBar()!= null){
            ((AppCompatActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(true);
            ((AppCompatActivity)getActivity()).getSupportActionBar().setDisplayShowHomeEnabled(true);
            ((AppCompatActivity)getActivity()).getSupportActionBar().setHomeButtonEnabled(true);
            ((AppCompatActivity)getActivity()).getSupportActionBar().setTitle("");
        }

       // MatchAdapter matAdapter = new MatchAdapter(maImg, size, context, discoverPage,
        //passAdapterVariable);

       // matAdapter.passAdapterVariable = this;
        //passAdapterVariable.passAdapterVariable(mname);
        //txt_matchname.setText(pname);

        MatchAdapter matAdapter = new MatchAdapter(pname, getContext());
        matAdapter.passAdapterVariable = this;
        passAdapterVariable(pname);
        txt_matchname.setText(pname);

        return view;
    }
    @Override
    public void passAdapterVariable(String mname) {
       this.pname = mname;

    }



    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == android.R.id.home) {
            this.getActivity().finish();
        }

       return super.onOptionsItemSelected(item);

    }

}

这是错误日志

08-13 21:53:49.341 12852-12852/com.test.jack E/AndroidRuntime: FATAL EXCEPTION: main
     Process: com.test.jack, PID: 12852

     java.lang.NullPointerException: Attempt to invoke interface method 'void com.test.jack.MatchAdapter$PassAdapterVariable.passAdapterVariable(java.lang.String)' on a null object reference
     at com.test.jack.MatchAdapter$MatchViewHolder.onClick(MatchAdapter.java:138)
     at android.view.View.performClick(View.java:5637)
     at android.view.View$PerformClick.run(View.java:22429)
     at android.os.Handler.handleCallback(Handler.java:751)
     at android.os.Handler.dispatchMessage(Handler.java:95)
     at android.os.Looper.loop(Looper.java:154)
     at android.app.ActivityThread.main(ActivityThread.java:6119)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

为了添加更多上下文，我有一个主要 Activity (UserMainPage)。底部导航菜单选择器用 fragment (DiscoverPage) 替换了 UserMainPage。

DiscoverPage 调用适配器(MatchAdapter)。单击 DiscoverPage 上的按钮会将 DiscoverPage 替换为子 fragment (ClickedMatch)。

我正在尝试将一个变量从 MatchAdapter 传递到 ClickedMatch。

Answer 1

我能够使用 Bundle 将数据从适配器传递到子 fragment 。

我修改了我的代码如下:

在我添加的适配器中:

fragment = new ClickedMatch();
Bundle bundle = new Bundle();
bundle.putString("matchname",mname);
fragment.setArguments(bundle); 

...

transaction.commit();

在我的子 fragment 中，我添加了:

Bundle bundle = this.getArguments();
if(bundle != null){
   pname = bundle.getString("matchname");
}

不需要使用接口(interface)。根据我的研究，我认为(我可能是错的)创建一个接口(interface)然后实现它更适合在 fragment 之间进行通信，而不是在适配器和子 fragment 之间进行通信。