java - Android Studio : NullPointerException when trying to click on a object in a list and start a new activity

Question

我有一个 Room 数据库，其中包含成员列表。列表中有不同的成员。我想单击某个特定成员，然后它应该启动一个新 Activity ，我可以在其中查看有关该成员的其他信息。但是当我点击某个成员时，应用程序崩溃了。

我的适配器代码:

public class MemberListAdapter extends RecyclerView.Adapter<MemberListAdapter.MemberViewHolder> {

    private List<Member> mMember;
    private MemberClickListener listener;


    public interface MemberClickListener {
        void onMemberClick(int position);
    }

    public MemberListAdapter(final MemberClickListener listener) {
        this.listener = listener;
    }


    @Override
    public MemberViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.recyclerview_member, parent, false);
        return new MemberViewHolder(itemView, listener);
    }


    void setMember(List<Member> members) {
        mMember = members;
        notifyDataSetChanged();
    }

    @Override
    public int getItemCount() {
        if (mMember != null)
            return mMember.size();
        else return 0;
    }


    @Override
    public void onBindViewHolder(MemberViewHolder holder, int position) {
        holder.bind(mMember.get(position), listener);
    }

    class MemberViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {

        private final MemberClickListener listener;
        private final TextView firstname;
        private final TextView surname;
        private final TextView balance;

        MemberViewHolder(View itemView, MemberClickListener listener) {
            super(itemView);
            firstname = itemView.findViewById(R.id.layout_firstname);
            surname = itemView.findViewById(R.id.layout_surname);
            balance = itemView.findViewById(R.id.layout_balance);
            this.listener = listener;

            itemView.setOnClickListener(this);
        }


        public void bind(final Member member, final MemberClickListener listener) {
            firstname.setText(member.getFirstname());
            surname.setText(member.getSurname());
            //balance.setText(toString(member.getBalance()));

            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Log.i("test1", "beforelistenercheck");
                    if (listener != null) {
                        Log.i("test2", "duringlistenercheck");
                        listener.onMemberClick(getAdapterPosition());
                        Log.i("test3", "aftergettingposition");
                    }
                }
            });
        }

        public void onClick(View v) {
            if (listener != null) {
                ********listener.onMemberClick(getAdapterPosition());********
            }
        }
    }
}

我的主要 Activity 代码:

public class MemberMainActivity extends AppCompatActivity implements MemberListAdapter.MemberClickListener{

    private MemberViewModel mMemberViewModel;
    private List<Member> mMember;

    void setMember(List<Member> members) {
        mMember = members;
    }

    public static final int NEW_MEMBER_ACTIVITY_REQUEST_CODE = 1;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_member);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MemberMainActivity.this, NewMemberActivity.class);
                startActivityForResult(intent, NEW_MEMBER_ACTIVITY_REQUEST_CODE);
            }
        });

        RecyclerView recyclerView = findViewById(R.id.recyclerviewcard_member);
        final MemberListAdapter adapter = new MemberListAdapter(this);
        recyclerView.setAdapter(adapter);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        mMemberViewModel = ViewModelProviders.of(this).get(MemberViewModel.class);

        mMemberViewModel.getAllMember().observe(this, new Observer<List<Member>>() {
            @Override
            public void onChanged(@Nullable final List<Member> members) {
                // Update the cached copy of the words in the adapter.
                adapter.setMember(members);
            }
        });
    }

    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == NEW_MEMBER_ACTIVITY_REQUEST_CODE && resultCode == RESULT_OK) {
            Member member = new Member(data.getStringExtra(NewMemberActivity.EXTRA_REPLY), data.getStringExtra(NewMemberActivity.EXTRA_REPLY2));
            mMemberViewModel.insert(member);
        } else {
            Toast.makeText(
                    getApplicationContext(),
                    R.string.empty_not_saved,
                    Toast.LENGTH_LONG).show();
        }
    }


    public void onMemberClick(int position) {
        **********Member member = mMember.get(position);**********
        MemberInfo.open(this, member.getId());
    }
}

点击成员(member)后应启动的 Activity 代码:

public class MemberInfo extends AppCompatActivity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_memberinfo);

        Intent intent = getIntent();
        if (intent != null && intent.hasExtra("MemberID")) {
            long memberid = intent.getLongExtra("MemberID", -1);
            // TODO: get customer details based on customer id
            Bundle bundle = new Bundle();
            bundle.putLong("MemberID", memberid);
            intent.putExtras(bundle);
            startActivity(intent);
        }
        else {
            Toast.makeText(
                    getApplicationContext(),
                    R.string.empty_not_saved,
                    Toast.LENGTH_LONG).show();
        }
    }

    public static void open(Activity activity, long memberid) {
        Intent intent = new Intent(activity, MemberInfo.class);
        intent.putExtra("MemberID", memberid);
        activity.startActivity(intent);
    }
}

错误:

logcat: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference
        at com.example.ahash.mainbuchhaltung.Member.MemberMainActivity.onMemberClick(MemberMainActivity.java:81)
        at com.example.ahash.mainbuchhaltung.Member.MemberListAdapter$MemberViewHolder$1.onClick(MemberListAdapter.java:86)
        at android.view.View.performClick(View.java:6256)
        at android.view.View$PerformClick.run(View.java:24701)
        at android.os.Handler.handleCallback(Handler.java:789)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6541)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

我用星号标记了错误所在的行。我还尝试通过以下问题的答案来解决我的问题:What is a NullPointerException, and how do I fix it?

我的适配器代码中有三个不同的日志，名为“test1”、“test2”和“test3”。我模拟了我的应用程序，“test1”和“test2”显示在我的 logcat 中，但“test3”未显示。

我已经为此工作了三个星期，但我仍然遇到与以前相同的问题。谁能帮我解决这个问题吗？

Answer 1

您的 mMember 在 onMemberClick 方法中为 null。