为什么我得到参数可能为空错误？

Question

package e.starf.project;

import android.content.Context;
import android.content.Intent;
import android.media.Image;
import android.support.annotation.NonNull;
import android.support.design.widget.NavigationView;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.Toolbar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.RecyclerView;
import android.view.Display;
import android.view.LayoutInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

import com.bumptech.glide.annotation.GlideModule;
import com.bumptech.glide.module.AppGlideModule;
import com.bumptech.glide.Glide;
import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.firebase.ui.database.FirebaseRecyclerOptions;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import com.google.firebase.storage.FirebaseStorage;
import com.google.firebase.storage.StorageReference;
import com.squareup.picasso.Picasso;

import de.hdodenhof.circleimageview.CircleImageView;

public class MainActivity extends AppCompatActivity {
    private NavigationView navigationView;
    private DrawerLayout drawerLayout;
    private ActionBarDrawerToggle actionBarDrawerToggle;
    private RecyclerView postList;
    private Toolbar toolbar;
    private ImageButton AddNewPostButton;
    private FirebaseAuth firebaseAuth;
    private DatabaseReference UsersRef, PostsRef, LikesRef;
    private StorageReference mStorageRef;

    String currentUserID;
    Boolean LikeChecker = false;


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



        firebaseAuth = FirebaseAuth.getInstance();
        UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");
        PostsRef = FirebaseDatabase.getInstance().getReference().child("Posts");
        LikesRef = FirebaseDatabase.getInstance().getReference().child("Likes");

        UsersRef.keepSynced(true);
        PostsRef.keepSynced(true);
        LikesRef.keepSynced(true);
        mStorageRef = FirebaseStorage.getInstance().getReference();

        toolbar = (Toolbar) findViewById(R.id.main_page_toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("Home");

        AddNewPostButton = (ImageButton) findViewById(R.id.add_new_post);

        drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        actionBarDrawerToggle = new ActionBarDrawerToggle(MainActivity.this, drawerLayout,
            R.string.drawer_open, R.string.drawer_close);
        drawerLayout.addDrawerListener(actionBarDrawerToggle);
        actionBarDrawerToggle.syncState();
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        navigationView = (NavigationView) findViewById(R.id.navigation_view);

        postList = (RecyclerView) findViewById(R.id.all_users_post_list);
        postList.setHasFixedSize(true);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
        linearLayoutManager.setReverseLayout(true);
        linearLayoutManager.setStackFromEnd(true);
        postList.setLayoutManager(linearLayoutManager);

        View navView = navigationView.inflateHeaderView(R.layout.navigration_header);

        navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                UserMenuSelector(item);
                return false;
            }
        });

        AddNewPostButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SendUserToPostActivity();
            }
        });
        DisplayAllUsersPosts();
    }

    private void DisplayAllUsersPosts() {
        final FirebaseRecyclerOptions < Posts options = new FirebaseRecyclerOptions.Builder < Posts().setQuery(PostsRef,
            Posts.class).build();

        FirebaseRecyclerAdapter < Posts, PostsViewHolder firebaseRecyclerAdapter = new FirebaseRecyclerAdapter < Posts,
            PostsViewHolder(options) {
                @NonNull
                @Override
                public PostsViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {

                    View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.all_posts_layout,
                        viewGroup, false);
                    return new PostsViewHolder(view);
                }

                @Override
                protected void onBindViewHolder(@NonNull final PostsViewHolder holder, int position, @NonNull final Posts model) {
                    final String PostKey = getRef(position).getKey();

                    holder.setName(model.getName());
                    holder.setTime(model.getTime());
                    holder.setDate(model.getDate());
                    holder.setDescription(model.getDescription());
                    holder.setPostimage(getApplicationContext(), model.getPostimage());

                    holder.setLikeButtonStatus(PostKey);

                    holder.mView.setOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View view) {
                            Intent clickPostIntent = new Intent(MainActivity.this, ClickPostActivity.class);
                            clickPostIntent.putExtra("PostKey", PostKey);
                            startActivity(clickPostIntent);
                        }
                    });

                    holder.CommentPostButton.setOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View view) {
                            Intent clickPostIntent = new Intent(MainActivity.this, CommentsActivity.class);
                            clickPostIntent.putExtra("PostKey", PostKey);
                            startActivity(clickPostIntent);
                        }
                    });

                    holder.LikePostButton.setOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View view) {
                            LikeChecker = true;

                            if (LikeChecker) {
                                LikesRef.addValueEventListener(new ValueEventListener() {
                                    @Override
                                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                                        if (LikeChecker.equals(true)) {
                                            if (dataSnapshot.child(PostKey).hasChild(currentUserID)) {
                                                LikesRef.child(PostKey).child(currentUserID).removeValue();
                                                LikeChecker = false;
                                            } else {
                                                LikesRef.child(PostKey).child(currentUserID).setValue(true);
                                                LikeChecker = false;
                                            }
                                        }

                                    }

                                    @Override
                                    public void onCancelled(@NonNull DatabaseError databaseError) {

                                    }
                                });


                            }

                        }
                    });

                }
            };
        postList.setAdapter(firebaseRecyclerAdapter);
        firebaseRecyclerAdapter.startListening();
    }

    public static class PostsViewHolder extends RecyclerView.ViewHolder {
        View mView;
        ImageButton LikePostButton, CommentPostButton;
        TextView DisplayNoOfLikes;
        int countLikes;
        String currentUserID;
        DatabaseReference LikesRef;

        public PostsViewHolder(@NonNull View itemView) {
            super(itemView);
            mView = itemView;

            LikePostButton = (ImageButton) mView.findViewById(R.id.like_button);
            CommentPostButton = (ImageButton) mView.findViewById(R.id.comment_button);
            DisplayNoOfLikes = (TextView) mView.findViewById(R.id.display_no_of_likes);

            LikesRef = FirebaseDatabase.getInstance().getReference().child("Likes");
            currentUserID = FirebaseAuth.getInstance().getCurrentUser().getUid();
        }

        public void setLikeButtonStatus(final String PostKey) {
            LikesRef.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    if (dataSnapshot.child(PostKey).hasChild(currentUserID)) {
                        countLikes = (int) dataSnapshot.child(PostKey).getChildrenCount();
                        LikePostButton.setImageResource(R.drawable.like);
                        DisplayNoOfLikes.setText(Integer.toString(countLikes) + (" Likes"));
                    } else {
                        countLikes = (int) dataSnapshot.child(PostKey).getChildrenCount();
                        LikePostButton.setImageResource(R.drawable.dislike);
                        DisplayNoOfLikes.setText(Integer.toString(countLikes) + (" Likes"));

                    }
                }


                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });

        }
        public void setName(String name) {
            TextView username = (TextView) mView.findViewById(R.id.post_user_name);
            username.setText(name);
        }
        public void setTime(String time) {
            TextView PostTime = (TextView) mView.findViewById(R.id.post_time);
            PostTime.setText(" " + time);
        }
        public void setDate(String date) {
            TextView PostDate = (TextView) mView.findViewById(R.id.post_date);
            PostDate.setText(" " + date);
        }
        public void setDescription(String description) {
            TextView PostDescription = (TextView) mView.findViewById(R.id.post_question);
            PostDescription.setText(description);
        }
        public void setPostimage(Context ctx, String postimage) {
            ImageView PostImage = (ImageView) mView.findViewById(R.id.post_image);
            Picasso.get().load(postimage).into(PostImage);
            //   Glide.with(ctx).load(postimage).into(PostImage);
        }
    }


    private void SendUserToPostActivity() {
        Intent addNewPostIntent = new Intent(MainActivity.this, PostActivity.class);
        startActivity(addNewPostIntent);
    }

    protected void onStart() {
        super.onStart();

        FirebaseUser currentUser = firebaseAuth.getCurrentUser();

        if (currentUser == null) {
            SendUserToLoginActivity();
        } else {
            CheckUserExistence();
        }
    }

    private void CheckUserExistence() {
        final String current_user_id = firebaseAuth.getCurrentUser().getUid();
        UsersRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if (!dataSnapshot.hasChild(current_user_id)) {
                    SendUserToSetupActivity();
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    private void SendUserToSetupActivity() {
        Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
        setupIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(setupIntent);
        finish();
    }

    private void SendUserToLoginActivity() {
        Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
        loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(loginIntent);
        finish();
    }

    private void SendUserToProfileActivity() {
        Intent profileIntent = new Intent(MainActivity.this, ProfileActivity.class);
        startActivity(profileIntent);
    }

    private void SendUserToAboutUsActivity() {
        Intent AboutUsIntent = new Intent(MainActivity.this, AboutUs.class);
        startActivity(AboutUsIntent);
    }
    private void SendUserToContactUsActivity() {
        Intent AboutUsIntent = new Intent(MainActivity.this, ContactUs.class);
        startActivity(AboutUsIntent);
    }

    public boolean onOptionsItemSelected(MenuItem item) {
        if (actionBarDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private void UserMenuSelector(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.nav_post:
                SendUserToPostActivity();
                break;
            case R.id.nav_menu1:
                Toast.makeText(this, "Home", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_menu2:
                Toast.makeText(this, "Discussion FOrum", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_menu3:
                SendUserToProfileActivity();
                break;
            case R.id.nav_menu4:
                SendUserToAboutUsActivity();
                break;
            case R.id.nav_menu5:
                Intent sendIntent = new Intent();
                sendIntent.setAction(Intent.ACTION_SEND);
                sendIntent.putExtra(Intent.EXTRA_TEXT, "PICT Placement.\n Forum to create awareness...\nLink:
                    https: //github.com/pragyagupta1898");
                    sendIntent.setType("text/plain"); startActivity(sendIntent); Toast.makeText(this, "Share", Toast.LENGTH_SHORT).show();
                    break;
                    case R.id.nav_contact:
                        SendUserToContactUsActivity();
                        break;
                    case R.id.nav_logout:
                        firebaseAuth.signOut(); SendUserToLoginActivity();
                        break;

                }
        }
    }

错误在这里：

if(dataSnapshot.child(PostKey).hasChild(currentUserID)) 

它说：

  

参数'PostKey'可能为null少...（Ctrl + F1）   此检查分析方法控制和数据流，以报告可能始终为真或为假的情况，其值被静态证明为常数的表达式以及可能导致违反可废性合同的情况。   标记为@Nullable或@NotNull的变量，方法参数和返回值被视为可为空（或分别为非空），并在分析期间用于检查可为空的合同，例如报告可能产生的NullPointerException（NPE）错误。   可以使用@Contract注释定义更复杂的合同，例如：   @Contract（“ ，null-null”）—如果第二个参数为null，则方法返回null @Contract（“ ，null-null; _，！null-！null”）—如果返回则为null它的第二个参数为null，否则为非null，否则@Contract（“ true-fail”）—一种典型的assertFalse方法，如果将true传递给它，则抛出异常   可以将检查配置为使用自定义@Nullable   @NotNull批注（默认情况下，将使用notifications.jar中的批注）

如何解决？

Answer 1

这不是错误，是警告。

由于似乎您没有阅读所引用的警告，因此我将其解释为：

DataSnapshot.child()方法具有@Nullable标志，这意味着它有可能返回null。由于您直接调用该方法的结果，因此Android Studio只是告诉您，由于child()可以为null，因此在其上运行hasChild()可能会导致NullPointerException。

要么检查null状态，要么忽略它。

Answer 2

尝试类似这样的方法。

if((PostKey != null) && dataSnapshot.child(PostKey).hasChild(currentUserID)) 

您也可以编写它以便处理空值情况

if (PostKey == null)  //do somthing
else if (dataSnapshot.child(PostKey).hasChild(currentUserID)) //do something
else //do something

Answer 3

如果在出现此警告的情况下调试应用，则会发现mUserID = null， 因此，您只需要进行初始化即可：

mUserID = mAuth.getCurrentUser().getUid();