尝试在空对象引用上调用虚拟方法'boolean java.lang.Object.equals（java.lang.Object）'

Question

[我正在追踪youtube上的一个教程，以制作火种的副本。我收到以下错误。有什么可能的解决方案？主要活动代码在android studio中未显示任何错误，但在运行时显示错误。

[这是在android设备上测试应用程序时显示的错误。

 java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.Object.equals(java.lang.Object)' on a null object reference
    at com.techjd.hubu.MainActivity$6.onChildAdded(MainActivity.java:196)
    at com.google.firebase.database.core.ChildEventRegistration.fireEvent(com.google.firebase:firebase-database@@19.2.0:79)
    at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@19.2.0:63)
    at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@19.2.0:55)
    at android.os.Handler.handleCallback(Handler.java:815)
    at android.os.Handler.dispatchMessage(Handler.java:104)
    at android.os.Looper.loop(Looper.java:207)
    at android.app.ActivityThread.main(ActivityThread.java:5740)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:766)

这是主要活动。

  import android.content.Context;
  import android.content.Intent;

  import android.os.Bundle;
  import android.util.Log; 
  import android.view.View; 
  import android.widget.ArrayAdapter;
  import android.widget.ListView; 
  import android.widget.Toast;

  import androidx.annotation.NonNull;
  import androidx.appcompat.app.AppCompatActivity;

  import com.google.firebase.auth.FirebaseAuth;
  import com.google.firebase.auth.FirebaseUser;
  import com.google.firebase.database.ChildEventListener;
  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.lorentzos.flingswipe.SwipeFlingAdapterView;

  import java.util.ArrayList;
  import java.util.List;

  public class MainActivity extends AppCompatActivity {
  private cards cards_data[];
  private ArrayAdapter arrayAdapter;
  private int i;

  private FirebaseAuth mAuth;
  private String currentUId;
  private DatabaseReference usersDb;

  ListView listView;
  List<cards> rowItems;
  @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);



    usersDb = FirebaseDatabase.getInstance().getReference().child("Users");

    mAuth = FirebaseAuth.getInstance();

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

    checkUserSex();



    rowItems = new ArrayList<cards>();

    arrayAdapter = new arrayAdapter(this, R.layout.item, rowItems );

    SwipeFlingAdapterView flingContainer = (SwipeFlingAdapterView) findViewById(R.id.frame);

    flingContainer.setAdapter(arrayAdapter);
    flingContainer.setFlingListener(new SwipeFlingAdapterView.onFlingListener() {
        @Override
        public void removeFirstObjectInAdapter() {
            // this is the simplest way to delete an object from the Adapter (/AdapterView)
            Log.d("LIST", "removed object!");
            rowItems.remove(0);
            arrayAdapter.notifyDataSetChanged();
        }

        @Override
        public void onLeftCardExit(Object dataObject) {

            cards obj = (cards) dataObject;
            String userId = obj.getUserId();
            usersDb.child(oppositeUserSex).child(userId).child("connections").child("nope").child(currentUId).setValue(true);
            Toast.makeText(MainActivity.this, "Left", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onRightCardExit(Object dataObject) {

            cards obj = (cards) dataObject;
            String userId = obj.getUserId();
            usersDb.child(oppositeUserSex).child(userId).child("connections").child("yes").child(currentUId).setValue(true);
            isConnectionMatch(userId);
            Toast.makeText(MainActivity.this, "Right", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onAdapterAboutToEmpty(int itemsInAdapter) {
        }

        @Override
        public void onScroll(float scrollProgressPercent) {
        }
    });


    // Optionally add an OnItemClickListener
    flingContainer.setOnItemClickListener(new SwipeFlingAdapterView.OnItemClickListener() {
        @Override
        public void onItemClicked(int itemPosition, Object dataObject) {
            Toast.makeText(MainActivity.this, "Item Clicked", Toast.LENGTH_SHORT).show();
        }
    });

}

private void isConnectionMatch(String userId) {

    DatabaseReference currentUserConnectionsDB = usersDb.child(userSex).child(currentUId).child("connections").child("yes").child(userId);
    currentUserConnectionsDB.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists()) {
                Toast.makeText(MainActivity.this, "new Connection", Toast.LENGTH_LONG).show();
                usersDb.child(oppositeUserSex).child(dataSnapshot.getKey()).child("connections").child("matches").child(currentUId).setValue(true);
                usersDb.child(userSex).child(currentUId).child("connections").child("matches").child(dataSnapshot.getKey()).setValue(true);
            }

        }

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

        }
    });
}

private String userSex;
private String oppositeUserSex;
public void checkUserSex(){
    final FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();

    DatabaseReference maleDb = FirebaseDatabase.getInstance().getReference().child("Users").child("Male");
    maleDb.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            if (dataSnapshot.getKey().equals(user.getUid())){
                userSex = "Male";
                oppositeUserSex = "Female";
                getOppositeSexUsers();
            }
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });

    DatabaseReference femaleDb = FirebaseDatabase.getInstance().getReference().child("Users").child("Female");
    femaleDb.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            if (dataSnapshot.getKey().equals(user.getUid())){
                userSex = "Female";
                oppositeUserSex = "Male";
                getOppositeSexUsers();
            }
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });
}

public void getOppositeSexUsers(){
    DatabaseReference oppositeSexDb = FirebaseDatabase.getInstance().getReference().child("Users").child(oppositeUserSex);
    oppositeSexDb.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            if (dataSnapshot.exists() && !dataSnapshot.child("connections").child("nope").hasChild(currentUId) && !dataSnapshot.child("connections").child("yes").hasChild(currentUId)){
                String profileImageUrl = "default";
                    if (!dataSnapshot.child("profileImageUrl").getValue().equals("default")){
                        profileImageUrl = dataSnapshot.child("profileImageUrl").getValue().toString();
                    }
                cards item = new cards(dataSnapshot.getKey(), dataSnapshot.child("name").getValue().toString(), profileImageUrl);
                rowItems.add(item);
                arrayAdapter.notifyDataSetChanged();
            }
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });

}


public void logoutUser(View view) {
    mAuth.signOut();
    Intent intent = new Intent(MainActivity.this, ChooseLoginRegistrationActivity.class);
    startActivity(intent);
    finish();
    return;
}

public void goToSettings(View view) {
    Intent intent = new Intent(MainActivity.this, SettingsActivity.class);
    intent.putExtra("userSex",userSex);
    startActivity(intent);

    return;
}

}

第196行是if（！dataSnapshot.child（“ profileImageUrl”）。getValue（）。equals（“ default”））

Answer 1

您的profileImageUrl的getValue函数返回null。因此，当您对其调用.equals时，它不知道您的意思，因为该函数不存在于null上。

您可以简单地在调用.equals之前先检查它是否为null。

Object profileImageUrlData = dataSnapshot.child("profileImageUrl").getValue();

if (profileImageUrlData != null && !profileImageUrlData.equals("default")){
    profileImageUrl = profileImageUrlData.toString();
}

也许只是更改profileImageUrlData的类型，所以它不是对象。我只是不确定会是什么。

Answer 2

您在此行上收到NullPointerException

 if (!dataSnapshot.child("profileImageUrl").getValue().equals("default")){

您的代码已到达该行，因此dataSnapshot不能为null，因为它已在上一行执行了dataSnapshot.exists()，没有任何错误。

此时您的代码获得NPE的唯一原因是dataSnapshot.child（）为null，并且无法在null对象上调用.getValue（）方法，或者.getValue（）返回null。

比较字符串时，最好将静态字符串（如果有）放在比较的左侧。将您的代码更改为

 if (dataSnapshot.child("profileImageUrl") != null &&
 !"default".equals(dataSnapshot.child("profileImageUrl").getValue())){