Android NullPointer 尝试在空对象引用上调用虚拟方法 'boolean java.util.ArrayList.add(java.lang.Object)'

Question

我得到这个错误:java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference，只是为了说清楚，我知道其他有同样问题的人也问了同样的问题，我已经阅读了解决方案，通常你必须实例化你的ArrayList，但我显然在做，但仍然收到这个错误，所以有人知道我如何能够纠正这个错误吗?

AddListActivity.java

public class AddListActivity extends AppCompatActivity {
private RecyclerView listsRecyclerView;
RecyclerView.LayoutManager layoutManager;
ListsRecyclerViewAdapter listsRecyclerViewAdapter;
EditText listEditText;
View addListButton;
View backButton;
//Instantiating the ArrayList right here
ArrayList<Category> categories = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_add_list);
listsRecyclerView = findViewById(R.id.listsRecyclerView);
listEditText = findViewById(R.id.listEditText);
addListButton = findViewById(R.id.addListButton);
backButton = findViewById(R.id.backButton);
Intent intent = getIntent();
categories = intent.getParcelableExtra(MainActivity.KEY_NAME);
initRecyclerView();
listEditText.setOnEditorActionListener(editorActionListener);
addListButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v)
{onAddListClick();}
});
backButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {finish();}
});
}
private void initRecyclerView()
{
layoutManager = new LinearLayoutManager(this);
listsRecyclerView.setLayoutManager(layoutManager);
listsRecyclerViewAdapter = new ListsRecyclerViewAdapter(categories);
listsRecyclerView.setAdapter(listsRecyclerViewAdapter);
new ItemTouchHelper(itemTouchHelperCallback).attachToRecyclerView(listsRecyclerView);
DividerItemDecoration dividerItemDecoration = new DividerItemDecoration(this, DividerItemDecoration.VERTICAL);
dividerItemDecoration.setDrawable(getResources().getDrawable(R.drawable.divider_line, null));
listsRecyclerView.addItemDecoration(dividerItemDecoration);
}
private TextView.OnEditorActionListener editorActionListener = new TextView.OnEditorActionListener() {
@Override
public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
onAddListClick();
return true;
}
};
private void onAddListClick()
{
final DatabaseReference myRef = FirebaseDatabase.getInstance().getReference("CategoryList");
final String text = listEditText.getText().toString();
myRef.orderByValue().equalTo(text).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
if(dataSnapshot.exists())
{
Toast.makeText(AddListActivity.this, "This list already exists", Toast.LENGTH_SHORT).show();
} else
{
if (text.trim().length() > 0)
{
String autoGeneratedId = myRef.push().getKey();
//This following line is where I am getting the error!
categories.add(new Category(text, autoGeneratedId));
listsRecyclerViewAdapter.notifyDataSetChanged();
listEditText.getText().clear();
myRef.child(autoGeneratedId).setValue(text);
Toast.makeText(AddListActivity.this, "New list added", Toast.LENGTH_SHORT).show();
finish();
} else
{
Toast.makeText(AddListActivity.this, "You did not enter any text", Toast.LENGTH_SHORT).show();
}
}
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
}
});
}
ItemTouchHelper.SimpleCallback itemTouchHelperCallback = new ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT) {
@Override
public boolean onMove(@NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, @NonNull RecyclerView.ViewHolder target) {
return false;
}
@Override
public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {
final DatabaseReference myRef = FirebaseDatabase.getInstance().getReference("CategoryList");
String categoryId = categories.get(viewHolder.getAdapterPosition()).getCategoryId();
categories.remove(viewHolder.getAdapterPosition());
myRef.child(categoryId).removeValue();
listsRecyclerViewAdapter.notifyDataSetChanged();
}
};
}

运行时错误:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.testapplication, PID: 12703
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
at com.example.testapplication.AddListActivity$4.onDataChange(AddListActivity.java:111)
at com.google.firebase.database.Query$1.onDataChange(com.google.firebase:firebase-database@@19.2.1:179)
at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@19.2.1:75)
at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@19.2.1:63)
at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@19.2.1:55)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:226)
at android.os.Looper.loop(Looper.java:313)
at android.app.ActivityThread.main(ActivityThread.java:8669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:571)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1135)

编辑:

如何从MainActivity.java发送:

Intent intent = new Intent(this, AddListActivity.class);
intent.putExtra(KEY_NAME, categories);
startActivity(intent);

Answer 1

通过使用putParcelableArrayListExtra()而不是putExtra()和getParcelableArrayListExtra()而不是getParcelableExtra()来解决，因为我使用的是ArrayList