出现错误 java.lang.NullPointerException：尝试在空对象引用上调用接口方法“int java.util.List.size()”[重复]

Question

我收到错误：java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference。我试图找出确切的问题，但无法确定确切的位置。希望有人可以提供帮助。我有 2 个单独的布局，带有 2 个单独的适配器和列表视图。一直在为此苦苦挣扎，希望有解决方案。非常感谢！

    public class viewContacts extends AppCompatActivity {

      private static final String TAG = viewContacts.class.getSimpleName();
      ListView listView_Android_Contacts;
      ListView viewContacts;
      View currentSelected;
      Cursor c;
      String contactName;
      CardView saveContacts;
      String phoneNumber;
      CheckBox cb;
      String currentContactName;
      ArrayList<String> selectedContacts;
      ArrayList<String> contacts;
      ArrayList<String> listMessages;
      LinearLayout linearMain;
      ListView currentContactsList;
      String contactEntry;
      public List<String> savedContacts;

      @Override
      protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_contacts_list);
        final ListView currentContactsList = findViewById(R.id.currentContactsList);
        setContentView(R.layout.activity_view_contacts);
        saveContacts = findViewById(R.id.saveContacts);
        final ListView viewContacts = findViewById(R.id.contactsView);
        Dexter.withActivity(this)
                .withPermission(Manifest.permission.READ_CONTACTS)
                .withListener(new PermissionListener() {
                    @Override
                    public void onPermissionGranted(PermissionGrantedResponse response) {
                        get();
                    }


                    @Override
                    public void onPermissionDenied(PermissionDeniedResponse response) {
                        // check for permanent denial of permission
                        if (response.isPermanentlyDenied()) {
                            // navigate user to app settings
                        }
                    }


                    @Override
                    public void onPermissionRationaleShouldBeShown(PermissionRequest permission, PermissionToken token) {
                        token.continuePermissionRequest();
                    }
                }).check();
        final ArrayAdapter<String> adapter = new ArrayAdapter<String>(
                this, android.R.layout.simple_list_item_multiple_choice, contacts
        );
        final ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_2, selectedContacts);
        viewContacts.setAdapter(adapter);
        currentContactsList.setAdapter(arrayAdapter);
        viewContacts.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String selectedFromList = (String) viewContacts.getItemAtPosition(position);
                Toast.makeText(viewContacts.this, selectedFromList,Toast.LENGTH_SHORT).show();
            }
        });
        saveContacts.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                selectedContacts = new ArrayList<>();
                List<Integer> checkedIDs = new ArrayList<Integer>();
                final SparseBooleanArray checkedItems = viewContacts.getCheckedItemPositions();
                for (int i = 0; i < checkedItems.size(); i++){
                    final boolean isChecked = checkedItems.valueAt(i);
                    if (isChecked){
                        final int position = checkedItems.keyAt(i);
                        String s = (String) viewContacts.getItemAtPosition(position);
                        selectedContacts.add(s);
                        checkedIDs.add(position);
                    }

                }
                startActivity(new Intent(viewContacts.this, contactsList.class));
            }
        });
    }

    public void get(){

        c = getContentResolver().query(ContactsContract.CommonDataKinds.Phone.CONTENT_URI, null, null, null, ContactsContract.Contacts.DISPLAY_NAME );
        contacts = new ArrayList<>();
        while(c.moveToNext()){
            contactName = c.getString(c.getColumnIndex(ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME));
            phoneNumber = c.getString(c.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER));
            contacts.add( contactName + "\n" + phoneNumber);

            Log.d(TAG, "show contacts:");
        }
        c.close();
    }
}

Answer 1

你在代码中调用size()的唯一地方是这里：

        for (int i = 0; i < checkedItems.size(); i++){

假设这是引发异常的地方，checkedItems 必须是 null。

因此viewContacts.getCheckedItemPositions() 必须返回null。

这是我们所能提供的信息。

剩下的诊断工作交给你。

---

更新

您似乎以不同的视图调用了两次setContentView。那是行不通的。 Activity 仅具有一个内容视图。

这可能解释了 NPE。两个视图之一不会（不能）连接到 Activity，这可能就是它返回 null 的原因。

您需要创建一个包含/组合您的两个布局的布局。或者也许在两种布局之间切换。 （不确定……）