函数中的android nullpointerexception

Question

我是 Java/android 的新手，所以很多这些术语都是外国的，但我愿意学习。我不打算详细介绍该应用程序，因为我认为它不相关。我目前的问题是，我使用了博客中的教程和代码 fragment ，并使我的代码可以正常工作。试图清理和组织我的代码时，当我移动一行(创建我的 autocompletetextview)时，我得到了一个 nullpointer 异常。下面是我用过的代码。我遇到问题的第一行代码是

AutoCompleteTextView companyAutoComplete = (AutoCompleteTextView) addAddressDialog.findViewById(R.id.add_record_dialog_autocomplete);

当我将它移到函数开头的右侧时，它出错了，但当它留在原处时，它就像一个魅力。我想了解这是为什么。

public void addAddress() {
    final Dialog addAddressDialog = new Dialog(this);
    final int[] to = new int[] { android.R.id.text1 };
    final String[] from = new String[] { "CompanyName" };

    // Create a SimpleCursorAdapter for the CompanyName field.
    SimpleCursorAdapter adapter =  new SimpleCursorAdapter(this, android.R.layout. select_dialog_item, null, from, to);

    addAddressDialog.setContentView(R.layout.add_record_dialog);
    addAddressDialog.setTitle(getString(R.string.add_record_dialog_address_title));
    addAddressDialog.setCancelable(true);

    final EditText text1 = (EditText) addAddressDialog.findViewById(R.id.add_record_dialog_edittext);
    text1.setHint(getString(R.string.add_record_dialog_company_hint));

    Button buttonOK1 = (Button) addAddressDialog.findViewById(R.id.add_record_dialog_ok);
    buttonOK1.setText(getString(R.string.add_record_dialog_ok_button));

    Button buttonCancel1 = (Button) addAddressDialog.findViewById(R.id.add_record_dialog_cancel);
    buttonCancel1.setText(getString(R.string.add_record_dialog_cancel_button));

    buttonOK1.setOnClickListener(new OnClickListener(){
        public void onClick(View v) {
            Bundle addressBundle = new Bundle();
            addressBundle.putString("CompanyName", text1.getText().toString());

            Intent intent = new Intent(MenuActivity.this, AddAddressActivity.class);
            intent.putExtras(addressBundle);
            startActivity(intent);

            addAddressDialog.dismiss();
        }   
    });

    buttonCancel1.setOnClickListener(new OnClickListener(){
        public void onClick(View v) {
            Toast.makeText(getBaseContext(), "Cancel button clicked", Toast.LENGTH_SHORT).show();
            addAddressDialog.dismiss();
        }   
    });
    AutoCompleteTextView companyAutoComplete = (AutoCompleteTextView) addAddressDialog.findViewById(R.id.add_record_dialog_autocomplete);

    companyAutoComplete.setAdapter(adapter);

    // Set an OnItemClickListener, to update dependent fields when
    // a choice is made in the AutoCompleteTextView.
    companyAutoComplete.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> listView, View view,
                    int position, long id) {
            // Get the cursor, positioned to the corresponding row in the
            // result set
            Cursor cursor = (Cursor) listView.getItemAtPosition(position);

            // Get the CompanyID from this row in the database.
            String companyID = cursor.getString(cursor.getColumnIndexOrThrow("_id"));

            // test to make sure CompanyID returned
            Toast.makeText(getBaseContext(), companyID, Toast.LENGTH_SHORT).show();
        }
    });

    // Set the CursorToStringConverter, to provide the labels for the
    // choices to be displayed in the AutoCompleteTextView.
    adapter.setCursorToStringConverter(new CursorToStringConverter() {
        public String convertToString(android.database.Cursor cursor) {
            // Get the label for this row out of the "CompanyName" column
            final int columnIndex = cursor.getColumnIndexOrThrow("CompanyName");
            final String str = cursor.getString(columnIndex);
            return str;
        }
    });

    // Set the FilterQueryProvider, to run queries for choices
    // that match the specified input.
    adapter.setFilterQueryProvider(new FilterQueryProvider() {
        public Cursor runQuery(CharSequence constraint) {
            Cursor cursorReturn = dbAdapter.getCompanies(constraint != null ? constraint.toString() : null);

            startManagingCursor(cursorReturn);
            return cursorReturn;
        }
        });

    addAddressDialog.show();
}

Answer 1

发生这种情况是因为您稍后调用了 setContentView。

setContentView 设置 addAddressDialog 对话框的布局。如果您不调用 setContentView，它没有布局项，因此 addAddressDialog.findViewById(...); 将为 null，并且，显然你不能将它转换为任何东西，也不能对其调用 setHint。

这行代码在您的方法中的什么位置无关紧要，只要您的 setContentView 行在之前被调用即可。