我必须在Android中制作应用。我已成功调试了一些错误，但我不明白这个：
FATAL EXCEPTION: main
Process: com.example.android.bookapp, PID: 7450
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)'
on a null object reference
我可以注意到Android Studio提供橙色背景提醒我，我的代码的某些部分可能会产生java.lang.NullPointerException，但我看不出要改变什么来避免这种情况，我甚至不明白为什么运行我的应用程序时会发生此错误。
这是我的代码：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
/** find the button view, set an onclick listener on it, define & execute the bookListAsyncTask */
Button searchButton = (Button) findViewById(R.id.search_button);
searchButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
BookListAsyncTask bookListAsyncTask = new BookListAsyncTask();
bookListAsyncTask.execute();
}
});
}
/**
* define the URL
*/
public String makeUrl() {
EditText searchBookEditText = (EditText) findViewById(R.id.search_edit_text);
String searchBook = searchBookEditText.getText().toString();
String searchBookModified = searchBook.replaceAll(" ", "+");
final String REQUEST_URL = "https://www.googleapis.com/books/v1/volumes?q=" + searchBookModified + "&maxResults=3&printType=books";
return REQUEST_URL;
}
private class BookListAsyncTask extends AsyncTask<URL, Void, ArrayList<Book>> {
@Override
protected ArrayList<Book> doInBackground(URL... urls) {
// Create URL object
URL url = createUrl(makeUrl());
// Perform HTTP request to the URL and receive a JSON response back
String jsonResponse = "";
try {
jsonResponse = makeHttpRequest(url);
} catch (IOException e) {
}
// Extract relevant fields from the JSON response and create an ArrayList<Book> object
ArrayList<Book> allBooks = extractFeatureFromJson(jsonResponse);
return allBooks;
}
/**
* Update the screen with the given books (which was the result of the
* {@link BookListAsyncTask}).
*/
@Override
protected void onPostExecute(ArrayList<Book> allBooks) {
if (allBooks == null) {
return;
}
BookAdapter bookAdapter = new BookAdapter(MainActivity.this, allBooks);
ListView listView = (ListView) findViewById(R.id.list_view);
listView.setAdapter(bookAdapter);
}
/**
* Returns new URL object from the given string URL.
*/
private URL createUrl(String stringUrl) {
URL url = null;
try {
url = new URL(stringUrl);
} catch (MalformedURLException exception) {
Log.e("MainActivity", "Error with creating URL", exception);
return null;
}
return url;
}
/**
* Make an HTTP request to the given URL and return a String as the response.
*/
private String makeHttpRequest(URL url) throws IOException {
String jsonResponse = "";
HttpURLConnection urlConnection = null;
InputStream inputStream = null;
try {
urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.setReadTimeout(10000 /* milliseconds */);
urlConnection.setConnectTimeout(15000 /* milliseconds */);
urlConnection.connect();
inputStream = urlConnection.getInputStream();
int responseCode = urlConnection.getResponseCode();
if (responseCode == 200) {
jsonResponse = readFromStream(inputStream);
} else {
jsonResponse = "";
Log.e("MainActivity", "error response code" + String.valueOf(responseCode));
}
} catch (IOException e) {
// TODO: Handle the exception
Log.e("MainActivity", "Problem retrieving the earthquake JSON results", e);
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
if (inputStream != null) {
// function must handle java.io.IOException here
inputStream.close();
}
}
return jsonResponse;
}
/**
* Convert the {@link InputStream} into a String which contains the
* whole JSON response from the server.
*/
private String readFromStream(InputStream inputStream) throws IOException {
StringBuilder output = new StringBuilder();
if (inputStream != null) {
InputStreamReader inputStreamReader = new InputStreamReader(inputStream, Charset.forName("UTF-8"));
BufferedReader reader = new BufferedReader(inputStreamReader);
String line = reader.readLine();
while (line != null) {
output.append(line);
line = reader.readLine();
}
}
return output.toString();
}
/**
* Return a book ArrayList
*/
public ArrayList<Book> extractFeatureFromJson(String bookJSON) {
ArrayList<Book> allBooks = new ArrayList<>();
try {
JSONObject baseJsonResponse = new JSONObject(bookJSON);
JSONArray items = baseJsonResponse.getJSONArray("items");
// If there are results in the features array
if (items.length() > 0) {
for (int i = 0; i < items.length(); i++)
// Parse the JSON and fill the allBooks ArrayList)
{
JSONObject currentBook = items.getJSONObject(i);
JSONObject currentVolumeInfo = currentBook.getJSONObject("volumeInfo");
String currentTitle = currentVolumeInfo.getString("title");
String currentFirstAuthor = "Author Unknown";
Book book = new Book(currentTitle, currentFirstAuthor);
allBooks.add(book);
}
return allBooks;
}
} catch (JSONException e) {
Log.e("MainActivity", "Problem parsing the earthquake JSON results", e);
}
return null;
}
}
public class BookAdapter extends ArrayAdapter<Book> {
public BookAdapter(Activity context, ArrayList<Book> books) {
super(context, 0, books);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
// Check if the existing view is being reused, otherwise inflate the view
View listItemView = convertView;
if (listItemView == null) {
listItemView = LayoutInflater.from(getContext()).inflate(
R.layout.list_item_view, parent, false);
}
Book currentBook = getItem(position);
TextView title = (TextView) findViewById(R.id.title_text_view);
title.setText(currentBook.getTitle());
TextView author = (TextView) findViewById(R.id.author_text_view);
author.setText(currentBook.getAuthor());
return listItemView;
}
}
}
和书类，我修改了一些get方法，但没有帮助：
public class Book {
private String mTitle;
private String mAuthor;
public Book(String title, String author){
mTitle = title;
mAuthor = author;
}
public String getTitle(){
if (mTitle == null){
return mTitle = "";}
else {return mTitle;}}
public String getAuthor(){
if (mAuthor == null){
return mAuthor = "";
} else {return mAuthor;}}
}
你看到了什么问题吗？
答案 0 :(得分：4)
更改行
WebDriverError: unknown error: [ng:test] no injector found for element argument to getTestability
http://errors.angularjs.org/1.5.8/ng/test
到
TextView title = (TextView) findViewById(R.id.title_text_view);
TextView author = (TextView) findViewById(R.id.author_text_view);