尝试在书籍列表应用程序项目中对空对象引用调用虚拟方法?

Question

我正在将我的项目作为我的 udacity 网络课程的一部分。我在(com.example.android.flavor.MainActivity.onCreate(MainActivity.java:54)).处收到错误 我不知道如何解决这个问题。

主活动.java

public class MainActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<List<BookList>> {
public static final String LOG_TAG = MainActivity.class.getName();
/**
* Constant value for the Book loader ID. We can choose any integer.
* This really only comes into play if you're using multiple loaders.
*/
private Adapter mAdapter;
private static final int BOOK_LOADER_ID = 1;
private String GOOGLE_BOOK_URL;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Fetching the bundle from the searchbar activity
Bundle searchBundle = getIntent().getExtras();
String searchQuery = searchBundle.getString("searchQuery");
searchQuery = searchQuery.replaceAll("","+");
Log.v("replace",searchQuery);
//Make a new String builder to produce URL
StringBuilder searchUrlBuilder = new StringBuilder();
searchUrlBuilder.append("https://www.googleapis.com/books/v1/volumes?q=" + searchQuery +
"&maxResults= 20");
Log.v("URL", String.valueOf(searchUrlBuilder));
//GOOG...URL assumes value of searchUrlBuilder
GOOGLE_BOOK_URL = String.valueOf(searchUrlBuilder);

// Find a reference to the {@link ListView} in the layout
final ListView bookListView = (ListView) findViewById(R.id.listview_book);
// Create a new adapter that takes an empty list of earthquakes as input
mAdapter = new BookListAdapter(this, new ArrayList<BookList>());
// Set the adapter on the {@link ListView}
// so the list can be populated in the user interface
bookListView.setAdapter((ListAdapter) mAdapter);
//Get a reference to the LoaderManager, in order to interact with loaders.
LoaderManager loaderManager = getLoaderManager();
// Initialize the loader. Pass in the int ID constant defined above and pass in null for
// the bundle. Pass in this activity for the LoaderCallbacks parameter (which is valid
// because this activity implements the LoaderCallbacks interface).
loaderManager.initLoader(BOOK_LOADER_ID, null,  this);
}
@Override
public Loader<List<BookList>> onCreateLoader(int i, Bundle bundle) {
//Create a new Loader for the given URL this is where GOOGLE_BOOKS_REQUEST_URL was
return new BookLoader(this, GOOGLE_BOOK_URL);
}
@Override
public void onLoadFinished(Loader<List<BookList>> loader, List<BookList> books) {
//Clear the adapter of previous data
books.clear();
if (books != null && !books.isEmpty()) {
books.addAll(books);
}
}
@Override
public void onLoaderReset(Loader<List<BookList>> loader) {
// Loader reset, so we can clear out our existing data.
}

}

日志猫：

enter code here
09-18 17:05:58.961 23093-23093/com.example.android.flavor E/AndroidRuntime: FATAL EXCEPTION: main
      Process: com.example.android.flavor, PID: 23093
      java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.flavor/com.example.android.flavor.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2426)
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
          at android.app.ActivityThread.-wrap11(ActivityThread.java)
          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)
          at android.os.Handler.dispatchMessage(Handler.java:102)
          at android.os.Looper.loop(Looper.java:148)
          at android.app.ActivityThread.main(ActivityThread.java:5443)
          at java.lang.reflect.Method.invoke(Native Method)
          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
       Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
          at com.example.android.flavor.MainActivity.onCreate(MainActivity.java:54)
          at android.app.Activity.performCreate(Activity.java:6259)
          at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1130)
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379)
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490) 
          at android.app.ActivityThread.-wrap11(ActivityThread.java) 
          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354) 
          at android.os.Handler.dispatchMessage(Handler.java:102) 
          at android.os.Looper.loop(Looper.java:148) 
          at android.app.ActivityThread.main(ActivityThread.java:5443) 

Answer 1

要在活动中接收捆绑包，这意味着当您启动此活动时，您必须已将该捆绑包添加到意向中。您必须手动指定要传递给起始活动的内容。阅读您的帖子，我可以想象您的主要活动是您的第一个启动活动，对吗？

传递捆绑包的方法错误

您将捆绑包错误地传递给您的活动。试试这个：

final Intent openResultActivity = new Intent(this,MainActivity.class);
final EditText searchQuery = (EditText) findViewById(R.id.search_edit_text);
Button searchButton=(Button)findViewById(R.id.search_button);
searchButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v){
final String searchQueryString=searchQuery.getText().toString();
Log.v("SearchinSBA",searchQueryString);
Bundle bundle=new Bundle();
bundle.putString("searchQuery",searchQueryString);
openResultActivity.putExtras(bundle);
startActivity(openResultActivity);
}
});

错误的启动活动

我的第一个答案是正确的：您从MainActivity开始启动您的应用程序。因此，SearchBarActivity中的代码(您putString执行的代码等)永远不会执行。必须首先执行该代码，以便它可以启动MainActivity将您的捆绑包传递给它。

您的问题在于您的AndroidManifest.xml。您必须按如下方式更改启动器活动：

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.android.flavor">
<uses-permission android:name="android.permission.INTERNET" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme">
<activity
android:name=".MainActivity"
android:label="@string/app_name">
</activity>
<activity android:name=".SearchBarActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

Answer 2

您没有收到捆绑包。请在访问其方法之前检查空值

String searchQuery;
if(searchBundle!=null){
searchQuery = searchBundle.getString("searchQuery");
}

或者，如果您错误地传递捆绑包，请执行此操作

Intent intent = new Intent(YourActivityName.this, MainActivity.class);
Bundle bundle = new Bundle();
bundle.putString("searchQuery", yourSearchQuery);
intent.putExtras(bundle);
startActivity(intent);