书签活动中出现NullpointException错误

Question

我在执行应用程序时遇到了这个错误。其目的是将一个活动添加到列表中，并通过按下列表中的按钮来访问该活动。

我从教程中获得了代码，活动代码中没有错误。我可以将另一个活动中列出的活动添加到"收藏夹"中。但当我尝试通过按钮输入该活动时，它会抛出NullPointException错误。

这是活动的代码：

public class BookmarksActivity extends Activity {
private TextView mEmptyText;
private LinearLayout mBookmarkLayout;
private Context mContext;
@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.bookmarks);
    mContext = getApplicationContext();
    mEmptyText = (TextView) findViewById(R.id.empty_textview);
    mBookmarkLayout = (LinearLayout) findViewById(R.id.bookmark_insert_point);
    getAllKeys();
}
private void getAllKeys()
{
    SharedPreferences sp = this.getSharedPreferences("bookmarks", MODE_PRIVATE);
    Map<String,?> keys = sp.getAll();
    int count = 0;
    for(Map.Entry<String,?> entry : keys.entrySet())
    {
        String value = entry.getValue().toString();
        System.out.println("!!!!!!!!!!!!!!!!!value = "+value);
        String delimiter = ",";
        String[] values_array = value.split(delimiter);
        addBookmark(values_array);
        count++; //keep track of the number of bookmarks
    }
    //if there are no bookmarks, display a text view saying so.  Otherwise, make the text view go away
    if (count == 0)
    {
        mEmptyText.setVisibility(View.VISIBLE);
        mEmptyText.setText(getString(R.string.no_bookmarks));
    }
    else
        mEmptyText.setVisibility(View.GONE);
}
private void addBookmark(String[] values_array)
{       
    LayoutInflater vi = (LayoutInflater) getApplicationContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    View v = vi.inflate(R.layout.single_bookmark, null);
    TextView text = (TextView) v.findViewById(R.id.bookmark_text);
    Button button = (Button) v.findViewById(R.id.bookmark_button);
    text.setText(values_array[1]);
    final String myClass = values_array[0];
    button.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            Class<?> cl = null;
            try {
                cl = Class.forName(myClass);
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
            Intent myIntent = new Intent(mContext, cl);
            startActivity(myIntent);
        }
    });
    // insert into main view
    mBookmarkLayout.addView(v, 0, new      LinearLayout.LayoutParams(LinearLayout.LayoutParams.FILL_PARENT,  LinearLayout.LayoutParams.FILL_PARENT));
    System.out.println("!!!!!!!!!!!!!!!!!!!!Just added a view");
}
}

我在以下位置得到错误：Intent myIntent=new Intent（mContext，cl）；

也许myClass是空的，但我不知道该怎么查。

谢谢！

编辑：添加LogCat

09-23 20:04:53.367: E/AndroidRuntime(629): FATAL EXCEPTION: main
09-23 20:04:53.367: E/AndroidRuntime(629): java.lang.NullPointerException
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.content.ComponentName.<init>(ComponentName.java:76)
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.content.Intent.<init>(Intent.java:3301)
09-23 20:04:53.367: E/AndroidRuntime(629):  at com.and.mememe.FavouritesActivity$1.onClick(FavouritesActivity.java:101)
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.view.View.performClick(View.java:4084)
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.view.View$PerformClick.run(View.java:16966)
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.os.Handler.handleCallback(Handler.java:615)
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.os.Handler.dispatchMessage(Handler.java:92)
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.os.Looper.loop(Looper.java:137)
09-23 20:04:53.367: E/AndroidRuntime(629):  at android.app.ActivityThread.main(ActivityThread.java:4745)
09-23 20:04:53.367: E/AndroidRuntime(629):  at java.lang.reflect.Method.invokeNative(Native Method)
09-23 20:04:53.367: E/AndroidRuntime(629):  at java.lang.reflect.Method.invoke(Method.java:511)
09-23 20:04:53.367: E/AndroidRuntime(629):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-23 20:04:53.367: E/AndroidRuntime(629):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-23 20:04:53.367: E/AndroidRuntime(629):  at dalvik.system.NativeStart.main(Native Method)

编辑2

button.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            Class<?> cl = null;
            try {
                cl = Class.forName(myClass);
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
            Intent myIntent = new Intent(mContext, cl);
            startActivity(myIntent);
        }
    });

Answer 1

这里的问题最终是myClass没有返回有效的类名。因此没有Activity可以实际启动。

此代码在此处

try {
            cl = Class.forName(myClass);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

正在捕获ClassNotFoundException，但代码继续尝试启动无效的Activity。要解决此问题，请将Intent代码放入try块中。这样，当它捕捉到异常时，可以对它执行一些操作，如日志记录、打印消息等。但是，最重要的是，它不会尝试启动无效的"活动"。需要检查myClass变量的有效性。它的路径错误或拼写错误。

try {
        cl = Class.forName(myClass);
        Intent myIntent = new Intent(mContext, cl);
        startActivity(myIntent);
     } catch (ClassNotFoundException e) {
        e.printStackTrace();
     }