我是android编程的新手。在下面的代码中，我尝试从主activity开始Listview ImageView点击Activity。当我跑它崩溃。请帮我解决这个问题。
执行v.getContext().startActivity(searchResults);时，错误就在行上。
package com.example.libraryhelpdesk;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.RadioButton;
import android.widget.Toast;
public class MainActivity extends ActionBarActivity {
// Creating the widget objects in the layout
ImageView ivSearch;
RadioButton rbBooks, rbAuthor, rbPub, rbKey;
EditText eSearchBar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Asiging ID to the widgets
ivSearch = (ImageView) findViewById(R.id.searchbtn);
rbBooks = (RadioButton) findViewById(R.id.rbBook);
rbAuthor = (RadioButton) findViewById(R.id.rbAuthor);
rbPub = (RadioButton) findViewById(R.id.rbPub);
rbKey = (RadioButton) findViewById(R.id.rbKeywords);
eSearchBar = (EditText) findViewById(R.id.seachbar);
// OnClickListner for ImageView
ivSearch.setOnClickListener(new View.OnClickListener() {
String flag = "";
@Override
public void onClick(View v) {
// Set Chosen option of search by the user
if (rbBooks.isChecked()) {
// Launch B_Search Class
flag = "B";
} else if (rbAuthor.isChecked()) {
// Launch ASearch Class
flag = "A";
} else if (rbPub.isChecked()) {
// Launch PSearch Class
flag = "P";
} else if (rbKey.isChecked()) {
// Launch KSearch Class
flag = "K";
} else {
Toast.makeText(getApplicationContext(),
"Please select any one option to search!!",
Toast.LENGTH_SHORT).show();
}
Log.d("CheckLib",flag);
if (eSearchBar.getText().toString().trim().length() > 0) {
// start activity to view the search results
Intent searchResults = new Intent(MainActivity.this, B_Search.class);
searchResults.putExtra("sValue", eSearchBar.getText()
.toString().trim());
searchResults.putExtra("sOption", flag);
v.getContext().startActivity(searchResults);
} else {
Toast.makeText(getApplicationContext(),
"Enter the search elements!!", Toast.LENGTH_SHORT)
.show();
}
}
});
}
}
我的清单文件......
<activity
android:name="com.example.libraryhelpdesk.MainActivity"
android:label="@string/app_name"
android:screenOrientation="portrait" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name="B_Search" android:label="@string/res_disp">
<intent-filter>
<category android:name="android.intent.category.DEFAULT"/>
<action android:name="com.libhlpdsk.resultview"/>
</intent-filter>
</activity>
logcat错误
07-04 09:29:56.855: D/AndroidRuntime(678): Shutting down VM
07-04 09:29:56.855: W/dalvikvm(678): threadid=1: thread exiting with uncaught exception (group=0xb1a0ad58)
07-04 09:29:56.875: E/AndroidRuntime(678): FATAL EXCEPTION: main
07-04 09:29:56.875: E/AndroidRuntime(678): Process: com.example.libraryhelpdesk, PID: 678
07-04 09:29:56.875: E/AndroidRuntime(678): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.libraryhelpdesk/com.example.libraryhelpdesk.B_Search}: java.lang.NullPointerException
07-04 09:29:56.875: E/AndroidRuntime(678): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2134)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2270)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.app.ActivityThread.access$800(ActivityThread.java:138)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1209)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.os.Handler.dispatchMessage(Handler.java:102)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.os.Looper.loop(Looper.java:136)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.app.ActivityThread.main(ActivityThread.java:5042)
07-04 09:29:56.875: E/AndroidRuntime(678): at java.lang.reflect.Method.invokeNative(Native Method)
07-04 09:29:56.875: E/AndroidRuntime(678): at java.lang.reflect.Method.invoke(Method.java:515)
07-04 09:29:56.875: E/AndroidRuntime(678): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:776)
07-04 09:29:56.875: E/AndroidRuntime(678): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
07-04 09:29:56.875: E/AndroidRuntime(678): at dalvik.system.NativeStart.main(Native Method)
07-04 09:29:56.875: E/AndroidRuntime(678): Caused by: java.lang.NullPointerException
07-04 09:29:56.875: E/AndroidRuntime(678): at com.example.libraryhelpdesk.B_Search.<init>(B_Search.java:28)
07-04 09:29:56.875: E/AndroidRuntime(678): at java.lang.Class.newInstanceImpl(Native Method)
07-04 09:29:56.875: E/AndroidRuntime(678): at java.lang.Class.newInstance(Class.java:1214)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
07-04 09:29:56.875: E/AndroidRuntime(678): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2125)
07-04 09:29:56.875: E/AndroidRuntime(678): ... 11 more
B_Search.java的代码
package com.example.libraryhelpdesk;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.app.ListActivity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.ListAdapter;
import android.widget.SimpleAdapter;
import android.widget.Toast;
public class B_Search extends ListActivity {
// Progress Bar while retrieving data
private ProgressDialog pDialog;
// Retrieving values from the previous activity
Bundle extras = getIntent().getExtras();
private static String sOption = null;
private static String sBy = null;
// URL to retrieve search from
private static String URL = null;
// Creating JSON Parser object
JSONParser jParser = new JSONParser();
ArrayList<HashMap<String, String>> booksList;
// JSON Node names
private static final String TAG_SUCCESS = "success";
private static final String TAG_BOOKS = "catalogues";
private static final String TAG_BNAME = "b_name";
private static final String TAG_BAUTHOR = "b_author";
private static final String TAG_BID = "b_id";
// JSON Array
JSONArray books = null;
@Override
public void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
Log.d("CheckLib","Reached activity B");
setContentView(R.layout.result_view_item);
// Initiate the static variables
sOption = extras.getString("sOption");
sBy = extras.getString("sValue");
URL = getURLVal(sOption);
// Hashmap for ListView
booksList = new ArrayList<HashMap<String, String>>();
// Loading products in Background Thread
new LoadResults().execute();
// Get listview
// ListView lv = getListView();
}
private static String getURLVal(String sval) {
// TODO Auto-generated method stub
String URL = "";
switch (sval) {
case "B":
URL = "http://10.0.2.2:1234/webservices/b_search.php";
case "A":
URL = "http://10.0.2.2:1234/webservices/a_search.php";
case "P":
URL = "http://10.0.2.2:1234/webservices/p_search.php";
case "K":
URL = "http://10.0.2.2:1234/webservices/k_search.php";
default:
Log.i("Error", "Option by not found");
}
return URL;
}
// Background Async Task to Load all product by making HTTP Request
class LoadResults extends AsyncTask<String, String, String> {
// Progress Dialog
@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
pDialog = new ProgressDialog(B_Search.this);
pDialog.setMessage("Loading results. Please wait...");
pDialog.setIndeterminate(false);
pDialog.setCancelable(false);
pDialog.show();
}
// Setting URL to be chosen
@Override
protected String doInBackground(String... args) {
// TODO Auto-generated method stub
List<NameValuePair> params = new ArrayList<NameValuePair>();
params.add(new BasicNameValuePair("searchvalue", sBy));
Log.d("request!", "starting");
// Make HTTP Request
JSONObject json = jParser.makeHttpRequest(URL, "POST", params);
Log.d("Login attempt", json.toString());
try {
// Checking for SUCCESS TAG
int success = json.getInt(TAG_SUCCESS);
if (success == 1) {
// Books found
// Getting Array of books
books = json.getJSONArray(TAG_BOOKS);
// looping through All books in array
for (int i = 0; i < books.length(); i++) {
JSONObject temp = books.getJSONObject(i);
// Storing each JSON item in variable
String id = temp.getString(TAG_BID);
String name = temp.getString(TAG_BNAME);
String author = temp.getString(TAG_BAUTHOR);
// creating new HashMap
HashMap<String, String> map = new HashMap<String, String>();
// adding each child node to HashMap key => value
map.put(TAG_BID, id);
map.put(TAG_BNAME, name);
map.put(TAG_BAUTHOR, author);
// adding HashList to ArrayList
booksList.add(map);
}
} else {
// no books found
// Launch Main Activity
Toast.makeText(getApplicationContext(),
"No Books Found. Try Again!!", Toast.LENGTH_LONG)
.show();
Intent mainact = new Intent(getApplicationContext(),
MainActivity.class);
// Closing all previous activities
startActivity(mainact);
}
} catch (JSONException e) {
e.printStackTrace();
}
return null;
}
// After completing background task Dismiss the progress dialog
protected void onPostExecute(String file_url) {
// dismiss the dialog after getting the results
pDialog.dismiss();
// updating UI from Background Thread
runOnUiThread(new Runnable() {
public void run() {
// Updating parsed JSON data into ListView
ListAdapter adapter = new SimpleAdapter(B_Search.this,
booksList, R.layout.result_view_item, new String[] {
TAG_BID, TAG_BNAME, TAG_BAUTHOR },
new int[] { R.id.b_id, R.id.b_name, R.id.b_author });
// updating listview
setListAdapter(adapter);
}
});
}
}
}
答案 0 :(得分：1)
Bundle extras = getIntent().getExtras();
您过早地调用getIntent()并返回null。将此代码移至onCreate()。
在尝试检索额外值之前，还需要检查getExtras()返回非null值。
答案 1 :(得分：0)
您正在使用ListActivity，相应的xml必须具有ListView且ID为android:id="@android:id/list"的xml文件应该如下所示
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<ListView
android:id="@android:id/list"
android:layout_height="wrap_content"
android:layout_width="match_parent">
</ListView>
</RelativeLayout>
答案 2 :(得分：-1)
将MainActivity.this更改为v.getContext()
我认为错误发生在android:name="B_Search"，我认为您需要在名称属性之前加上一段时间：android:name=".B_Search"