在添加异步任务之前，我对代码进行了一些更改，我的应用程序可以正确地从远程服务器验证用户名和密码，但在登录成功消息消失时无法启动其他 Activity 。有人建议我添加异步任务，现在我已经添加了该任务，但是当我输入正确的用户名和密码时，它会停止工作。当我输入错误的用户名和密码时，其工作正常，显示错误的用户名密码消息。如果有人知道会发生什么错误，请帮助我。
代码-
public class LoActivity extends Activity {
Intent i;
Button signin;
TextView error;
CheckBox check;
String name = "", pass = "";
byte[] data;
HttpPost httppost;
StringBuffer buffer;
HttpResponse response;
HttpClient httpclient;
InputStream inputStream;
SharedPreferences app_preferences;
List<NameValuePair> nameValuePairs;
EditText editTextId, editTextP;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.login);
signin = (Button) findViewById(R.id.signin);
editTextId = (EditText) findViewById(R.id.editTextId);
editTextP = (EditText) findViewById(R.id.editTextP);
app_preferences = PreferenceManager.getDefaultSharedPreferences(this);
check = (CheckBox) findViewById(R.id.check);
String Str_user = app_preferences.getString("username", "0");
String Str_pass = app_preferences.getString("password", "0");
String Str_check = app_preferences.getString("checked", "no");
if (Str_check.equals("yes")) {
editTextId.setText(Str_user);
editTextP.setText(Str_pass);
check.setChecked(true);
}
signin.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
name = editTextId.getText().toString();
pass = editTextP.getText().toString();
String Str_check2 = app_preferences.getString("checked", "no");
if (Str_check2.equals("yes")) {
SharedPreferences.Editor editor = app_preferences.edit();
editor.putString("username", name);
editor.putString("password", pass);
editor.commit();
}
if (name.equals("") || pass.equals("")) {
Toast.makeText(Lo.this, "Blank Field..Please Enter", Toast.LENGTH_SHORT).show();
} else {
new LoginTask().execute();
}
}
});
check.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// Perform action on clicks, depending on whether it's now
// checked
SharedPreferences.Editor editor = app_preferences.edit();
if (((CheckBox) v).isChecked()) {
editor.putString("checked", "yes");
editor.commit();
} else {
editor.putString("checked", "no");
editor.commit();
}
}
});
}
public void Move_to_next() {
startActivity(new Intent(this, QuestionnActivity.class));
}
@SuppressLint("NewApi")
private class LoginTask extends AsyncTask <Void, Void, String> {
@SuppressLint("NewApi")
@Override
protected void onPreExecute()
{
super.onPreExecute();
// Show progress dialog here
}
@Override
protected String doInBackground(Void... arg0) {
try {
httpclient = new DefaultHttpClient();
httppost = new HttpPost("http://abc.com/login1.php");
// Add your data
nameValuePairs = new ArrayList<NameValuePair>(2);
nameValuePairs.add(new BasicNameValuePair("UserEmail", name.trim()));
nameValuePairs.add(new BasicNameValuePair("Password", pass.trim()));
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
// Execute HTTP Post Request
response = httpclient.execute(httppost);
inputStream = response.getEntity().getContent();
data = new byte[256];
buffer = new StringBuffer();
int len = 0;
while (-1 != (len = inputStream.read(data))) {
buffer.append(new String(data, 0, len));
}
inputStream.close();
return buffer.toString();
}
catch (Exception e)
{
e.printStackTrace();
}
return "";
}
@SuppressLint("NewApi")
@Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
// Hide progress dialog here
if (buffer.charAt(0) == 'Y') {
Toast.makeText(LoActivity.this, "login successfull", Toast.LENGTH_SHORT).show();
Move_to_next();
} else {
Toast.makeText(LoActivity.this, "Invalid Username or password", Toast.LENGTH_SHORT).show();
}
}
}
}
登录猫-
09-24 07:59:32.818: E/AndroidRuntime(17602): FATAL EXCEPTION: main
09-24 07:59:32.818: E/AndroidRuntime(17602): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.abc.cyk/com.abc.cyk.QuestionActivity}: java.lang.NullPointerException
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.os.Handler.dispatchMessage(Handler.java:99)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.os.Looper.loop(Looper.java:137)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.ActivityThread.main(ActivityThread.java:5041)
09-24 07:59:32.818: E/AndroidRuntime(17602): at java.lang.reflect.Method.invokeNative(Native Method)
09-24 07:59:32.818: E/AndroidRuntime(17602): at java.lang.reflect.Method.invoke(Method.java:511)
09-24 07:59:32.818: E/AndroidRuntime(17602): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
09-24 07:59:32.818: E/AndroidRuntime(17602): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
09-24 07:59:32.818: E/AndroidRuntime(17602): at dalvik.system.NativeStart.main(Native Method)
09-24 07:59:32.818: E/AndroidRuntime(17602): Caused by: java.lang.NullPointerException
09-24 07:59:32.818: E/AndroidRuntime(17602): at com.abc.cyk.QuestionActivity.processScreen(QuestionActivity.java:41)
09-24 07:59:32.818: E/AndroidRuntime(17602): at com.abc.cyk.QuestionActivity.onCreate(QuestionActivity.java:33)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.Activity.performCreate(Activity.java:5104)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
09-24 07:59:32.818: E/AndroidRuntime(17602): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
09-24 07:59:32.818: E/AndroidRuntime(17602): ... 11 more
09-24 07:59:38.557: I/Process(17602): Sending signal. PID: 17602 SIG: 9
问题 Activity -
public class QuestionActivity extends Activity implements OnClickListener{
private Question currentQ;
private GamePlay currentGame;
private CountDownTimer counterTimer;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.question);
processScreen();
}
/**
* Configure current game and get question
*/
private void processScreen()
{
currentGame = ((CYKApplication)getApplication()).getCurrentGame();
currentQ = currentGame.getNextQuestion();
Button nextBtn1 = (Button) findViewById(R.id.answer1);
nextBtn1.setOnClickListener(this);
Button nextBtn2 = (Button) findViewById(R.id.answer2);
nextBtn2.setOnClickListener(this);
Button nextBtn3 = (Button) findViewById(R.id.answer3);
nextBtn3.setOnClickListener(this);
Button nextBtn4 = (Button) findViewById(R.id.answer4);
nextBtn4.setOnClickListener(this);
Button nextBtn5 = (Button) findViewById(R.id.answer5);
nextBtn5.setOnClickListener(this);
/**
* Update the question and answer options..
*/
setQuestions();
}
/**
* Method to set the text for the question and answers from the current games
* current question
*/
private void setQuestions() {
//set the question text from current question
String question = Utility.capitalise(currentQ.getQuestion());
TextView qText = (TextView) findViewById(R.id.question);
qText.setText(question);
//set the available options
List<String> answers = currentQ.getQuestionOptions();
TextView option1 = (TextView) findViewById(R.id.answer1);
option1.setText(Utility.capitalise(answers.get(0)));
TextView option2 = (TextView) findViewById(R.id.answer2);
option2.setText(Utility.capitalise(answers.get(1)));
TextView option3 = (TextView) findViewById(R.id.answer3);
option3.setText(Utility.capitalise(answers.get(2)));
TextView option4 = (TextView) findViewById(R.id.answer4);
option4.setText(Utility.capitalise(answers.get(3)));
int score = currentGame.getScore();
String scr = String.valueOf(score);
TextView score1 = (TextView) findViewById(R.id.score);
score1.setText(scr);
counterTimer=new CountDownTimer(15000, 1000) {
public void onFinish() {
if(currentGame.getRound()==20)
System.exit(0);
currentGame.decrementScore();
processScreen();
}
public void onTick(long millisUntilFinished) {
TextView time = (TextView) findViewById(R.id.timers);
time.setText( ""+millisUntilFinished/1000);
}
};
counterTimer.start();
}
@Override
public void onResume() {
super.onResume();
}
@Override
public void onClick(View arg0) {
//Log.d("Questions", "Moving to next question");
if(arg0.getId()==R.id.answer5)
{
new AlertDialog.Builder(this)
.setMessage("Are you sure?")
.setCancelable(true)
.setPositiveButton("Yes",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog,
int id) {
finish();
}
}).setNegativeButton("No", null).show();
}
else
{
if(!checkAnswer(arg0)) return;
/**
* check if end of game
*/
if (currentGame.isGameOver()){
//Log.d("Questions", "End of game! lets add up the scores..");
//Log.d("Questions", "Questions Correct: " + currentGame.getRight());
//Log.d("Questions", "Questions Wrong: " + currentGame.getWrong());
Intent i = new Intent(this, EndgameActivity.class);
startActivity(i);
finish();
}
else
{
Intent i = new Intent(this, QuestionActivity.class);
finish();
startActivity(i);
}
}
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event)
{
switch (keyCode)
{
case KeyEvent.KEYCODE_BACK :
return true;
}
return super.onKeyDown(keyCode, event);
}
/**
* Check if a checkbox has been selected, and if it
* has then check if its correct and update gamescore
*/
private boolean checkAnswer(View v) {
final Button b = (Button) v;
String answer = b.getText().toString();
counterTimer.cancel();
b.setBackgroundResource(R.drawable.ans);
b.setEnabled(false);
//Log.d("Questions", "Valid Checkbox selection made - check if correct");
if (currentQ.getAnswer().equalsIgnoreCase(answer))
{
b.setBackgroundResource(R.drawable.ansgreen);
//Log.d("Questions", "Correct Answer!");
currentGame.incrementScore();
}
else{
b.setBackgroundResource(R.drawable.ansred);
//Log.d("Questions", "Incorrect Answer!");
currentGame.decrementScore1();
}
return true;
}
}
最佳答案
将评论转换为答案
通过查看 logcat，我们可以看到问题出在 QuestionActivity 的第 41 行。所以我们知道 currentGame 是 null。
您需要在此之前设置断点，并查看是什么导致该断点为 null。您可能已经发现其他变量/对象为null，这最终导致NPE。
关于java - 无法启动 Activity ？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19031225/