我正在为我们的顶点项目做一个Android应用程序。我正在尝试将我放在主屏幕中的唯一按钮用于登录目的。显然，我的代码还没有完成，因为这只是用于检查用户帐户，这意味着我还没有使用意图去另一个屏幕。单击此特定按钮（id为loginBtn）后，我正在捕获“空指针异常”。任何帮助将不胜感激。
这是MainActivity类
package com.feufern.apms;
import java.util.ArrayList;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.ResponseHandler;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.BasicResponseHandler;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import android.os.Bundle;
import android.os.Handler;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.animation.Animation;
import android.view.animation.AnimationUtils;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends Activity implements OnClickListener{
//login
Button b;
EditText et,pass;
TextView tv;
RadioGroup radioUserGroup;
RadioButton radioUserButton;
//json
HttpPost httppost;
StringBuffer buffer;
HttpResponse response;
HttpClient httpclient;
List<NameValuePair> nameValuePairs;
ProgressDialog dialog = null;
//session
public static String student_id;
//slideshow variables
public int imageIndex=0;
Timer timer;
TimerTask task;
ImageView slidingImages;
//end of slideshow variables
private int[] IMAGE_ID = {R.drawable.slide1, R.drawable.slide2, R.drawable.slide3, R.drawable.slide4};
@Override
protected void onCreate(Bundle savedInstanceState) {
try{
super.onCreate(savedInstanceState);
this.setTitle("Welcome");
setContentView(R.layout.activity_main);
//login codes
b = (Button) findViewById(R.id.loginBtn);
et = (EditText)findViewById(R.id.username);
pass= (EditText)findViewById(R.id.password);
b.setOnClickListener(this);
}catch(Exception e){
Toast.makeText(getApplicationContext(),
"Error..." + e.getMessage(), Toast.LENGTH_LONG).show();
}
try{
//start of code for slideshow
final Handler mHandler = new Handler();
final Runnable mUpdateResults = new Runnable() {
public void run() {
SlideShow();
}
};
int delay = 200; //1sec
int period = 4000; //3sec
Timer timer = new Timer();
timer.scheduleAtFixedRate(new TimerTask() {
public void run() {
mHandler.post(mUpdateResults);
}
}, delay, period);
//end of code for slideshow
}catch(Exception e){
Toast.makeText(getApplicationContext(),
"Error..." + e.getMessage(), Toast.LENGTH_LONG).show();
}
}
private void SlideShow() {
slidingImages = (ImageView)findViewById(R.id.slideBanner);
slidingImages.setImageResource(IMAGE_ID[imageIndex%IMAGE_ID.length]);
imageIndex++;
Animation fadeInAnimation = AnimationUtils.loadAnimation(this, R.anim.fade_in);
slidingImages.startAnimation(fadeInAnimation);
}
void login(){
try{
httpclient=new DefaultHttpClient();
httppost= new HttpPost("http://10.0.2.2/mobile/check.php");
//add your data
nameValuePairs = new ArrayList<NameValuePair>(2);
// Always use the same variable name for posting i.e the android side variable name and php side variable name should be similar,
nameValuePairs.add(new BasicNameValuePair("username",et.getText().toString().trim())); // $Edittext_value = $_POST['Edittext_value'];
nameValuePairs.add(new BasicNameValuePair("password",pass.getText().toString().trim()));
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
//Execute HTTP Post Request
response=httpclient.execute(httppost);
ResponseHandler<String> responseHandler = new BasicResponseHandler();
final String response = httpclient.execute(httppost, responseHandler);
System.out.println("Response : " + response);
if(response.equalsIgnoreCase("User Found")){
runOnUiThread(new Runnable() {
public void run() {
Toast.makeText(MainActivity.this,"Login Success", Toast.LENGTH_SHORT).show();
}
});
startActivity(new Intent(MainActivity.this, MenuPage.class));
student_id = et.getText().toString().trim();
}else{
startActivity(new Intent(MainActivity.this, MenuPage.class));
student_id = et.getText().toString().trim();
}
}catch(Exception e){
dialog.dismiss();
System.out.println("Exception : " + e.getMessage());
}
}
void showAlert(){
try{
MainActivity.this.runOnUiThread(new Runnable() {
public void run() {
AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
builder.setTitle("Login Error.");
builder.setMessage("User not Found.")
.setCancelable(false)
.setPositiveButton("OK", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
}
});
AlertDialog alert = builder.create();
alert.show();
}
});
}catch(Exception e){
Toast.makeText(getApplicationContext(),
"Error..." + e.toString(), Toast.LENGTH_LONG).show();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
switch(v.getId()){
case R.id.loginBtn:
try{
login();
}catch(Exception e){
Toast.makeText(getApplicationContext(),
"Error..." + e.getMessage(), Toast.LENGTH_LONG).show();
}
}
}
}
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_gravity="center|center_vertical"
android:background="@color/firstbgcolor"
android:orientation="vertical" >
<ImageView
android:id="@+id/imageView1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:adjustViewBounds="true"
android:src="@drawable/banner" />
<ImageView
android:id="@+id/slideBanner"
android:layout_width="match_parent"
android:layout_height="177dp"
android:adjustViewBounds="true"
android:src="@drawable/slide1" />
<FrameLayout
android:layout_width="match_parent"
android:layout_height="23dp" >
</FrameLayout>
<FrameLayout
android:layout_width="match_parent"
android:layout_height="0dip"
android:layout_weight="0.51" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent" >
<FrameLayout
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="0.08" >
</FrameLayout>
<ScrollView
android:id="@+id/scrollView1"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="0.18" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical" >
<TextView
android:id="@+id/textView1"
android:layout_width="103dp"
android:layout_height="wrap_content"
android:text="Username"
android:textAppearance="?android:attr/textAppearanceSmall" />
<EditText
android:id="@+id/username"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<requestFocus />
</EditText>
<TextView
android:id="@+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Password"
android:textAppearance="?android:attr/textAppearanceSmall" />
<EditText
android:id="@+id/password"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:ems="10"
android:inputType="textPassword" />
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="0.74"
android:text="Login as: "
android:textAppearance="?android:attr/textAppearanceSmall" />
</LinearLayout>
<Button
android:id="@+id/loginBtn"
style="?android:attr/buttonStyleSmall"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:text="Login" />
</LinearLayout>
</ScrollView>
<FrameLayout
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="0.08" >
</FrameLayout>
</LinearLayout>
</FrameLayout>
</LinearLayout>
和清单文件
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.feufern.apms"
android:versionCode="1"
android:versionName="1.0" xmlns:tools="http://schemas.android.com/tools">
<uses-sdk
android:minSdkVersion="10"
android:targetSdkVersion="14" tools:ignore="OldTargetApi"/>
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
<application
android:allowBackup="true"
android:icon="@drawable/logo"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:screenOrientation="portrait"
android:name="com.feufern.apms.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:screenOrientation="portrait"
android:name=".MenuPage"
android:label="@string/app_name" />
</application>
</manifest>
我不知道为什么我会收到这个错误。 :(请帮忙。谢谢！
答案 0 :(得分：1)
你在下面的行
获得NullPointerExceptiondialog.dismiss();
由于您尚未初始化对话框。
进一步捕捉一般异常不是一个好习惯。它抑制了一些其他的bug。因此，请检查您的日志，为什么您的代码进入了异常处理程序，其中调用了dismiss