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我正在爲我們的頂點項目做一個android應用程序。我正在嘗試將唯一的按鈕放在主屏幕中用於登錄目的。顯然，我的代碼還沒有完成，因爲這只是爲了檢查用戶帳戶，這意味着我沒有使用意圖還沒有去另一個屏幕。點擊這個特定的按鈕（id是loginBtn），我正在捕捉一個「空指針異常」。任何幫助將不勝感激。點擊按鈕顯示空指針異常錯誤
這裏是在MainActivity類
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();
}
}
}
}
的activity_main.xml中
<?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>
和清單文件
<?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>
我不知道爲什麼我得到這個錯誤。 :(請幫忙，謝謝！
你確定你得到一個NPE，而不是NetworkOnMainThreadException？ – Blackbelt
這樣一長段代碼沒有行號和沒有異常堆棧指定異常行。 Ahhhhh痛苦。在您的最後搜索該行。並在調用其中的任何方法之前查看該語句中的對象已初始化並且不爲null。 – Javanator
@blackbelt是的，我敢肯定，因爲我使用了try/catch塊 –