Javascript Android应用程序在收到意向包时崩溃？

首先，我要说我是一个完全的新手，这是我的第一个项目，我已经尝试到处搜索来回答我的问题

我在android上设置了一个登录屏幕。一旦用户点击登录按钮并且登录名/密码正确（在PHP/SQL中验证），它将向下一个活动发送一个意图/捆绑包

以下是发送意向/捆绑包的代码：

Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
Intent intent2 = new Intent(this, Homepage.class);
intent2.putExtra("username2", username2);
startActivity(intent);

以下是在下一个活动中接收意向包的代码：

   Intent intent2 = getIntent(); 
   String usernamefromlogin = intent2.getExtras().getString("username2");
   String url5 = url.concat(usernamefromlogin);
   TextView text = (TextView) findViewById(R.id.errorchecking);
   text.setText(url5);

Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
intent.putExtra("username2", username2);
startActivity(intent);

我使用concat（）将URL放在意图包之前，因为该URL将用作PHP文件的_GET命令（但我离题了）。目前，我已将其设置为将文本放置到文本视图中，以便查看最终结果，但在我到达该点之前，应用程序崩溃

当我点击登录按钮时，我的应用程序崩溃。这是我得到的日志：

11-30 20:06:39.449: E/AndroidRuntime(4578): FATAL EXCEPTION: main
11-30 20:06:39.449: E/AndroidRuntime(4578): Process: com.sencide, PID: 4578
11-30 20:06:39.449: E/AndroidRuntime(4578): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sencide/com.sencide.Homepage}: java.lang.NullPointerException
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.ActivityThread.access$700(ActivityThread.java:135)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.os.Handler.dispatchMessage(Handler.java:102)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.os.Looper.loop(Looper.java:137)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.ActivityThread.main(ActivityThread.java:4998)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at java.lang.reflect.Method.invokeNative(Native Method)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at java.lang.reflect.Method.invoke(Method.java:515)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at dalvik.system.NativeStart.main(Native Method)
11-30 20:06:39.449: E/AndroidRuntime(4578): Caused by: java.lang.NullPointerException
11-30 20:06:39.449: E/AndroidRuntime(4578):     at com.sencide.Homepage.onCreate(Homepage.java:46)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.Activity.performCreate(Activity.java:5243)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
11-30 20:06:39.449: E/AndroidRuntime(4578):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
11-30 20:06:39.449: E/AndroidRuntime(4578):     ... 11 more
11-30 20:06:42.569: I/Process(4578): Sending signal. PID: 4578 SIG: 9

这是我的全部代码：

AndroidLogin.java

public class AndroidLogin extends Activity implements OnClickListener {


Button ok,back,exit;
TextView result;
String thisisausername;



/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
   setContentView(R.layout.main);



    // Login button clicked
    ok = (Button)findViewById(R.id.btn_login);
    ok.setOnClickListener(this);


    result = (TextView)findViewById(R.id.tbl_result);



}









public void postLoginData() {
    // Create a new HttpClient and Post Header
    HttpClient httpclient = new DefaultHttpClient();

   // login.php returns true if username and password match in db 
    HttpPost httppost = new HttpPost("http://10.0.2.2/login.php");

    try {




        // Add user name and password
        EditText uname = (EditText)findViewById(R.id.txt_username);
        String username = uname.getText().toString();



        EditText pword = (EditText)findViewById(R.id.txt_password);
        String password = pword.getText().toString();

        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
        nameValuePairs.add(new BasicNameValuePair("username", username));
        nameValuePairs.add(new BasicNameValuePair("password", password));
        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

        // Execute HTTP Post Request
        Log.w("SENCIDE", "Execute HTTP Post Request");
        HttpResponse response = httpclient.execute(httppost);

        String str = inputStreamToString(response.getEntity().getContent()).toString();
        Log.w("SENCIDE", str);

        if(str.toString().equalsIgnoreCase("true"))
        {
            Log.w("SENCIDE", "TRUE");
            result.setText("Login Successful! Please Wait...");   
        }else
        {
            Log.w("SENCIDE", "FALSE");
            result.setText(str);                
        }

    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
} 

private StringBuilder inputStreamToString(InputStream is) {
    String line = "";
    StringBuilder total = new StringBuilder();
    // Wrap a BufferedReader around the InputStream
    BufferedReader rd = new BufferedReader(new InputStreamReader(is));
    // Read response until the end
    try {
        while ((line = rd.readLine()) != null) { 
            total.append(line); 
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
    // Return full string
    return total;
}


public void RegisterButton(View view) {
    String url = "http://10.0.2.2/register.php";
    Intent i = new Intent(Intent.ACTION_VIEW);
    i.setData(Uri.parse(url));
    startActivity(i);   
}




@Override
public void onClick(View view) {
            postLoginData();




            // turns the text in the textview "Tbl_result" into a text string called "tblresult"
            TextView tblresult = (TextView) findViewById(R.id.tbl_result);




    // If "tblresult" text string matches the string "Login Successful! Please Wait..." exactly, it will switch to next activity
            if (tblresult.getText().toString().equals("Login Successful! Please Wait...")) {
                  Intent intent = new Intent(this, Homepage.class);
                  EditText uname2 = (EditText)findViewById(R.id.txt_username);
                  String username2 = uname2.getText().toString();
                  intent.putExtra("username2", username2);
                  startActivity(intent);
               }    






}



}

JSONParser.java

public class JSONParser {

static InputStream is = null;
static JSONObject jObj = null;
static String json = "";

// constructor
public JSONParser() {

}

public JSONObject getJSONFromUrl(String url) {

    // Making HTTP request
    try {
        // defaultHttpClient
        DefaultHttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(url);

        HttpResponse httpResponse = httpClient.execute(httpPost);
        HttpEntity httpEntity = httpResponse.getEntity();
        is = httpEntity.getContent();           

    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(
                is, "iso-8859-1"), 8);
        StringBuilder sb = new StringBuilder();
        String line = null;
        while ((line = reader.readLine()) != null) {
            sb.append(line + "\n");
        }
        is.close();
        json = sb.toString();
    } catch (Exception e) {
        Log.e("Buffer Error", "Error converting result " + e.toString());
    }

    // try parse the string to a JSON object
    try {
        jObj = new JSONObject(json);
    } catch (JSONException e) {
        Log.e("JSON Parser", "Error parsing data " + e.toString());
    }

    // return JSON String
    return jObj;

}
}

Main.XML（第一个登录屏幕）

reshomepage.XML（我试图将意图发送到的第二个屏幕）

---

这里的问题是，您创建了两个意图（intent和intent2），然后将用户名保存在intent2中，但正在调用startActivity（intent）

因此，基本上，删除其中一个意图，您的代码应该如下所示：

Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
intent.putExtra("username2", username2);
startActivity(intent);

---

让我知道这是如何进行的

这里的问题是您已将用户名2附加到intent2，但您没有使用intent2

您启动活动，然后在传入的意图中查找额外的“username2”，因为您发送的意图不包含您从login获得的额外“username2”，并为usernamefromlogin分配了null值，从而导致堆栈跟踪中出现null指针异常

只需去掉第二个意图，并在开始活动时使用的字符串中添加额外的字符串：

   Intent intent2 = getIntent(); 
   String usernamefromlogin = intent2.getExtras().getString("username2");
   String url5 = url.concat(usernamefromlogin);
   TextView text = (TextView) findViewById(R.id.errorchecking);
   text.setText(url5);

Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
intent.putExtra("username2", username2);
startActivity(intent);

为了安全起见，在你跑掉并使用它之前，也许要确保它有额外的意图

Intent intent = getIntent(); 
if (intent.hasExtra("username2") {
   String usernamefromlogin = intent.getExtras().getString("username2");
   String url5 = url.concat(usernamefromlogin);
   TextView text = (TextView) findViewById(R.id.errorchecking);
   text.setText(url5);
} else {
    // ...appropriate error handling goes here!
}

---

祝你好运

感谢您的快速回复，我现在在Logcat中发现了这个错误：有什么想法吗？我添加了全部代码以及第49行：

user=json.getJSONArray（TAG_user）？如果是，请粘贴json数据的演示。如果第49行与ui有关，请粘贴布局xml代码。非常感谢@Aftab的帮助。我刚刚发布了我剩下的代码。main.xml文件中的第49行是第一个活动中的登录按钮。我指的是
HomePage.java
中的第49行。不确定是否添加了额外的行或删除了任何行。LogCat显示导致错误的行，这使得修复变得容易。因此，如果您对代码进行了任何更改，请再次运行该应用程序，并从logCat中给出导致错误的行。应该是<代码>由…引起的。。。。（HomePage.Java:49）

看起来像“text.setText（url5）；”是第49行。所以它在将文本设置为文本视图时遇到问题？这可能是什么原因？我甚至试着删除“TextView text=（TextView）findViewById（R.id.errorchecking）；”并用“errorchecking.setText（url5）；”替换setText，但我得到的错误仍然在同一行。我的猜测是字符串url5的格式不正确，有没有办法找出原因？

Intent intent = getIntent(); 
if (intent.hasExtra("username2") {
   String usernamefromlogin = intent.getExtras().getString("username2");
   String url5 = url.concat(usernamefromlogin);
   TextView text = (TextView) findViewById(R.id.errorchecking);
   text.setText(url5);
} else {
    // ...appropriate error handling goes here!
}