android應用程序由於java.lang.NullPointerException而崩潰

Question

我正在遵循《 andorid開發指南》，在嘗試注冊帳戶時似乎遇到了錯誤，我知道問題所在和位置（第197行），但似乎無法解決問題，請在下面的logcat副本中查找和我的注冊碼，如果您還有其他信息，請發表評論，我將盡力提供幫助。

04-24 21:24:32.735: E/AndroidRuntime(30368): FATAL EXCEPTION: main
04-24 21:24:32.735: E/AndroidRuntime(30368): java.lang.NullPointerException
04-24 21:24:32.735: E/AndroidRuntime(30368):    at com.loggedin.Register$ProcessRegister.onPostExecute(Register.java:197)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at com.loggedin.Register$ProcessRegister.onPostExecute(Register.java:1)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at android.os.AsyncTask.finish(AsyncTask.java:631)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at android.os.Handler.dispatchMessage(Handler.java:99)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at android.os.Looper.loop(Looper.java:176)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at android.app.ActivityThread.main(ActivityThread.java:5419)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at java.lang.reflect.Method.invokeNative(Native Method)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at java.lang.reflect.Method.invoke(Method.java:525)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
04-24 21:24:32.735: E/AndroidRuntime(30368):    at dalvik.system.NativeStart.main(Native Method)
04-24 21:25:05.435: I/Process(30368): Sending signal. PID: 30368 SIG: 9

注冊java

 import android.app.Activity;
 import android.app.ProgressDialog;
 import android.content.Context; 
 import android.content.Intent;
 import android.net.ConnectivityManager;
 import android.net.NetworkInfo;
 import android.os.AsyncTask;
 import android.os.Bundle;
 import android.view.View;  
 import android.widget.Button;
 import android.widget.EditText;
 import android.widget.TextView;
 import android.widget.Toast;
 import com.loggedin.internal.DatabaseHandler;
 import com.loggedin.internal.UserFunctions;
 import org.json.JSONException;
 import org.json.JSONObject;
 import java.io.IOException;
 import java.net.HttpURLConnection;
 import java.net.MalformedURLException;
 import java.net.URL;

 public class Register extends Activity {
 /**
 *  JSON Response node names.
 **/
 private static String KEY_SUCCESS = "success";
 private static String KEY_UID = "id";
 private static String KEY_FIRSTNAME = "FirstName";
 private static String KEY_LASTNAME = "LastName";
 private static String KEY_USERNAME = "Username";
 private static String KEY_EMAIL = "email";
 private static String KEY_DOB = "DOB";
 private static String KEY_CREATED_AT = "created_at";
 private static String KEY_ERROR = "error";
 /**
 * Defining layout items.
 **/
 EditText inputFirstName;
 EditText inputLastName;
 EditText inputUsername;
 EditText inputEmail;
 EditText inputDOB;
 EditText inputPassword;
 Button registerbtn;
 TextView register_error;
 /**
 * Called when the activity is first created.
 */
 @Override
 public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_register);
 /**
 * Defining all layout items
 **/
    inputFirstName = (EditText) findViewById(R.id.FirstName);
    inputLastName = (EditText) findViewById(R.id.LastName);
    inputUsername = (EditText) findViewById(R.id.Username);
    inputEmail = (EditText) findViewById(R.id.email);
    inputDOB = (EditText) findViewById(R.id.DOB);
    inputPassword = (EditText) findViewById(R.id.Password);
    registerbtn = (Button) findViewById(R.id.registerbtn);
    register_error = (TextView) findViewById(R.id.register_error);
 /**
 * Button which Switches back to the login screen on clicked
 **/
    Button login = (Button) findViewById(R.id.bktologinbtn);
    login.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view) {
            Intent myIntent = new Intent(view.getContext(), Login.class);
            startActivityForResult(myIntent, 0);
            finish();
        }
    });
    /**
     * Register Button click event.
     * A Toast is set to alert when the fields are empty.
     * Another toast is set to alert Username must be 5 characters.
     **/
    registerbtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (  ( !inputUsername.getText().toString().equals("")) && ( !inputPassword.getText().toString().equals("")) && ( !inputFirstName.getText().toString().equals("")) && ( !inputLastName.getText().toString().equals(""))  && ( !inputDOB.getText().toString().equals("")) && ( !inputEmail.getText().toString().equals("")) )
            {
                if ( inputUsername.getText().toString().length() > 5 ){
                InternetAsync(view);
                }
                else
                {
                    Toast.makeText(getApplicationContext(),
                            "Username should be minimum 5 characters", Toast.LENGTH_SHORT).show();
                }
            }
            else
            {
                Toast.makeText(getApplicationContext(),
                        "One or more fields are empty", Toast.LENGTH_SHORT).show();
            }
        }
    });
   }
  /**
  * Async Task to check whether internet connection is working
  **/
   private class InternetCheck extends AsyncTask<String, Boolean, Boolean>   {
    private ProgressDialog nDialog;
    @Override
    protected void onPreExecute(){
        super.onPreExecute();
        nDialog = new ProgressDialog(Register.this);
        nDialog.setMessage("Loading..");
        nDialog.setTitle("Checking Network");
        nDialog.setIndeterminate(false);
        nDialog.setCancelable(true);
        nDialog.show();
    }
    @Override
    protected Boolean doInBackground(String... args){
 /**
  * Gets current device state and checks for working internet connection by trying Google.
  **/
        ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo netInfo = cm.getActiveNetworkInfo();
        if (netInfo != null && netInfo.isConnected()) {
            try {
                URL url = new URL("http://www.google.com");
                HttpURLConnection urlc = (HttpURLConnection) url.openConnection();
                urlc.setConnectTimeout(3000);
                urlc.connect();
                if (urlc.getResponseCode() == 200) {
                    return true;
                }
            } catch (MalformedURLException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        return false;
    }
    @Override
    protected void onPostExecute(Boolean th){
        if(th == true){
            nDialog.dismiss();
            new ProcessRegister().execute();
        }
        else{
            nDialog.dismiss();
            register_error.setText("Error in Network Connection");
        }
    }
   }
 private class ProcessRegister extends AsyncTask <String, String, JSONObject>{
 /**
  * Defining Process dialog
  **/
    private ProgressDialog pDialog;
    String email,Password,FirstName,LastName,DOB,Username;
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        inputUsername = (EditText) findViewById(R.id.Username);
        inputPassword = (EditText) findViewById(R.id.Password);
           FirstName = inputFirstName.getText().toString();
           LastName = inputLastName.getText().toString();
            email = inputEmail.getText().toString();
            DOB = inputDOB.getText().toString();
            Username= inputUsername.getText().toString();
            Password = inputPassword.getText().toString();
        pDialog = new ProgressDialog(Register.this);
        pDialog.setTitle("Contacting Servers");
        pDialog.setMessage("Registering ...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(true);
        pDialog.show();
    }
    @Override
    protected JSONObject doInBackground(String... args) {
    UserFunctions userFunction = new UserFunctions();
    JSONObject json = userFunction.registerUser(FirstName, LastName, DOB, email, Username, Password);
        return json;
    }
   @Override
    protected void onPostExecute(JSONObject json) {
   /**
    * Checks for success message.
    **/
       if (json ==null){

       }
            try {
                if (json.getString(KEY_SUCCESS) != null) {
                    register_error.setText("");
                    String res = json.getString(KEY_SUCCESS);
                    String red = json.getString(KEY_ERROR);
                    if(Integer.parseInt(res) == 1){
                        pDialog.setTitle("Getting Data");
                        pDialog.setMessage("Loading Info");
                        register_error.setText("Successfully Registered");
                        DatabaseHandler db = new DatabaseHandler(getApplicationContext());
                        JSONObject json_user = json.getJSONObject("user");
                        /**
                         * Removes all the previous data in the SQlite database
                         **/
                        UserFunctions logout = new UserFunctions();
                        logout.logoutUser(getApplicationContext());
                        db.addUser(json_user.getString(KEY_FIRSTNAME),json_user.getString(KEY_LASTNAME),json_user.getString(KEY_EMAIL),json_user.getString(KEY_USERNAME),json_user.getString(KEY_UID),json_user.getString(KEY_CREATED_AT));
                        /**
                         * Stores registered data in SQlite Database
                         * Launch Registered screen
                         **/
                        Intent registered = new Intent(getApplicationContext(), Registered.class);
                        /**
                         * Close all views before launching Registered screen
                        **/
                        registered.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                        pDialog.dismiss();
                        startActivity(registered);
                          finish();
                    }
                    else if (Integer.parseInt(red) ==2){
                        pDialog.dismiss();
                        register_error.setText("User already exists");
                    }
                    else if (Integer.parseInt(red) ==3){
                        pDialog.dismiss();
                        register_error.setText("Invalid Email id");
                    }
                }
                    else{
                    pDialog.dismiss();
                        register_error.setText("Error occured in registration");
                    }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
   }
    public void InternetAsync(View view){
        new InternetCheck().execute();
    }
    }

Answer 1

我可以看到三種可能性：

if (json ==null){

}

這什么也不做，json可以為null，

json.getString(KEY_SUCCESS)

可能是罪魁禍首。

另一種可能是pDialog或register_error 。 確保在引用之前檢查每個資源，您會看到每個人都在猜測它是哪一個。 實際上，NPE是最常見的例外之一，因此請確保在使用數據之前先對其進行驗證，以免使您頭疼。

解：

if (json ==null){
    return; //You can potentially Log the error too, if it helps.
}

要么

if(json != null) {
    try{
        if (json.getString(KEY_SUCCESS) != null) {
        //...
}

另外，添加以下檢查：

if(pDialog != null) {
    //Manipulate the element to do what you want.
}

和

if(register_error != null) {
    //Manipulate the element to do what you want.
}

Answer 2

好像json對象為null。 您應該以正確的方式進行處理。 現在，您只是忽略它。 這里：

   /**
   * Checks for success message.
   **/
   if (json ==null){
       return; // <-- just a sample, handle null in yuor way, don't ignore it
   }