获取致命异常：主错误

Question

我一直在设计我的应用程序的登录和注册屏幕。更改我的主要活动中的操作栏设置后，我似乎无法登录到应用程序。登录和注册屏幕加载完美。但后来它说：“不幸的是，____已经停止了”这是我得到的错误，我还包含了我的LoginActivity.java，因为它说错误在哪里。但我不知道要引用哪个其他文件，所以如果您需要另一个文件，请告诉我，我会更新它。谢谢！

1493-1493/com.example.aalok.chirp E/AndroidRuntime﹕ FATAL EXCEPTION: main 
Process: com.example.aalok.chirp, PID: 1493 
java.lang.NullPointerException 
     at com.example.aalok.chirp.LoginActivity$2.onClick(LoginActivity.java:58) 
     at android.view.View.performClick(View.java:4438) 
     at android.view.View$PerformClick.run(View.java:18422) 
     at android.os.Handler.handleCallback(Handler.java:733) 
     at android.os.Handler.dispatchMessage(Handler.java:95) 
     at android.os.Looper.loop(Looper.java:136) 
     at android.app.ActivityThread.main(ActivityThread.java:5017) 
     at java.lang.reflect.Method.invokeNative(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:515) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
     at dalvik.system.NativeStart.main(Native Method) 


    package com.example.aalok.chirp; 

import android.app.ActionBar; 
import android.app.Activity; 
import android.app.AlertDialog; 
import android.content.Intent; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.Window; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 

import com.parse.LogInCallback; 
import com.parse.Parse; 
import com.parse.ParseException; 
import com.parse.ParseUser; 
import com.parse.SignUpCallback; 


public class LoginActivity extends Activity { 

    protected EditText mUsername; 
    protected EditText mPassword; 
    protected Button mLoginButton; 
    protected TextView mSignUpTextView; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS); 
     setContentView(R.layout.activity_login); 

     ActionBar actionBar = getActionBar(); 
     actionBar.hide(); 

     //Set up SignUp text 
     mSignUpTextView = (TextView) findViewById(R.id.signUpText); 
     mSignUpTextView.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       Intent intent = new Intent(LoginActivity.this, SignUpActivity.class); 
       startActivity(intent); 
      } 
     }); 

     mUsername = (EditText) findViewById(R.id.usernameField); 
     mPassword = (EditText) findViewById(R.id.passwordField); 

     //Set up Login button 
     mLoginButton = (Button) findViewById(R.id.loginButton); 
     mLoginButton.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       String username = mUsername.getText().toString(); 
       String password = mPassword.getText().toString(); 

       username = username.trim(); 
       password = password.trim(); 

       if (username.isEmpty() || password.isEmpty()) { 
        AlertDialog.Builder builder = new AlertDialog.Builder(LoginActivity.this); 
        builder.setMessage(R.string.login_error_message) 
          .setTitle(R.string.login_error_title) 
          .setPositiveButton(android.R.string.ok, null); 
        AlertDialog dialog = builder.create(); 
        dialog.show(); 
       } else { 
        //LOGIN 
        setProgressBarIndeterminateVisibility(true); 

        ParseUser.logInInBackground(username, password, new LogInCallback() { 
         @Override 
         public void done(ParseUser user, ParseException e) { 
          setProgressBarIndeterminateVisibility(false); 
          if (e == null) { 
           //SUCCESS! 
           Intent intent = new Intent(LoginActivity.this, MainActivity.class); 
           intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); 
           intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK); 
           startActivity(intent); 
          } else { 
           AlertDialog.Builder builder = new AlertDialog.Builder(LoginActivity.this); 
           builder.setMessage(e.getMessage()) 
             .setTitle(R.string.login_error_title) 
             .setPositiveButton(android.R.string.ok, null); 
           AlertDialog dialog = builder.create(); 
           dialog.show(); 
          } 
         } 
        }); 
       } 

      } 
     }); 
    } 
} 

Answer 1

Process: com.example.aalok.chirp, PID: 1493 
java.lang.NullPointerException 
     at com.example.aalok.chirp.LoginActivity$2.onClick(LoginActivity.java:58) 
     at android.view.View.performClick(View.java:4438) 
     at android.view.View$PerformClick.run(View.java:18422) 

这意味着什么是行空58

mUsername = (EditText) findViewById(R.id.usernameField); 
mPassword = (EditText) findViewById(R.id.passwordField); 

//Set up Login button 
mLoginButton = (Button) findViewById(R.id.loginButton); 
mLoginButton.setOnClickListener(new View.OnClickListener() { 
    @Override 
    public void onClick(View view) { 
     String username = mUsername.getText().toString(); 
     String password = mPassword.getText().toString(); //this is line 58 

但根据你的代码，你初始化mPassword，所以如果它在布局中存在，它不能null。因此，我怀疑R.id.passwordField不存在R.layout.activity_login