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问题:

I am making an android app using android 2.2 and eclipse.

There are two workflows of the app:

WF1: CoverPageApp -> LoginActivity -> Dashboard.

WF2: CoverPageApp -> RegisterActivity -> Dashboard.

But as I click on the Start Button in CoverPageApp to go on another activity, i.e LoginActivity, the app force closes. I have also included the LogCat which shows error of Null Exception and in the LoginActivity Java File it points on the line 51: btnLinkToRegistrScrn = (Button) findViewById(R.id.LinkToRegisterScreen);

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android"     package="com.app.android"     android:versionCode="1"     android:versionName="1.0" >      <uses-sdk android:targetSdkVersion="15" android:minSdkVersion="8" />      <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />     <uses-permission android:name="android.permission.INTERNET" />      <application         android:icon="@drawable/ic_launcher"         android:label="@string/app_name" >          <activity             android:enabled="true"             android:name=".PageApp"             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:enabled="true"             android:name=".LoginActivity1"             android:label="Login Activity" >         </activity>          <activity             android:enabled="true"             android:name=".CAActivity"             android:label="Register Activity" >         </activity>          <activity             android:enabled="true"             android:name=".DashboardActivity"             android:label="Dashboard Activity" >         </activity>      </application>    </manifest>

PageApp.java

import android.app.Activity; import android.os.Bundle; //import android.content.Context; import android.content.Intent; import android.widget.Button; import android.view.View;  public class PageApp extends Activity {      Button startbutton;      /** Called when the activity is first created. */     @Override     public void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.coverpage);         addListenerOnButton();    }       public void addListenerOnButton() {          //final Context context1 = this;          startbutton = (Button) findViewById(R.id.button1);          startbutton.setOnClickListener(new View.OnClickListener() {              @Override             public void onClick(View arg0) {                 Intent intent1 = new Intent(arg0.getContext(), LoginActivity1.class);              //    intent1.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);                 startActivity(intent1);                  finish();                 }             });      }  }

Log.java

import android.app.Activity; //import android.content.Context; import android.content.Intent; import android.os.Bundle; import android.view.View; //import android.view.View.OnClickListener; import android.widget.Button; //import android.widget.TextView;   public class Log extends Activity {      Button btnLinkToRegistrScrn;     Button loginbtn1;      /** Called when the activity is first created. */     @Override     public void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);      setContentView(R.layout.login);      addListenerOnButton(); //     btnLinkToRegistrScrn = (Button) findViewById(R.id.LinkToRegisterScreen);      }       public void addListenerOnButton() {          //final Context context2 = this;              loginbtn1 = (Button) findViewById(R.id.btnLogin);              loginbtn1.setOnClickListener(new View.OnClickListener() {              @Override             public void onClick(View arg0) {                  Intent intent = new Intent(arg0.getContext(), DashboardActivity.class);                 intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);                          startActivity(intent);                }          });     }       {          // Link to Register Screen          btnLinkToRegistrScrn = (Button) findViewById(R.id.LinkToRegisterScreen);          btnLinkToRegistrScrn.setOnClickListener(new View.OnClickListener() {             @Override             public void onClick(View v) {                  Intent i = new Intent(v.getContext(), CAaactivity.class);                //  i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);                 startActivity(i);                 }         });       }        }

LOGCAT

05-31 17:53:19.691: D/AndroidRuntime(1958): Shutting down VM 05-31 17:53:19.710: W/dalvikvm(1958): threadid=1: thread exiting with uncaught exception (group=0x4001d800) 05-31 17:53:19.730: E/AndroidRuntime(1958): FATAL EXCEPTION: main 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.ActivityThread.access$2300(ActivityThread.java:125) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.os.Handler.dispatchMessage(Handler.java:99) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.os.Looper.loop(Looper.java:123) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.ActivityThread.main(ActivityThread.java:4627) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at java.lang.reflect.Method.invokeNative(Native Method) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at java.lang.reflect.Method.invoke(Method.java:521) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at dalvik.system.NativeStart.main(Native Method) 05-31 17:53:19.730: E/AndroidRuntime(1958): Caused by: java.lang.NullPointerException 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.Activity.findViewById\untime(1958):  at java.lang.Class.newInstanceImpl(Native Method) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at java.lang.Class.newInstance(Class.java:1429) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.Instrumentation.newActivity(Instrumentation.java:1021) 05-31 17:53:19.730: E/AndroidRuntime(1958):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577) 05-31 17:53:19.730: E/AndroidRuntime(1958):     ... 11 more

回答1:

Change Your LoginActivity1 Activity as:

public class LoginActivity1 extends Activity {      Button btnLinkToRegistrScrn;     Button loginbtn1;      /** Called when the activity is first created. */     @Override     public void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);      setContentView(R.layout.login);      addListenerOnButton();         // Link to Register Screen          btnLinkToRegistrScrn = (Button) findViewById(R.id.LinkToRegisterScreen);          btnLinkToRegistrScrn.setOnClickListener(new View.OnClickListener() {             @Override             public void onClick(View v) {                  Intent i = new Intent(v.getContext(), TrekEyesAndroidActivity.class);                //  i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);                 startActivity(i);                 }         });     }     public void addListenerOnButton() {          //final Context context2 = this;              loginbtn1 = (Button) findViewById(R.id.btnLogin);              loginbtn1.setOnClickListener(new View.OnClickListener() {              @Override             public void onClick(View arg0) {                  Intent intent = new Intent(arg0.getContext(), DashboardActivity.class);                 intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);                          startActivity(intent);                }          });     }    }

and in xml change TextView to Button as

<Button          android:id="@+id/LinkToRegisterScreen"             android:layout_width="fill_parent"             android:layout_height="wrap_content"             android:layout_gravity="center"             android:layout_marginTop="25dip"              android:textColor="#21dbd4"             android:textStyle="bold"              android:text="@string/noAccountRegisterME" />

回答2:

in Login.xml you declared

<TextView     android:id="@+id/LinkToRegisterScreen"     android:layout_width="fill_parent"     android:layout_height="wrap_content"/>

and while mapping LoginActivity1.java

 btnLinkToRegistrScrn = (Button) findViewById(R.id.LinkToRegisterScreen);

so just change the TextView to Button

<Button     android:id="@+id/LinkToRegisterScreen"     android:layout_width="fill_parent"     android:layout_height="wrap_content"/>

回答3:

In your login.xml

<TextView             android:id="@+id/LinkToRegisterScreen"             android:layout_width="fill_parent"             android:layout_height="wrap_content"             android:layout_gravity="center"             android:layout_marginTop="25dip"              android:textColor="#21dbd4"             android:textStyle="bold"              android:text="@string/noAccountRegisterME" />

android:id="@+id/LinkToRegisterScreen" and it is EditText and you are doing btnLinkToLoginScrn = (Button) findViewById(R.id.LinkToLoginScreen); so it would be null pointer..

so just change Button instead of TextView in your login.xml

回答4:

i think u should concentrate the braces.you give the code to add listeneraddListenerOnButton()

文章来源: Android app force closes while going on another activity

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