当我运行任何布局时，默认情况下应首先运行启动活动布局，让我登录，然后转到我点击的页面/布局。登录或注册后，模拟器崩溃说＆＃34;不幸的是，app_name已停止工作。＆＃34;
这是布局之一
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.bignerdranch.android.assignment2.EmailReceiptActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text=" "
android:id="@+id/textView3"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="27dp"
android:text="@string/fragment_login_message"
android:layout_gravity="left|top"
android:id="@+id/fragment_login_message"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/username"
android:layout_gravity="center_horizontal|top"
android:layout_alignTop="@+id/fragment_login_message"
android:layout_toRightOf="@+id/fragment_login_message"
android:layout_toEndOf="@+id/fragment_login_message"
android:id="@+id/textView2"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="Send Receipt To:"
android:id="@+id/send_receipt_to"
android:layout_marginTop="40dp"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"/>
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="textEmailAddress"
android:ems="10"
android:id="@+id/email_send_receipt"
android:layout_below="@+id/send_receipt_to"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginTop="47dp"
android:text="Email Address:"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Send"
android:id="@+id/send_receipt_button"
android:layout_below="@+id/email_send_receipt"
android:layout_centerHorizontal="true"
android:layout_marginTop="35dp"/>
</RelativeLayout>
这是该布局的Java类
package com.bignerdranch.android.assignment2;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
public class EmailReceiptActivity extends AppCompatActivity {
Button btn_send;
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main_options_menu, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.logoff_menu_item) {
super.onOptionsItemSelected(item);
Toast.makeText(this, "Logging You Off...", Toast.LENGTH_LONG).show();
startActivity(new Intent(EmailReceiptActivity.this, LaunchActivity.class));
}
return true;
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_email_receipt);
btn_send = (Button)findViewById(R.id.send_receipt_button);
btn_send.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v){
//Use the name of this class, and the name class where you want to be taken when the button is clicked.
Intent intent = new Intent(EmailReceiptActivity.this, SuccessActivity.class);
startActivity(intent);
}
});
}
}
选项菜单
<menu
xmlns:android="http://schemas.android.com/apk/res/android">
<item
android:id="@+id/logoff_menu_item"
android:title="@string/logoff_menu_item">
</item>
</menu>
启动活动布局
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_horizontal"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.bignerdranch.android.assignment2.LaunchActivity">
<!-- Login progress -->
<ProgressBar
android:id="@+id/login_progress"
style="?android:attr/progressBarStyleLarge"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:visibility="gone"/>
<ScrollView
android:id="@+id/login_form"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:id="@+id/email_login_form"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<android.support.design.widget.TextInputLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<AutoCompleteTextView
android:id="@+id/email"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="@string/prompt_email"
android:inputType="textEmailAddress"
android:maxLines="1"
android:singleLine="true"/>
</android.support.design.widget.TextInputLayout>
<android.support.design.widget.TextInputLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<EditText
android:id="@+id/password"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="@string/prompt_password"
android:imeActionId="@+id/login"
android:imeActionLabel="@string/action_sign_in_short"
android:imeOptions="actionUnspecified"
android:inputType="textPassword"
android:maxLines="1"
android:singleLine="true"/>
</android.support.design.widget.TextInputLayout>
<Button
android:id="@+id/email_sign_in_button"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="16dp"
android:text="Sign In"
android:textStyle="bold"/>
<Button
android:id="@+id/email_register_button"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Register"
android:textStyle="bold"/>
</LinearLayout>
</ScrollView>
</LinearLayout>
LogCat错误：
09-12 07:23:57.680 21430-21430/com.bignerdranch.android.assignment2 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.bignerdranch.android.assignment2, PID: 21430
java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.MenuItem.setIntent(android.content.Intent)' on a null object reference
at com.bignerdranch.android.assignment2.UserAccountActivity.onCreateOptionsMenu(UserAccountActivity.java:22)
at android.app.Activity.onCreatePanelMenu(Activity.java:2823)
at android.support.v4.app.FragmentActivity.onCreatePanelMenu(FragmentActivity.java:340)
at android.support.v7.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:85)
at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.onCreatePanelMenu(AppCompatDelegateImplBase.java:258)
at android.support.v7.app.AppCompatDelegateImplV7.preparePanel(AppCompatDelegateImplV7.java:1320)
at android.support.v7.app.AppCompatDelegateImplV7.doInvalidatePanelMenu(AppCompatDelegateImplV7.java:1600)
at android.support.v7.app.AppCompatDelegateImplV7.access$100(AppCompatDelegateImplV7.java:92)
at android.support.v7.app.AppCompatDelegateImplV7$1.run(AppCompatDelegateImplV7.java:131)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
09-12 07:24:15.778 21645-21645/com.bignerdranch.android.assignment2 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.bignerdranch.android.assignment2, PID: 21645
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bignerdranch.android.assignment2/com.bignerdranch.android.assignment2.NewSessionActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.bignerdranch.android.assignment2.NewSessionActivity.onCreate(NewSessionActivity.java:40)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
09-12 07:30:02.938 24027-24027/com.bignerdranch.android.assignment2 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.bignerdranch.android.assignment2, PID: 24027
java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.MenuItem.setIntent(android.content.Intent)' on a null object reference
at com.bignerdranch.android.assignment2.UserAccountActivity.onCreateOptionsMenu(UserAccountActivity.java:22)
at android.app.Activity.onCreatePanelMenu(Activity.java:2823)
at android.support.v4.app.FragmentActivity.onCreatePanelMenu(FragmentActivity.java:340)
at android.support.v7.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:85)
at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.onCreatePanelMenu(AppCompatDelegateImplBase.java:258)
at android.support.v7.app.AppCompatDelegateImplV7.preparePanel(AppCompatDelegateImplV7.java:1320)
at android.support.v7.app.AppCompatDelegateImplV7.doInvalidatePanelMenu(AppCompatDelegateImplV7.java:1600)
at android.support.v7.app.AppCompatDelegateImplV7.access$100(AppCompatDelegateImplV7.java:92)
at android.support.v7.app.AppCompatDelegateImplV7$1.run(AppCompatDelegateImplV7.java:131)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)