实现工具栏调用null对象引用

Question

我在实现操作栏(我认为它现在被工具栏取代了)时遇到了问题。我想要的是一个有3个点的小下拉菜单，在那里我可以有一些选项，比如设置和积分。我试着把它设置成菜单出现在homeistview.java页面上。我觉得我一直在尝试不同的解决方案，每次都会出现新的错误，而且它总是指向下面提到的同一位置。

setSupportActionBar(toolbar);

我最初认为这是因为我没有包含setContentView(R.layout.activity_main)；但它对包含2个内容视图并不满意。如有任何帮助，我们将不胜感激。

这是我的密码。

styles.xml

<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
<!-- Customize your theme here. -->
<item name="colorPrimary">@color/colorPrimary</item>
<item name="colorPrimaryDark">@color/colorPrimaryDark</item>
<item name="colorAccent">@color/colorAccent</item>
</style>
<style name="AppTheme.NoActionBar">
<item name="windowActionBar">false</item>
<item name="windowNoTitle">true</item>
</style>
<style name="AppTheme.AppBarOverlay" parent="ThemeOverlay.AppCompat.Dark.ActionBar" />
<style name="AppTheme.PopupOverlay" parent="ThemeOverlay.AppCompat.Light" />

---------------menu_main.xml-------------------

<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
tools:context="cs495capstone.edu.bsu.myapplication.MainActivity">
<item
android:id="@+id/action_settings"
android:orderInCategory="100"
android:title="@string/action_settings"
app:showAsAction="never" />
<item
android:id="@+id/action_credits"
android:orderInCategory="100"
android:title="@string/action_credits"
app:showAsAction="never" />
<item
android:id="@+id/action_lovely"
android:orderInCategory="100"
android:title="@string/action_lovely"
app:showAsAction="never" />
</menu>

--------------activity_main.xml------------------

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:context="cs495capstone.edu.bsu.myapplication.HomeActivityListview">
<android.support.design.widget.AppBarLayout
android:layout_height="wrap_content"
android:layout_width="match_parent"
android:theme="@style/AppTheme.AppBarOverlay">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:popupTheme="@style/AppTheme.PopupOverlay" />
</android.support.design.widget.AppBarLayout>
<include layout="@layout/activity_add_appointment" />
<include layout="@layout/activity_add_history" />
<include layout="@layout/activity_appointments" />
<include layout="@layout/activity_dog_profile" />
<include layout="@layout/activity_history" />
<include layout="@layout/activity_home" />
<include layout="@layout/activity_home_activity_listview" />
<include layout="@layout/activity_login" />
<include layout="@layout/activity_new_dog" />
<include layout="@layout/activity_registration" />
<include layout="@layout/activity_splash_screen" />
<include layout="@layout/activity_update_dog" />
</android.support.design.widget.CoordinatorLayout>

-----------------------homeactivitylistview.xml------------------------

public class HomeActivityListview extends AppCompatActivity {
ListView lv;
Context context;
ArrayList dogName;
ArrayList dogID;
public static int [] dogImages={R.drawable.dogpic};
public static String [] dogNames={};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_home_activity_listview);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
context=this;
lv=(ListView) findViewById(R.id.listView);
lv.setAdapter(new CustomAdapter(this, dogImages));
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}

public void sendProfile(View view) {
Intent intent = new Intent(getApplicationContext(), DogProfileActivity.class);
startActivity(intent);
}
public void NumberOne(View view) {
AlertDialog.Builder alertDlg = new AlertDialog.Builder(this);
alertDlg.setMessage("Confirm Dog went number one");
alertDlg.setCancelable(false);
alertDlg.setPositiveButton("Confirm", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.setNegativeButton("Change", new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.create().show();
}
public void NumberTwo(View view) {
AlertDialog.Builder alertDlg = new AlertDialog.Builder(this);
alertDlg.setMessage("Confirm Dog went number two");
alertDlg.setCancelable(false);
alertDlg.setPositiveButton("Confirm", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.setNegativeButton("Change", new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.create().show();
}
public void AddNewDog(View view) {
Intent intent = new Intent(getApplicationContext(), NewDogActivity.class);
startActivity(intent);
}
}

------------------main_activity.java------------------

public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}

--------------homeistview.java------------------

public class HomeActivityListview extends AppCompatActivity {
ListView lv;
Context context;
ArrayList dogName;
ArrayList dogID;
public static int [] dogImages={R.drawable.dogpic};
public static String [] dogNames={};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_home_activity_listview);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
context=this;
lv=(ListView) findViewById(R.id.listView);
lv.setAdapter(new CustomAdapter(this, dogImages));
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}

public void sendProfile(View view) {
Intent intent = new Intent(getApplicationContext(), DogProfileActivity.class);
startActivity(intent);
}
public void NumberOne(View view) {
AlertDialog.Builder alertDlg = new AlertDialog.Builder(this);
alertDlg.setMessage("Confirm Dog went number one");
alertDlg.setCancelable(false);
alertDlg.setPositiveButton("Confirm", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.setNegativeButton("Change", new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.create().show();
}
public void NumberTwo(View view) {
AlertDialog.Builder alertDlg = new AlertDialog.Builder(this);
alertDlg.setMessage("Confirm Dog went number two");
alertDlg.setCancelable(false);
alertDlg.setPositiveButton("Confirm", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.setNegativeButton("Change", new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int which) {
//HomeActivity.super.onBackPressed();
}
});
alertDlg.create().show();
}
public void AddNewDog(View view) {
Intent intent = new Intent(getApplicationContext(), NewDogActivity.class);
startActivity(intent);
}
}

---------------androidmanifest------------------

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="cs495capstone.edu.bsu.doggydid" >
<uses-permission android:name="android.permission.INTERNET" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme" >
<activity android:name=".SplashScreenActivity" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".RegistrationActivity"
android:label="Registration" />
<activity
android:name=".NewDogActivity"
android:label="Dog Information" />
<activity
android:name=".LoginActivity"
android:label="Login" />
<activity
android:name=".HomeActivity"
android:label="@string/title_activity_home" />
<activity
android:name=".DogProfileActivity"
android:label="@string/title_activity_dog_profile" />
<activity
android:name=".HistoryActivity"
android:label="@string/title_activity_records" />
<activity
android:name=".addHistoryActivity"
android:label="@string/title_activity_add_record" />
<activity
android:name=".EventsActivity"
android:label="@string/title_activity_events" />
<activity
android:name=".addEventsActivity"
android:label="@string/title_activity_add_events" />
<activity
android:name=".UpdateDogActivity"
android:label="@string/title_activity_update_dog" />
<activity
android:name=".HomeActivityListview"
android:label="@string/title_activity_home_activity_listview"
android:theme="@style/AppTheme.NoActionBar">
</activity>
</application>
</manifest>

-----------------------错误读数--------------------

04-14 13:35:36.905 3117-3117/cs495capstone.edu.bsu.doggydid E/AndroidRuntime：致命异常：main流程：cs495capstone.edu.bsu.doggydid，PID:3117java.lang.RuntimeException：无法启动活动ComponentInfo｛cs495capstone.edu.bsu.doggydid/cs495capstone.edu.bsu.doggydid.HomeActivityListview｝：java.lang.NullPointerException：尝试在null对象引用上调用虚拟方法"java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()"在android.app.ActivityThread.performLaunchActivity(ActivityThreads.java：2298)在android.app.ActivityThread.handleLaunchActivity(ActivityThreads.java:2360)在android.app.ActivityThread.access上$800(ActivityThreads.java:144)在android.app.ActivityThread$H.handleMessage(ActivityThreads.java:1278)在android.os.Handler.dispatchMessage(Handler.java:102)在android.os.Looper.loop(Looper.java:135)在android.app.ActivityThread.main(ActivityThreads.java:5221)位于java.lang.reflect.Method.ioke(本机方法)位于java.lang.reflect.Method.ioke(Method.java:372)网址：com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)网址：com.android.internal.os.ZygoteInit.main(ZygoteNit.java:694)由以下原因引起：java.lang.NullPointerException：试图对null对象引用调用虚拟方法"java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()"在android.support.v7.widget.ToolbarWidgetWrapper上。(ToolbarWidgetWrapper.java:98)在android.support.v7.widget.ToolbarWidgetWrapper上。(ToolbarWidgetWrapper.java:91)在android.support.v7.app.ToolbarActionBar.(ToolbarActionBar.java:73)在android.support.v7.app.AppCompatDelegateImplV7.setSupportActionBar(AppCompatDeleateImplV7.java:205)位于android.support.v7.app.AppCompatActivity.setSupportActionBar(AppCompatActivity.java:99)位于cs495capstone.edu.bsu.doggydid.HomeActivityListview.onCreate(HomeActivityListview.java:31)在android.app.Activity.performCreate(Activity.java:55933)在android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)在android.app.ActivityThread.performLaunchActivity(ActivityThreads.java:2251)在android.app.ActivityThread.handleLaunchActivity(ActivityThreads.java:2360)在android.app.ActivityThread.access上$800(ActivityThreads.java:144)在android.app.ActivityThread$H.handleMessage(ActivityThreads.java:1278)在android.os.Handler.dispatchMessage(Handler.java:102)在android.os.Looper.loop(Looper.java:135)在android.app.ActivityThread.main(ActivityThreads.java:5221)位于java.lang.reflect.Method.ioke(本机方法)位于java.lang.reflect.Method.ioke(Method.java:372)网址：com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)网址：com.android.internal.os.ZygoteInit.main(ZygoteNit.java:694)

Answer 1

您的Toolbar工具栏仅在onCreate中声明/初始化，因此作用域超出了该方法。。。

将对象定义为活动变量，并在onCreate中对其进行初始化

public class HomeActivityListview extends AppCompatActivity {
ListView lv;
Context context;
ArrayList dogName;
ArrayList dogID;
public static int [] dogImages={R.drawable.dogpic};
public static String [] dogNames={};
private Toolbar toolbar;  //define it  here
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_home_activity_listview);
toolbar = (Toolbar) findViewById(R.id.toolbar);  //but init here
setSupportActionBar(toolbar);
context=this;
lv=(ListView) findViewById(R.id.listView);
lv.setAdapter(new CustomAdapter(this, dogImages));
}