Question

我知道这个问题已经存在，但我尝试了几个解决方案，没有一个对我有效。

就我而言，我需要在ActionBar中使用Switch按钮。

我尝试手动添加minSdk Verision
我尝试在定义的xml

item

编辑：代码中的异常点标有注释。它是主要的活动。：）

以下是我的一些代码：

mainactivity：

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_minepedia_main);

        switchButton = (Switch) findViewById(R.id.switch_button);

        //The following line throws an exception, because switchButton
        //is null, even though I tried to take it on the upper line.
        switchButton.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {

        @Override
        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {

        //The following is a local boolean variable
                if (isChecked) {
                    ONLINE_MODE_ENABLED = true;
                } else {
                    ONLINE_MODE_ENABLED = false;
                }

            }
        });
    }

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; --> main_actions.xml
    getMenuInflater().inflate(R.menu.main_actions, menu);
    return super.onCreateOptionsMenu(menu);
}

mainactivity_layout：

<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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".Minepedia_main">

    <LinearLayout
        android:layout_width="200dp"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:layout_centerInParent="true"
        android:gravity="center" >
        <Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/items_button"
            android:onClick="startItemsMenu" />
        <Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/about_button"
            android:onClick="startAboutScreen" />
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="bottom"
            android:paddingTop="50dp"
            android:orientation="vertical">
            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="@string/update_offline_button"
                android:onClick="startOfflineUpdate"/>
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textAppearance="?android:attr/textAppearanceSmall"
                android:text="@string/offline_sync_info"
                android:id="@+id/textView"
                android:layout_gravity="center_horizontal" />
        </LinearLayout>
    </LinearLayout>
</RelativeLayout>

main_actions.xml（菜单）：

<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:yourapp="http://schemas.android.com/apk/res-auto" >
    <!-- Search, should appear as action button -->
    <item
        android:id="@+id/myswitch"
        android:title=""
        android:showAsAction="always"
        android:actionLayout="@layout/switch_layout"
        />
</menu>

switch_layout.xml：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <Switch
        android:id="@+id/switch_button"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:text=""
        android:textOff="Offline"
        android:textOn="Online"/>
</RelativeLayout>

堆栈跟踪：

08-20 17:45:52.902    5534-5534/com.nubage.minepedia E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.nubage.minepedia, PID: 5534
    java.lang.NullPointerException
            at com.nubage.minepedia.Minepedia_main.onCreateOptionsMenu(Minepedia_main.java:63)
            at android.app.Activity.onCreatePanelMenu(Activity.java:2538)
            at com.android.internal.policy.impl.PhoneWindow.preparePanel(PhoneWindow.java:489)
            at com.android.internal.policy.impl.PhoneWindow.doInvalidatePanelMenu(PhoneWindow.java:853)
            at com.android.internal.policy.impl.PhoneWindow$1.run(PhoneWindow.java:273)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
            at android.view.Choreographer.doCallbacks(Choreographer.java:574)
            at android.view.Choreographer.doFrame(Choreographer.java:543)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5081)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

由于Switch按钮位于main_actions.xml内，因此您应使用menu.findItem中的onCreateOptionsMenu来访问菜单中的视图：

@Override
public boolean onCreateOptionsMenu(Menu menu) { 
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.main_actions, menu);

    MenuItem item = menu.findItem(R.id.switch_button);
    switchButton = (Switch)item.getActionView();

    return super.onCreateOptionsMenu(menu);
}

Answer 2

按照'ρяσѕρєяK'的回答，但改变他的以下代码行：

MenuItem item = menu.findItem(R.id.switch_button);

到此：

MenuItem item = menu.findItem(R.id.myswitch); //the id was changed!

那应该摆脱NullPointerException，希望这有助于：）

所以你应该有这样的东西：

@Override 
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_minepedia_main);
} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.main_actions, menu);

    MenuItem item = menu.findItem(R.id.myswitch);
    switchButton = (Switch)item.getActionView();

    switchButton.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener({

    @Override 
    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {

    //The following is a local boolean variable 
            if (isChecked) {
                ONLINE_MODE_ENABLED = true; 
            } else { 
                ONLINE_MODE_ENABLED = false; 
            } 

        } 
    }); 

    return super.onCreateOptionsMenu(menu);
}

另外更改此行：

android:actionLayout="@layout/switch_layout"

到此：

android:actionViewClass="android.widget.Switch"

这是在您的菜单xml中。如果您应用该更改，那么您正在使用android提供的切换按钮，并且必须正常工作。

Switch（Button）抛出NullPointerException

2 个答案: