我创建了一个带有Backarrow作为ImageButton的服务器工具栏。我想获得对这个Button的引用，但它总是返回Null。两个小时前它工作，但我改变了代码，我不知道现在有什么问题..如果你可以帮助我会很棒！
MainActivity.java
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Always cast your custom Toolbar here, and set it as the ActionBar.
Toolbar tb = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(tb);
// Get the ActionBar here to configure the way it behaves.
ActionBar ab = getSupportActionBar();
//ab.setHomeAsUpIndicator(R.drawable.ic_menu); // set a custom icon for the default home button
ab.setDisplayShowHomeEnabled(true); // show or hide the default home button
ab.setDisplayHomeAsUpEnabled(true);
ab.setDisplayShowCustomEnabled(true); // enable overriding the default toolbar layout
ab.setDisplayShowTitleEnabled(true); // disable the default title element here
ImageButton backar = (ImageButton) findViewById(R.id.backarrow);
backar.setOnClickListener(this);
backar.setOnClickListener(new View.OnClickListener()
{
public void onClick(View arg0)
{
MainActivity.super.onBackPressed();
}
});
activity_main.xml中
?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="com.example.marc.stadtcoach.FreeActivity">
<android.support.design.widget.AppBarLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="@style/AppTheme.AppBarOverlay">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="56dp"
android:background="?attr/colorPrimary"
app:popupTheme="@style/AppTheme.PopupOverlay" />
</android.support.design.widget.AppBarLayout>
<RelativeLayout
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#FFFFFF"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.example.marc.stadtcoach.MainActivity">
<Button
android:id="@+id/zeitspiel"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="30dp"
android:background="@drawable/lockedroundbutton"
android:text="Zeitspiel"
android:textColor="#FFFFFF"
android:layout_below="@+id/spiel"
android:layout_alignLeft="@+id/spiel"
android:layout_alignStart="@+id/spiel"
android:layout_alignRight="@+id/spiel"
android:layout_alignEnd="@+id/spiel" />
<Button
android:id="@+id/statistik"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="26dp"
android:background="@drawable/roundbutton"
android:text="Statistik"
android:textColor="#FFFFFF"
android:layout_below="@+id/zeitspiel"
android:layout_alignLeft="@+id/spiel"
android:layout_alignStart="@+id/spiel"
android:layout_alignRight="@+id/zeitspiel"
android:layout_alignEnd="@+id/zeitspiel" />
<TextView
android:id="@+id/coinView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="0"
android:textSize="16dp"
android:layout_alignBottom="@+id/imageView1"
android:layout_alignRight="@+id/spiel"
android:layout_alignEnd="@+id/spiel" />
<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/coin"
android:layout_marginRight="10dp"
android:layout_marginEnd="10dp"
android:layout_alignParentTop="true"
android:layout_toLeftOf="@+id/coinView"
android:layout_toStartOf="@+id/coinView" />
<Button
android:id="@+id/spiel"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="108dp"
android:background="@drawable/startbutton"
android:text="Lernspiel"
android:textColor="#FFFFFF"
android:layout_below="@+id/coinView"
android:layout_centerHorizontal="true" />
</RelativeLayout>
</android.support.design.widget.CoordinatorLayout>
costum工具栏menu_back.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
app:layout_scrollFlags="scroll|enterAlways"
app:layout_collapseMode="pin">
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- This is a custom left side button -->
<ImageButton
android:id="@+id/backarrow"
android:layout_width="?attr/actionBarSize"
android:layout_height="?attr/actionBarSize"
android:layout_marginRight="?attr/actionBarSize"
android:layout_gravity="start|center_vertical"
android:visibility="visible"
android:src="@drawable/backarrow"
style="@style/Widget.AppCompat.ActionButton" />
</FrameLayout>
</android.support.v7.widget.Toolbar>
最后是错误信息
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.marc.stadtcoach, PID: 386
Theme: themes:{default=overlay:grovelet.s7.galaxys7.galaxy.launcher.theme, iconPack:grovelet.s7.galaxys7.galaxy.launcher.theme, fontPkg:grovelet.s7.galaxys7.galaxy.launcher.theme, com.android.systemui=overlay:grovelet.s7.galaxys7.galaxy.launcher.theme, com.android.systemui.navbar=overlay:grovelet.s7.galaxys7.galaxy.launcher.theme}
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.marc.stadtcoach/com.example.marc.stadtcoach.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2450)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5461)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.marc.stadtcoach.MainActivity.onCreate(MainActivity.java:44)
at android.app.Activity.performCreate(Activity.java:6251)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2403)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5461)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
I/Process: Sending signal. PID: 386 SIG: 9
Application terminated.
问题在于
ImageButton backar = (ImageButton) findViewById(R.id.backarrow);
返回null。
如果有人可以帮助我，我会很高兴的！
答案 0 :(得分：0)
图像按钮位于工具栏布局内，尝试在工具栏中找到它。 ImageButton backar =（ImageButton）tb.findViewById（R.id.backarrow）; 或者首先获取工具栏framelayout并从中获取图像按钮。