您好我是一名年轻的比利时开发人员，并且目前正在关注YouTube上的教程来创建抽屉导航，但在启动模拟器时我会遇到错误。 This是教程。
以下是错误：
02-10 11:31:12.432 2297-2297/com.example.flori.mylordring E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.flori.mylordring, PID: 2297
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.flori.mylordring/com.example.flori.mylordring.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayShowHomeEnabled(boolean)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$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.support.v7.app.ActionBar.setDisplayShowHomeEnabled(boolean)' on a null object reference
at com.example.flori.mylordring.MainActivity.onCreate(MainActivity.java:31)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
主要代码：
package com.example.flori.mylordring;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ImageButton;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
private static ImageButton imgView;
private int test;
int [] images = {R.drawable.im1,R.drawable.im2,R.drawable.im3,R.drawable.im4,R.drawable.im5,R.drawable.im6,R.drawable.im7};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DrawerLayout drawerLayout = (DrawerLayout)findViewById(R.id.drawerLayout);
ActionBar actionBar = getSupportActionBar();
actionBar.setDisplayShowHomeEnabled(true);
actionBar.setDisplayHomeAsUpEnabled(true);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Rolololo", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
}
public void image(View view){
imgView=(ImageButton)findViewById(R.id.imageButton);
imgView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v){
test++;
test = test % images.length;
imgView.setImageResource(images[test]);
}
}
);
}
@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);
}
}
活动代码（我有2个不同的活动，这是content_main，我也有activity_main）：
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout
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:id="@+id/drawerLayout"
android:background="#e2656262"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.example.flori.mylordring.MainActivity"
tools:showIn="@layout/activity_main">
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/fragmentholder">
</FrameLayout>
<ListView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/navlist"
android:background="#dedede"
android:layout_gravity="start"
/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!"
android:id="@+id/textView"
android:layout_centerVertical="true"
android:layout_centerHorizontal="true" />
<ImageButton
android:onClick="image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/imageButton"
android:src="@drawable/im1"
android:background="@android:color/transparent"
android:layout_above="@+id/textView"
android:layout_centerHorizontal="true" />
</android.support.v4.widget.DrawerLayout>
清单代码：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.flori.mylordring">
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity
android:name=".MainActivity"
android:label="@string/app_name"
android:theme="@style/AppTheme.NoActionBar">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
**</manifest>
和my2 styles.xml： 这是我的style.xml：**
<resources>
<!-- 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" />
</resources>
Style.xml（V21）：
<resources>>
<style name="AppTheme.NoActionBar">
<item name="windowActionBar">false</item>
<item name="windowNoTitle">true</item>
<item name="android:windowDrawsSystemBarBackgrounds">true</item>
<item name="android:statusBarColor">@android:color/transparent</item>
</style>
</resources>
答案 0 :(得分：3)
在这部分代码中：
ActionBar actionBar = getSupportActionBar(); //will return null
actionBar.setDisplayShowHomeEnabled(true);
actionBar.setDisplayHomeAsUpEnabled(true);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);//why set it after you call getSupportActionBar?
如上所示，在将工具栏设置为操作栏之前，您将获得操作栏，该操作栏将返回null。
您需要切换它们，以便从工具栏中获取参考。
答案 1 :(得分：0)
或者换句话说，您可以使用getSupportActionBar()：
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); // First initialize the Toolbar
setSupportActionBar(toolbar); // Set as SupportActionBar
getSupportActionBar().setDisplayShowHomeEnabled(true); // do your stuffs
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
第二个问题是：
app:layout_behavior="@string/appbar_scrolling_view_behavior"
您为Drawer设置了它并且它不应该在那里，请查看此链接：
将
app:layout_behavior添加到RecyclerView或任何其他支持的视图 嵌套滚动的内容，例如NestedScrollView。支持库 包含特殊的字符串资源 映射到的@string/appbar_scrolling_view_behaviorAppBarLayout.ScrollingViewBehavior，用于通知 在此特定视图上发生滚动事件时的AppBarLayout