我有以下问题：
getActionBar().setDisplayHomeAsUpEnabled(true);
显示java.lang.NullPointerException
我知道有很多帖子都有同样的问题，但是我可以解决这个问题。我尝试了我找到的所有解决方案，但没有人帮助我，我工作这个片段和抽屉（Android）
这是代码：MainActivity.java
package com.example.erik.test2;
import android.app.Activity;
import android.os.Bundle;
import android.support.design.widget.NavigationView;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;
import android.view.View;
import android.view.Window;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private DrawerLayout mDrawer;
private Toolbar toolbar;
private ActionBarDrawerToggle mDrawerToggle;
@Override
protected void onCreate(Bundle savedInstanceState) {
//requestWindowFeature(Window.FEATURE_ACTION_BAR);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Set a Toolbar to replace the ActionBar.
toolbar = (Toolbar) findViewById(R.id.toolbar);
//setSupportActionBar(toolbar);
// getSupportActionBar().setDisplayHomeAsUpEnabled(true);
// Find our drawer view
mDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);
NavigationView nvDrawer = (NavigationView) findViewById(R.id.nvView);
setupDrawerContent(nvDrawer);
//---- Open Drawer on clic icon BUT CAUSE NULLPOINT EXEPTION--- //
//*
getActionBar().setDisplayHomeAsUpEnabled(true);
getActionBar().setHomeButtonEnabled(true);//*/
/*
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);//*/
//*
mDrawerToggle = new ActionBarDrawerToggle(this, mDrawer, R.drawable.logo_tam, R.string.ouverture, R.string.fermeture) {
public void onDrawerClosed(View view) {
getActionBar().setTitle(R.string.titre);
invalidateOptionsMenu();
}
public void onDrawerOpened(View drawerView) {
getActionBar().setTitle(R.string.titre_apres_ouverture);
invalidateOptionsMenu();
}
};
mDrawer.setDrawerListener(mDrawerToggle); //*
//-----------------------------------------------------------------------------------------------------------------
/*
mDrawerToggle = new ActionBarDrawerToggle(this, mDrawer, R.drawable.logo_tam, R.string.ouverture, R.string.fermeture ) {
public void onDrawerClosed(View view) {
super.onDrawerClosed(view);
getActionBar().setTitle("Fermer");
}
public void onDrawerOpened(View drawerView) {
super.onDrawerOpened(drawerView);
getActionBar().setTitle("Ouvert");
}
};
// Set the drawer toggle as the DrawerListener
mDrawer.setDrawerListener(mDrawerToggle);*/
// getActionBar().setDisplayHomeAsUpEnabled(true);
//getActionBar().setHomeButtonEnabled(true);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// The action bar home/up action should open or close the drawer.
switch (item.getItemId()) {
case android.R.id.home:
mDrawer.openDrawer(GravityCompat.START);
return true;
}
return super.onOptionsItemSelected(item);
}
// Make sure this is the method with just `Bundle` as the signature
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
}
private void setupDrawerContent(NavigationView navigationView) {
navigationView.setNavigationItemSelectedListener(
new NavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(MenuItem menuItem) {
selectDrawerItem(menuItem);
return true;
}
});
}
public void selectDrawerItem(MenuItem menuItem) {
// Create a new fragment and specify the planet to show based on
// position
Fragment fragment = null;
Class fragmentClass;
switch(menuItem.getItemId()) {
case R.id.nav_first_fragment:
fragmentClass = FirstFragment.class;
break;
case R.id.nav_second_fragment:
fragmentClass = SecondFragment.class;
break;
case R.id.nav_third_fragment:
fragmentClass = ThirdFragment.class;
break;
default:
fragmentClass = FirstFragment.class;
}
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
// Insert the fragment by replacing any existing fragment
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
// Highlight the selected item, update the title, and close the drawer
menuItem.setChecked(true);
setTitle(menuItem.getTitle());
mDrawer.closeDrawers();
}
}
styles.xml的代码（我已经尝试将主题更改为Theme.AppCompat.Light，但它不起作用）
<resources>
<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
<!-- Customize your theme here. -->
<item name="colorPrimary">@color/colorPrimary</item>
<item name="colorPrimaryDark">@color/colorPrimaryDark</item>
<item name="colorAccent">@color/colorAccent</item>
</style>
</resources>
toolbar.xml的代码
<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_height="wrap_content"
android:layout_width="match_parent"
android:fitsSystemWindows="true"
android:minHeight="?attr/actionBarSize"
app:theme="@style/Theme.AppCompat.Light"
android:background="?attr/colorPrimaryDark">
</android.support.v7.widget.Toolbar>
代码activity_main.xml
<!-- This DrawerLayout has two children at the root -->
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- This LinearLayout represents the contents of the screen -->
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<!-- The ActionBar displayed at the top -->
<!-- <include
layout="@layout/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content" /> -->
<!-- The main content view where fragments are loaded -->
<FrameLayout
android:id="@+id/flContent"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
<!-- The navigation drawer that comes from the left -->
<!-- Note that `android:layout_gravity` needs to be set to 'start' -->
<android.support.design.widget.NavigationView
android:id="@+id/nvView"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
android:background="@android:color/white"
app:menu="@menu/drawer_view"
app:headerLayout="@layout/nav_header"
/>
</android.support.v4.widget.DrawerLayout>
和清单代码
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.erik.test2">
<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">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
我在主题中尝试了很多选项，但是在评论中我看不到我尝试的所有测试，它在我的ActivityMain中不起作用。
你有两种方式：
1.使用getSupportActionBar()而不是getActionBar()
2.改变你的活动以扩展ActionBarActivity
getActionBar（）setDisplayHomeAsUpEnabled（真）。给出了NullPointerException，因为在styles.xml中你给了父Theme.AppCompat.Light.NoActionBar，这意味着没有ActionBar。
解：
在styles.xml中使用下面给出的样式更改样式
<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>
<item name="windowActionBar">false</item>
<item name="windowNoTitle">true</item>
</style>
现在，在MainAcitivity中使用此代码
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
如果这有助于您，请告诉我。
Bad Approach
使用弃用的代码。避免调用ActionBarActivity。
从版本22.1.0开始，不推荐使用ActionBarActivity类。您应该使用AppCompatActivity。
你来了
getActionBar（）setDisplayHomeAsUpEnabled（真）。显示java.lang.NullPointerException
在第一行评论这一行。
<include
layout="@layout/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
2，你可以使用getSupportActionBar()而不是getActionBar()并添加null检查
if (getSupportActionBar() != null)
{
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}
您可以更改主题，因为您使用的是NoActionBar
parent="Theme.AppCompat.Light.NoActionBar"
使用工具栏替换ActionBar非常简单，首先要确保活动扩展ActionBarActivity。还要确保此活动的主题是Theme.AppCompat.NoActionBar或Theme.AppCompat.Light.NoActionBar的子主题。
请检查#SO答案What to use instead of getSupportActionBar()
和
https://developer.android.com/intl/pt-br/reference/android/support/v7/widget/Toolbar.html
以这种方式做到这一点
// Set a Toolbar to replace the ActionBar.
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
// Find our drawer view
mDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);
NavigationView nvDrawer = (NavigationView) findViewById(R.id.nvView);
setupDrawerContent(nvDrawer);
//---- Open Drawer on clic icon BUT CAUSE NULLPOINT EXEPTION--- //
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);
也改变你的布局
<!-- This LinearLayout represents the contents of the screen -->
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<!-- The ActionBar displayed at the top -->
<include
layout="@layout/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
<!-- The main content view where fragments are loaded -->
<FrameLayout
android:id="@+id/flContent"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
<!-- The navigation drawer that comes from the left -->
<!-- Note that `android:layout_gravity` needs to be set to 'start' -->
<android.support.design.widget.NavigationView
android:id="@+id/nvView"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
android:background="@android:color/white"
app:menu="@menu/drawer_view"
app:headerLayout="@layout/nav_header"
/>
你没有得到动作栏，取消注释你的xml代码。
<!-- <include
layout="@layout/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content" /> -->
当前使用NoActionBar也使用默认主题
您正在使用AppCompat.So，使用此：
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
当您使用支持库中的AppCompactActivity时。所以你需要使用getSupportActionBar()获取ActionBar实例
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);
既然你已经给出了NoActionBar主题。取消注释工具栏包含您需要添加工具栏的相关内容。使用ActionBar赋予主题而不是NoActionBar。或者在工具栏中添加工具栏并添加它。
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- This LinearLayout represents the contents of the screen -->
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<!-- The ActionBar displayed at the top -->
<include
layout="@layout/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
<!-- The main content view where fragments are loaded -->
<FrameLayout
android:id="@+id/flContent"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
<!-- The navigation drawer that comes from the left -->
<!-- Note that `android:layout_gravity` needs to be set to 'start' -->
<android.support.design.widget.NavigationView
android:id="@+id/nvView"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
android:background="@android:color/white"
app:menu="@menu/drawer_view"
app:headerLayout="@layout/nav_header"
/>
</android.support.v4.widget.DrawerLayout>