我是android开发的全新手。目前我正在尝试复制我看到的应用程序，可以将多个图像（jpeg，png，jpg）转换为单个pdf文件。我试图通过调试尽我所能。但我不明白为什么还有这个NullPointer例外。
哦，顺便说一句，请原谅我，如果在我的问题中放置正确的标签时有些错误，这是我的第一个问题，谢谢。
这是 ConvertToPDFConverter.java的代码
package com.example.lenovo_thinkpad.mobilelectureboard;
import android.app.ActionBar;
import android.app.Activity;
import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.design.widget.NavigationView;
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.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
public class ConvertToPDFConverter extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
DrawerLayout drawer;
android.support.v7.app.ActionBar bar;
CustomActionBarDrawerToggle toggle;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_convert_to_pdfconverter);
bar = this.getSupportActionBar();
assert bar != null;
bar.setDisplayHomeAsUpEnabled(true);
bar.setHomeButtonEnabled(true);
bar.setDisplayShowTitleEnabled(false);
drawer = findViewById(R.id.drawer_layout);
drawer.setBackgroundColor(getResources().getColor(R.color.colorPrimary));
try {
toggle = new CustomActionBarDrawerToggle(this, drawer);
} catch (RuntimeException e) {
e.printStackTrace();
}
drawer.addDrawerListener(toggle);
// Set navigation drawer
// Set com.example.lenovo_thinkpad.mobilelectureboard.Home fragment
Fragment fragment = new Home();
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.content, fragment).commit();
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
Fragment fragment;
FragmentManager fragmentManager = getSupportFragmentManager();
switch (id) {
case R.id.nav_camera:
fragment = new Home();
fragmentManager.beginTransaction().replace(R.id.content, fragment).commit();
break;
case R.id.nav_gallery:
fragment = new ViewFiles();
fragmentManager.beginTransaction().replace(R.id.content, fragment).commit();
break;
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
}
private class CustomActionBarDrawerToggle extends ActionBarDrawerToggle {
CustomActionBarDrawerToggle(Activity mActivity,
DrawerLayout mDrawerLayout) {
super(mActivity, mDrawerLayout, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
}
@Override
public void onDrawerClosed(View view) {
bar.setTitle(getString(R.string.navigation_drawer_close));
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
@Override
public void onDrawerOpened(View drawerView) {
bar.setTitle(getString(R.string.navigation_drawer_open));
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
}
}
以及布局 activity_convert_to_pdfconverter.xml
<?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:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:openDrawer="start">
<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=".ConvertToPDFConverter">
<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="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:popupTheme="@style/AppTheme.PopupOverlay" />
</android.support.design.widget.AppBarLayout>
<FrameLayout
android:layout_marginTop="20dp"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:background="@color/mb_white"
android:id="@+id/content"/>
</android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.NavigationView
android:id="@+id/nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
android:fitsSystemWindows="true"
app:headerLayout="@layout/nav_header_main"
app:menu="@menu/activity_main_drawer" />
</android.support.v4.widget.DrawerLayout>
这是堆栈跟踪
02-13 15:12:17.976 1548-1548/com.example.lenovo_thinkpad.mobilelectureboard E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.lenovo_thinkpad.mobilelectureboard, PID: 1548
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lenovo_thinkpad.mobilelectureboard/com.example.lenovo_thinkpad.mobilelectureboard.ConvertToPDFConverter}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5001)
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:785)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.lenovo_thinkpad.mobilelectureboard.ConvertToPDFConverter.onCreate(ConvertToPDFConverter.java:34)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2148)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5001)
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:785)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
at dalvik.system.NativeStart.main(Native Method)
的AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.CAMERA" />
<uses-feature android:name="android.hardware.camera" />
<uses-feature
android:name="android.hardware.camera.autofocus"
android:required="false" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme_picker">
<activity
android:name=".MainActivity"
android:screenOrientation="landscape">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".WhiteBoardActivity" />
<activity android:name=".ConvertToPDFConverter" />
</application>
答案 0 :(得分：1)
由于您正在扩展AppCompatActivity，请尝试通过'getSupportActionbar（）'获取操作栏。