NullPointerException - I don't understand what's wrong

Question

I am a complete newbie in android development. At the moment I am trying to replicate an app I saw on where it is possible to convert multiple images (jpeg, png, jpg) into a single pdf file. I have tried to do the most I can with debugging. But I don't understand why there are still this NullPointer Exception.

Oh and by the way please pardon me if I have some mistakes in placing proper tags or such in my question this is my first question thank you.

Here's my code for the ConvertToPDFConverter.java

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()
    }
}

}

and for the layout 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>

Here's the stacktrace

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) 

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>

Answer 1

由于您正在扩展 AppCompatActivity，请尝试通过“getSupportActionbar()”获取操作栏。