NullPointerException：在空对象引用上使用ActionBar.SetDisplayHomeAsUpEnabled（boolean）“

Question

我在运行时得到这个nullPointerException：

原因：java.lang.NullPointerException：试图对空对象引用调用虚拟方法“void android.app.actionbar.SetDisplayHomeasUpenabled（boolean）”

来自MainActivity的代码：

package com.example.vasilis.spangreek;

import android.app.ActionBar;
import android.app.Activity;
import android.content.res.Configuration;
import android.content.res.TypedArray;
import android.os.Bundle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.util.DisplayMetrics;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ExpandableListView;

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

import adapter.ExpandableListViewAdapter;
import model.NavDrawerItem;


public class MainActivity extends Activity {

    private DrawerLayout mDrawerLayout;
    private ExpandableListView mExpandableListView;
    private ActionBarDrawerToggle mActionBarDrawerToggle;

    //nav drawer Title
    private CharSequence mDrawerTitle;

    //used to store app titles
    private CharSequence mTitles;

    //slide menu items
    private String[] navMenuItems;
    private String[] navSubMenuItems;
    private TypedArray  navMenuIcons;

    private List<NavDrawerItem> groupList;
    private List<NavDrawerItem> childList;
    private Map<NavDrawerItem, List<NavDrawerItem>> mapList;
    private ExpandableListViewAdapter mAdapter;
    ActionBar mActionBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mTitles = mDrawerTitle = getTitle();

        //nav drawer icons
        navMenuIcons = getResources().obtainTypedArray(R.array.nav_icons);

        mDrawerLayout = (DrawerLayout)findViewById(R.id.drawer_layout);

        createGroupList();
        CreateChildList();

        // Recycle the typed array
        navMenuIcons.recycle();

        mExpandableListView = (ExpandableListView)findViewById(R.id.list_slideMenu);
        mAdapter = new ExpandableListViewAdapter(this, mapList, groupList);
        mExpandableListView.setAdapter(mAdapter);
        mActionBar = getActionBar();
        // enabling action bar app icon and behaving it as toggle button
        mActionBar.setDisplayHomeAsUpEnabled(true);
        mActionBar.setHomeButtonEnabled(true);

        //toggle
        mActionBarDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
                R.string.app_name,   R.string.app_name) {

            public void onDrawerClosed(View drawerView) {
                mActionBar.setTitle(mDrawerTitle);

                // calling onPrepareOptionsMenu() to hide action bar icons
                invalidateOptionsMenu();
            }


            public void onDrawerOpened(View drawerView) {
                mActionBar.setTitle(mDrawerTitle);
                // calling onPrepareOptionsMenu() to hide action bar icons
                invalidateOptionsMenu();
            }
        };

        mActionBarDrawerToggle.setHomeAsUpIndicator(R.drawable.ic_drawer);

        mDrawerLayout.setDrawerListener(mActionBarDrawerToggle);

        if(savedInstanceState == null) {
            //displayView(0);
        }


    }

    /***
     * Called when invalidateOptionsMenu() is triggered
     */
    @Override
    public boolean onPrepareOptionsMenu(Menu menu) {
        // if nav drawer is opened, hide the action items
        boolean drawerOpen = mDrawerLayout.isDrawerOpen(mExpandableListView);
        menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
        return super.onPrepareOptionsMenu(menu);
    }

    @Override
    public void setTitle(CharSequence title) {
        mTitles = title;
        getActionBar().setTitle(mTitles);
    }

    /**
     * When using the ActionBarDrawerToggle, you must call it during
     * onPostCreate() and onConfigurationChanged()...
     */

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        mActionBarDrawerToggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        // Pass any configuration change to the drawer toggls
        mActionBarDrawerToggle  .onConfigurationChanged(newConfig);
    }
    @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) {

        // toggle nav drawer on selecting action bar app icon/title
        if (mActionBarDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        // Handle action bar actions click
        switch (item.getItemId()) {
            case R.id.action_settings:
                return true;
            default:
                return super.onOptionsItemSelected(item);

        }
    }

    private void createGroupList() {
        //load slide menu items
        navMenuItems = getResources().getStringArray(R.array.nav_items);

        groupList =new ArrayList<NavDrawerItem>();

        for (int i = 0 ; i <  navMenuItems.length ; i++ ) {
            groupList.add(i , new NavDrawerItem(navMenuItems[i], navMenuIcons.getResourceId(i, -1)));
        }
    }

    private void CreateChildList() {

        mapList = new LinkedHashMap<NavDrawerItem, List<NavDrawerItem>>();
        navSubMenuItems  = getResources().getStringArray(R.array.nav_sub_items);
        childList = new ArrayList<>();



        for ( NavDrawerItem item : groupList) {
            if(item.getTitle().equalsIgnoreCase("learning Spanish")) {
                for (int i = 0 ;  i < navSubMenuItems.length ; i ++) {
                    childList.add(i, new NavDrawerItem(navSubMenuItems[i]));
                }
            }

            mapList.put(item,childList);
        }

    }

    private void setGroupIndicatorToRight() {
        /* Get the screen width */
        DisplayMetrics dm = new DisplayMetrics();
        getWindowManager().getDefaultDisplay().getMetrics(dm);
        int width = dm.widthPixels;

        mExpandableListView.setIndicatorBounds(width - getDipsFromPixel(35), width
                - getDipsFromPixel(5));
    }

    // Convert pixel to dip
    public int getDipsFromPixel(float pixels) {
        // Get the screen's density scale
        final float scale = getResources().getDisplayMetrics().density;
        // Convert the dps to pixels, based on density scale
        return (int) (pixels * scale + 0.5f);
    }
}

出现问题的代码行是：

 mActionBar = getActionBar();
    // enabling action bar app icon and behaving it as toggle button
    mActionBar.setDisplayHomeAsUpEnabled(true);
    mActionBar.setHomeButtonEnabled(true);

我使用：style name=“AppTheme”parent=“Theme.AppCompat.Light.DarkActionBar”

我使用了getSupportActionBar（）,但没有找到任何解决方案

Answer 1

问题的原因是使用MainActivity extend activity支持主题样式name=“AppTheme”parent=“theme.AppCompat.Light.DarkActionBar”。这是不相容的东西。您需要哪一个min sdk？

在具有MainActivity extends activity的代码中，不需要AppCompatTheme。使用name=“AppTheme”parent=“android:theme.light”

如果您使用的是Theme.AppCompat.Light.DarkActionBar，则应从AppCompattivity扩展您的activity，并使用GetSupportActionBar()。

而不是：

public class MainActivity extends Activity {

使用：

public class MainActivity extends AppCompatActivity {

而不是：

getActionBar().setTitle(mTitles);

使用：

getSupportActionBar().setTitle(mTitles);