Question

我想在我的应用中导入导航抽屉。我在片段中编码，但我的应用程序在启动时崩溃，我收到以下错误：

错误：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void android.support.design.widget.NavigationView.setNavigationItemSelectedListener（android.support.design.widget.NavigationView $ OnNavigationItemSelectedListener）' p>

代码：

import android.content.res.Configuration;
import android.os.Bundle;
import android.support.design.widget.NavigationView;
import android.support.v4.widget.DrawerLayout;
import android.support.v4.app.FragmentManager;
import android.support.v4.view.GravityCompat;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarDrawerToggle;

public class NavigationDrawer extends AppCompatActivity {
    private DrawerLayout NavDrawer;
    private Toolbar toolbar;


    private ActionBarDrawerToggle setupDrawerToggle() {
        return new ActionBarDrawerToggle(this, NavDrawer, toolbar, R.string.drawer_open,  R.string.drawer_close);
    }

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


        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);


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

        NavigationView nvDrawer = (NavigationView) findViewById(R.id.nvView);

        setupDrawerContent(nvDrawer);

        ActionBarDrawerToggle toggle = setupDrawerToggle();

        NavDrawer.addDrawerListener(toggle);
    }

    private void setupDrawerContent(NavigationView navigationView) {
        navigationView.setNavigationItemSelectedListener(
            new NavigationView.OnNavigationItemSelectedListener() {
                @Override
                public boolean onNavigationItemSelected(MenuItem menuItem) {
                    selectDrawerItem(menuItem);
                    return true;
                }
            });
    }
}

我在代码中看不到错误...

//编辑以获得更好的信息...

好的，我已经做到了，但现在出现了一个新错误......

java.lang.NullPointerException：尝试在空对象引用上调用虚方法'android.view.Window $ Callback android.view.Window.getCallback（）'

但我不知道是谁的代码......

这里是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">

    <include
        layout="@layout/app_bar_navigation_drawer"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <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_navigation_drawer"
        app:menu="@menu/activity_navigation_drawer_drawer" />

</android.support.v4.widget.DrawerLayout>

//编辑确定我已经制作了......而错误

错误：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void android.support.design.widget.NavigationView.setNavigationItemSelectedListener（android.support.design.widget.NavigationView $ OnNavigationItemSelectedListener）' p>

已经......

public class NavigationDrawer extends AppCompatActivity {
    private DrawerLayout NavDrawer;
    private Toolbar toolbar;
    private ActionBarDrawerToggle toggle;
  
  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_navigation_drawer);

        toggle = new ActionBarDrawerToggle(this, NavDrawer,
                toolbar, R.string.drawer_open, R.string.drawer_close) {

            public void onDrawerClosed(View view) {
                super.onDrawerClosed(view);
                invalidateOptionsMenu(); 
            }

            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                invalidateOptionsMenu(); 
            }
        };

        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        NavDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);

        NavigationView nvDrawer = (NavigationView) findViewById(R.id.nvView);

        NavDrawer.setDrawerListener(toggle);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
 

        super.onOptionsItemSelected(item);

        switch (item.getItemId()) {
            case android.R.id.home:
                NavDrawer.openDrawer(GravityCompat.START);
                return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        toggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        toggle.onConfigurationChanged(newConfig);
    }

    private void selectDrawerItem(MenuItem menuItem) {
 
        Fragment fragment = null;
        Class fragmentClass;
        switch(menuItem.getItemId()) {
            case R.id.nav_home:
                fragmentClass = Home.class;
                break;
            case R.id.nav_artikel:
                fragmentClass = Artikel.class;
                break;
            case R.id.nav_facebook:
                fragmentClass = Facebook.class;
                break;
            case R.id.nav_instagram:
                fragmentClass = Instagram.class;
                break;
            case R.id.nav_twitter:
                fragmentClass = Twitter.class;
                break;
            case R.id.nav_manage:
                fragmentClass = Settings.class;
                break;
            case R.id.nav_share:
                fragmentClass = Share.class;
                break;
            case R.id.nav_send:
                fragmentClass = Send.class;
                break;
            default:
                fragmentClass = Home.class;
        }

        try {
            fragment = (Fragment) fragmentClass.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }

        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
        menuItem.setChecked(true);
        setTitle(menuItem.getTitle());
        NavDrawer.closeDrawers();
    }

}

这是Completted Navigation java ....

Answer 1

您的问题似乎是您没有从setupDrawerContent()返回任何内容。因此，当您指定ActionBarDrawerToggle toggle = setupDrawerToggle; toggle时，将为空值。然后，当您尝试addDrawerListener时，您将尝试设置NullPointerException以尝试设置为null的抽屉侦听器。

以下是Google's Creating a Navigation Drawer中用于正确设置抽屉监听器的示例。

ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, mDrawerLayout,
            R.drawable.ic_drawer, R.string.drawer_open, R.string.drawer_close) {

        /** Called when a drawer has settled in a completely closed state. */
        public void onDrawerClosed(View view) {
            super.onDrawerClosed(view);
            getActionBar().setTitle(mTitle);
            invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
        }

        /** Called when a drawer has settled in a completely open state. */
        public void onDrawerOpened(View drawerView) {
            super.onDrawerOpened(drawerView);
            getActionBar().setTitle(mDrawerTitle);
            invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
        }
    };

需要有关导航抽屉错误的帮助

1 个答案: