Android 错误：尝试在空对象引用上调用虚拟方法“void androidx.appcompat.app.ActionBar.hide()”[重复]答案

【问题标题】：Android Error : Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.hide()' on a null object reference [duplicate]Android 错误：尝试在空对象引用上调用虚拟方法“void androidx.appcompat.app.ActionBar.hide()”[重复]
【发布时间】：2021-01-22 07:16:52
【问题描述】：

所以我正在编写将网站转变为 Web 视图应用程序的代码，它在开启暗模式时工作正常，在关闭时崩溃，但是，我还没有编写任何代码来保持这些模式我的想法。

我收到以下错误：

2021-01-22 11:51:40.868 7271-7271/com.shopitapp.shopit E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.shopitapp.shopit, PID: 7271
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.shopitapp.shopit/com.shopitapp.shopit.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.hide()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.hide()' on a null object reference
        at com.shopitapp.shopit.MainActivity.onCreate(MainActivity.java:23)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
2021-01-22 11:51:40.926 7271-7271/com.shopitapp.shopit I/Process: Sending signal. PID: 7271 SIG: 9

MainActivity.java

package com.shopitapp.shopit;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.KeyEvent;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class MainActivity extends AppCompatActivity {

    WebView web;


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



        getActionBar().hide();
        web = findViewById(R.id.webView);
        WebSettings webSettings = web.getSettings();
        webSettings.setJavaScriptEnabled(true);

        web.setWebViewClient(new Callback());
        web.loadUrl("https://shopit.app/");
    }

    private class Callback extends WebViewClient {
        @Override
        public boolean shouldOverrideKeyEvent(WebView view, KeyEvent event) {
            return false;
        }
    }

    /*@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(event.getAction()==KeyEvent.ACTION_DOWN)
        {
            switch(keyCode) {
                case KeyEvent.KEYCODE_BACK:
                if(web.canGoBack())
                    web.goBack();
                else
                    finish();
            }
        }
        return super.onKeyDown(keyCode, event);
    }*/

    @Override
    public void onBackPressed() {
        if(web.canGoBack())
            web.goBack();
        else
        super.onBackPressed();
    }
}

SplashScreen.java

package com.shopitapp.shopit;

import android.content.Intent;
import android.os.Bundle;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

public class SplashScreen extends AppCompatActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        /*Bitmap bitmapImage = BitmapFactory.decodeFile("src\main\res\drawable\logo.png");
        int nh = (int) ( bitmapImage.getHeight() * (512.0 / bitmapImage.getWidth()) );
        Bitmap scaled = Bitmap.createScaledBitmap(bitmapImage, 512, nh, true);
        ImageView your_imageview = null;
        your_imageview.setImageBitmap(scaled);*/

        startActivity(new Intent(this,MainActivity.class));
        finish();
    }

}

Android 清单：


<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.shopitapp.shopit">

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:networkSecurityConfig="@xml/network_security_config"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.Shopit">
        <activity android:name="com.shopitapp.shopit.SplashScreen"
            android:theme="@style/SplashScreen">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <activity android:name="com.shopitapp.shopit.MainActivity"
            android:theme="@style/Theme.Shopit"></activity>
    </application>

</manifest>

主题.xml

<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Theme.Shopit" parent="Theme.MaterialComponents.DayNight.NoActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/purple_500</item>
        <item name="colorPrimaryVariant">@color/purple_700</item>
        <item name="colorOnPrimary">@color/white</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/teal_200</item>
        <item name="colorSecondaryVariant">@color/teal_700</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>
    
    <style name="SplashScreen" parent="Theme.AppCompat.Light.NoActionBar">
        <item name="android:windowBackground">@drawable/background_splashscreen</item>
    </style>
</resources>

themes.xml（夜晚）


<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Theme.Shopit" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/purple_200</item>
        <item name="colorPrimaryVariant">@color/purple_700</item>
        <item name="colorOnPrimary">@color/black</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/teal_200</item>
        <item name="colorSecondaryVariant">@color/teal_200</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>
</resources>

【问题讨论】：

请在此处包含所有相关代码，而不是在场外资源上
另外，android studio 只是一个 IDE，它只是帮助您编写代码，所以除非您专门询问 IDE 的功能，否则不需要包含标签
@a_local_nobody 你现在可以检查一下吗？我已经按照你说的做了。
getActionBar().hide(); 这是导致您崩溃的代码。在使用它之前，您必须检查 getActionBar 是否不为空。为什么它为 null 可能与您的主题有关，您使用的是 NoActionBar 和 DarkActionBar
嘿，确实如此。非常感谢伙计。祝你有美好的一天。

标签： android

【解决方案1】：

在您的MainActivity 上，您试图使用代码getActionBar().hide 隐藏Action Bar，

但问题是您的MainActivity 没有Action Bar，因为您的Main Activity 正在使用称为Theme.Shopit 的Theme，它默认删除Action Bar，因为它的父级是Theme.MaterialComponents.DayNight.NoActionBar。

因此，要解决此问题，您只需从 MainActivity 中删除代码 getActionBar().hide。

【讨论】：