ClassNotFoundException，该怎么办？

简而言之，我是一名学习JAVA / Android开发的学生 . 在学习课程的过程中，出现错误，但我似乎无法找到问题 .

不断收到此错误：

05-04 12:50:18.569 6836-6836/com.lucianvibez.flashchatnewfirebase E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.lucianvibez.flashchatnewfirebase, PID: 6836
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.lucianvibez.flashchatnewfirebase/com.online.lucianvibez.flashchatnewfirebase.LoginActivity}: java.lang.ClassNotFoundException: Didn't find class "com.online.lucianvibez.flashchatnewfirebase.LoginActivity" on path: DexPathList[[zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/base.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_dependencies_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_0_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_1_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_2_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_3_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_4_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_5_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_6_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_7_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_8_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_9_apk.apk"],nativeLibraryDirectories=[/data/app/com.lucianvibez.flashchatnewfirebase-1/lib/x86, /system/lib, /vendor/lib]]
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6077)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
 Caused by: java.lang.ClassNotFoundException: Didn't find class "com.online.lucianvibez.flashchatnewfirebase.LoginActivity" on path: DexPathList[[zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/base.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_dependencies_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_0_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_1_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_2_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_3_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_4_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_5_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_6_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_7_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_8_apk.apk", zip file "/data/app/com.lucianvibez.flashchatnewfirebase-1/split_lib_slice_9_apk.apk"],nativeLibraryDirectories=[/data/app/com.lucianvibez.flashchatnewfirebase-1/lib/x86, /system/lib, /vendor/lib]]

这是下面的app模块build.gradle文件：

apply plugin: 'com.android.application'

android {
    compileSdkVersion 27

    defaultConfig {
        applicationId "com.lucianvibez.flashchatnewfirebase"
        minSdkVersion 16
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    testImplementation 'junit:junit:4.12'
    implementation 'com.android.support:appcompat-v7:27.0.2'
    implementation 'com.android.support:design:27.0.2'
    implementation 'com.google.firebase:firebase-core:15.0.0'
    implementation 'com.google.firebase:firebase-database:15.0.1'
    implementation 'com.google.firebase:firebase-auth:15.1.0'



}

apply plugin: 'com.google.gms.google-services'