java – Android Studio v7-少数类导入错误(无法解析符号)

我是 Android编程的初学者.我正在尝试使用Android Studio创建一个基本的Hello World应用程序.在按照创建Hello World程序的初始步骤后,我立即在Java文件中遇到以下错误.

"Cannot resolve Symbol FloatingActionButton"
"Cannot resolve Symbol Snackbar"
"Cannot resolve Symbol AppCompatActivity"
"Cannot resolve Symbol Toolbar"

正如您在附件Image中看到的那样,错误是红色的.
请帮帮我.

我的构建文件内容：

defaultConfig {
        applicationId "com.example.helloworld.helloworld"
        minSdkVersion 15
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'),'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs',include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:design:23.0.1'
    compile 'com.android.support:appcompat-v7:23.0.1'
    compile 'com.android.support:design:23.0.1'
}

好像我已经添加了所有相关的依赖项.

apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.1"

    defaultConfig {
        applicationId "com.example.networkingtest"
        minSdkVersion 17
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'),include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:23.1.0'
    compile 'com.android.support:design:23.1.0'
}