RecyclerView“无法解决符号”错误，-Android Studio

Question

我创建了一个虚拟项目来理解基于卡片的布局。不幸的是，我无法解决RecyclerView上的符号错误。我对编程还是个新手，不明白哪里出了问题。将依赖项添加到生成文件：

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    implementation 'com.android.support:recyclerview-v7:28.0.0'
    implementation 'com.android.support:cardview-v7:28.0.0'
}

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

    public class MainActivity extends AppCompatActivity {

        RecyclerView recyclerView;

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

Answer 1

当您想要使用一个类时，您应该导入它的库。

在本例中，您还没有导入recyclerview库。要解决此问题，只需将活动代码更改为：

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.RecyclerView; //recyclerview library

    public class MainActivity extends AppCompatActivity {

        RecyclerView recyclerView;

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