“未解析的引用: userRecyclerView”

import android.support.v7.widget.RecyclerView;

RecyclerView userRecyclerView = findViewById(R.id.user_recycler_view);

<!-- res/layout/activity_main.xml -->
<android.support.v7.widget.RecyclerView
    android:id="@+id/user_recycler_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

setContentView(R.layout.activity_main);

dependencies {
    implementation 'androidx.recyclerview:recyclerview:1.2.1'
}

<!-- res/layout/activity_main.xml -->
<android.support.v7.widget.RecyclerView
    android:id="@+id/user_recycler_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

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

public class MainActivity extends AppCompatActivity {
    private RecyclerView userRecyclerView;

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

        userRecyclerView = findViewById(R.id.user_recycler_view);
        // 设置LayoutManager和Adapter
        userRecyclerView.setLayoutManager(new LinearLayoutManager(this));
        userRecyclerView.setAdapter(new UserAdapter());
    }
}