问这是我用android写的代码studio.It显示找不到符号类TextView.It也显示包R不存在

EN

package com.example.android.favoritetoys;

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

public class MainActivity extends AppCompatActivity {

    // TODO (1) Declare a TextView variable called mToysListTextView
    TextView mToysListTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mToysListTextView = (TextView).findViewById(R.id.tv_toy_names);
        // TODO (3) Use findViewById to get a reference to the TextView from the layout

        // TODO (4) Use the static ToyBox.getToyNames method and store the names in a String array
        String[] toyNames = ToyBox.getToyNames();
        // TODO (5) Loop through each toy and append the name to the TextView (add \n for spacing)
        for (String toyName : toyNames) {
            mToysListTextView.append(toyName + "\n\n\n");
        }