安卓studio出现“无法找到符号”的错误

Question

代码中第17行R.id.listview的listview变成红的了。运行时报错“找不到符号”。
clean project还有清除缓存的方法都试了，不好使。
这是从网上复制的代码，小白求教，急。感谢大佬。
package com.example.listtest;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

import android.widget.ListView;

import java.util.ArrayList;

import java.util.List;
public class MainActivity extends AppCompatActivity {
private List fruitList = new ArrayList();
private ListView listView = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits();
FruitAdapter adapter = new FruitAdapter(MainActivity.this, R.layout.fruit_item, fruitList);
listView = (ListView) findViewById(R.id.listview);
listView.setAdapter(adapter);
}
private void initFruits() {
Fruit apple = new Fruit("Apple", R.drawable.apple);
fruitList.add(apple);
Fruit banana = new Fruit("Banana", R.drawable.banana);
fruitList.add(banana);
Fruit orange = new Fruit("Orange", R.drawable.orange);
fruitList.add(orange);
Fruit watermelon = new Fruit("Watermelon", R.drawable.watermelon);
fruitList.add(watermelon);
Fruit pear = new Fruit("Pear", R.drawable.pear);
fruitList.add(pear);
Fruit grape = new Fruit("Grape", R.drawable.grape);
fruitList.add(grape);
Fruit pineapple = new Fruit("Pineapple", R.drawable.pineapple);
fruitList.add(pineapple);
Fruit strawberry = new Fruit("Strawberry", R.drawable.strawberry);
fruitList.add(strawberry);
Fruit cherry = new Fruit("Cherry", R.drawable.cherry);
fruitList.add(cherry);
Fruit mango = new Fruit("Mango", R.drawable.mango);
fruitList.add(mango);
}
}

Answer 1

检查包名是否正确 package com.example.
AndroidManifest中是否有相应的activity

            <category android:name="android.intent.category.LAUNCHER" />