findViewById() is returning unable to find symbol, but the ID is defined in the layout?

Question

I'm working on learning some Android app development by myself and I'm following a video tutorial on how to make a simple ToDo list application. However, I hit a compile error and from what limited knowledge I have, everything seems to be in order. Below is both the main class and the layout:

Main.java: http://pastebin.com/vfuANx0p

package com.example.exampletodo;

import android.R;
import android.os.Bundle;
import android.app.Activity;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnKeyListener;
import android.widget.EditText;
import android.widget.Button;
import android.widget.ListView;
import java.util.ArrayList;
import android.widget.ArrayAdapter;

public class Main extends Activity implements OnClickListener, OnKeyListener {

    EditText txtItem;
    Button btnAdd;
    ListView listItems;

    ArrayList<String> toDoItems;
    ArrayAdapter<String> aa;

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

        txtItem = (EditText)findViewById(R.id.txtItem);
        btnAdd = (Button)findViewById(R.id.btnAdd);
        listItems = (ListView)findViewById(R.id.listItems);

        btnAdd.setOnClickListener(this);
        txtItem.setOnKeyListener(this);

        toDoItems = new ArrayList<String>();
        aa = new ArrayAdapter<String>(this, R.layout.simple_list_item_1, toDoItems);
        listItems.setAdapter(aa);
    }

    private void addItem(String item){
        if(item.length() > 0){
            this.toDoItems.add(item);
            this.aa.notifyDataSetChanged();
            this.txtItem.setText("");
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    public void onClick(View v){
        if(v == this.btnAdd){
            this.addItem(this.txtItem.getText().toString());
        }
    }

    public boolean onKey(View v, int keyCode, KeyEvent event){
        if(event.getAction() == KeyEvent.ACTION_DOWN && keyCode == KeyEvent.KEYCODE_DPAD_CENTER){
            this.addItem(this.txtItem.getText().toString());
        }
        return false;
    }

}

main.xml: http://pastebin.com/WcWg692v

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:paddingLeft="@dimen/activity_horizontal_margin"
   android:paddingRight="@dimen/activity_horizontal_margin"
   android:paddingTop="@dimen/activity_vertical_margin"
   android:paddingBottom="@dimen/activity_vertical_margin"
   tools:context=".Main">

    <EditText
           android:layout_width="fill_parent"
           android:layout_height="wrap_content"
           android:id="@+id/txtItem" android:layout_alignParentTop="true"
           android:layout_alignParentRight="true" android:layout_alignParentLeft="true" android:focusable="true"/>
    <Button
           android:layout_width="fill_parent"
           android:layout_height="wrap_content"
           android:text="@string/add"
           android:id="@+id/btnAdd" android:layout_below="@+id/txtItem" android:layout_alignRight="@+id/txtItem"
           android:layout_alignLeft="@+id/txtItem"/>
    <ListView
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:id="@+id/listItems"
           android:layout_alignParentRight="true" android:layout_alignParentLeft="true"
           android:layout_alignParentBottom="true" android:layout_below="@+id/btnAdd"/>
</RelativeLayout>

The error is:

 /home/rohan/ExampleToDo/ExampleToDo/src/main/java/com/example/exampletodo/Main.java
 Gradle: cannot find symbol variable main Gradle: cannot find symbol
 variable txtItem Gradle: cannot find symbol variable btnAdd Gradle:
 cannot find symbol variable listItems Gradle: cannot find symbol
 variable main

Thank you so much to anyone who is willing to help!

Answer 1

You import wrong R file, you import android.R and you should import your package.R

com.example.exampletodo.R

EDIT: And don't forget clean and build is sometimes a must. So better do it your self than count on automatic R creation. And you can always check your R file whether it contains your ID-s.

part 2:

change this

ArrayAdapter<String>(this, R.layout.simple_list_item_1, toDoItems);

into

ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, toDoItems);

because simple_list_item_1 is part of android.R not your R

Hope this helps and enjoy your work.