FindViewById () returns the inability to find the character, but is the identifier defined in the layout?

Question

I myself study some Android application development, and I follow the video tutorial on how to make a simple ToDo list application. However, I got into a compilation error and from what I have, everything seems to be in order. Below are 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> 

Error:

  /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 very much to everyone who is ready to help!

Answer 1

You import the wrong R file, you import android.R, and you have to import your .R package

 com.example.exampletodo.R 

EDIT: And don't forget that clean and assembly is sometimes necessary. Therefore, it is better to do it yourself than rely on the automatic creation of R. And you can always check your R file whether it contains your identifiers.

part 2:

change this value

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

in

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

because simple_list_item_1 is part of android.R and not your R

Hope this helps and enjoy your work.