Android Studio - error: cannot find symbol - setContentView(R.layout.activity_main)

Hi,

I'm learning to create something for Android in an Android studio and I'm trying to dissect one finished project to connect an Android application to a MySQL database using RestAPI. However, the application reports an error:

error: cannot find symbol
setContentView(R.layout.activity_main);
^
symbol: variable activity_main
location: class layout

Please help

REPLY

From what I've read, setContentView (R.layout.activity_main) means this:
R - resource
layout - The layout directory
activity_main - activity_main.xml template

But I don't understand why it doesn't work, because the activity_main.xml template is in the correct layout directory ...

However, I managed to solve it and here is the whole process of resolving the errors:


1.)
-------------------------------------------------- ----------------
error: cannot find symbol
getMenuInflater (). inflate (R.menu.menu_activity_main, menu);
^
symbol: variable menu
location: class R


2.)
-------------------------------------------------- ----------------
I added
import android.R;

# new error:
error: cannot find symbol
setContentView (R.layout.activity_main);
^
symbol: variable activity_main
location: class layout


3.)
-------------------------------------------------- ----------------
# I removed
import android.R;
# I gave a Clean project + Rebuild project

4.)
-------------------------------------------------- ----------------
# I added
import cz.muj_balicek.R;

# this returned an error
error: cannot find symbol
getMenuInflater (). inflate (R.menu.menu_activity_main, menu);
^
symbol: variable menu
location: class R

5.)
-------------------------------------------------- ----------------
# I made
res / menu
# and pasted there
menu_activity_main.xml

6.)
-------------------------------------------------- ----------------
# New error
error: cannot find symbol
if (id == R.id.action_settings) {
^
symbol: variable action_settings
location: class id

# I repaired
rest / menu / menu_activity_main.xml

xmlns: app = "http://schemas.android.com/apk/res-auto">
android: id = "@ + id / action_settings"
android: orderInCategory = "100"
app: showAsAction = "never" />

7.)
-------------------------------------------------- ----------------
# another error
error: cannot find symbol
btnSave = (Button) findViewById (R.id.btnSave);
^
symbol: variable btnSave
location: class id

# I added a button to res / layout / edit_product.xml
btn s id btnSave

8.)
-------------------------------------------------- ----------------
Miracle!!! The Android app has finally launched!