I am a new to Java Android . And I wanna help to my project . I want to solve the errors which have say ( Cannot resolve symbol R and the others )

Question

package com.example.abdoanany.sqliteapp;

import android.app.AlertDialog;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    DatabaseHelper myDb;
    EditText editName,editSurname,editMarks ,editTextId;
    Button btnAddData;
    Button btnviewAll;
    Button btnDelete;
    Button btnviewUpdate;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myDb = new DatabaseHelper(this);
        editName = (EditText)findViewById(R.id.editText_name);
        editSurname = (EditText)findViewById(R.id.editText_surname);

        editMarks = (EditText)findViewById(R.id.editText_Marks);
        editTextId = (EditText)findViewById(R.id.editText_id);
        btnAddData = (Button)findViewById(R.id.button_add);
        btnviewAll = (Button)findViewById(R.id.button_viewAll);
        btnviewUpdate= (Button)findViewById(R.id.button_update);
        btnDelete= (Button)findViewById(R.id.button_delete);
        AddData();
        viewAll();
        UpdateData();
        DeleteData();
    }
    public void DeleteData() {
        btnDelete.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {

                        Integer deletedRows = myDb.deleteData(editTextId.getText().toString());
                        if(deletedRows > 0)
                            Toast.makeText(MainActivity.this,"Data Deleted",Toast.LENGTH_LONG).show();
                        else
                            Toast.makeText(MainActivity.this,"Data not Deleted", Toast.LENGTH_LONG).show();

                    }
                }
        );
    }
    public void UpdateData() {
        btnviewUpdate.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        boolean isUpdate = myDb.updateData(editTextId.getText().toString(),
                                editName.getText().toString(),
                                editSurname.getText().toString(),editMarks.getText().toString());
                        if(isUpdate == true)
                            Toast.makeText(MainActivity.this,"Data Update",Toast.LENGTH_LONG).show();
                        else
                            Toast.makeText(MainActivity.this,"Data not Updated",Toast.LENGTH_LONG).show();
                    }
                }
        );
    }
    public void AddData() {
        btnAddData.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        boolean isInserted = myDb.insertData(editName.getText().toString(),
                                editSurname.getText().toString(),
                                editMarks.getText().toString() );
                        if(isInserted == true)
                            Toast.makeText(MainActivity.this,"Data Inserted",Toast.LENGTH_LONG).show();
                        else
                            Toast.makeText(MainActivity.this,"Data not Inserted",Toast.LENGTH_LONG).show();
                    }
                }
        );
    }

    public void viewAll() {
        btnviewAll.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        Cursor res = myDb.getAllData();
                        if(res.getCount() == 0) {
                            // show message
                            showMessage("Error","Nothing found");
                            return;
                        }

                        StringBuffer buffer = new StringBuffer();
                        while (res.moveToNext()) {
                            buffer.append("Id :"+ res.getString(0)+"\n");
                            buffer.append("Name :"+ res.getString(1)+"\n");
                            buffer.append("Surname :"+ res.getString(2)+"\n");
                            buffer.append("Marks :"+ res.getString(3)+"\n\n");
                        }

                        // Show all data
                        showMessage("Data",buffer.toString());
                    }
                }
        );
    }

    public void showMessage(String title,String Message){
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setCancelable(true);
        builder.setTitle(title);
        builder.setMessage(Message);
    builder.show();

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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return `super.onOptionsItemSelected`(item);
    }
}

Answer 1

If Project - Clean or/and Project - Rebuild would not help, most probable reason, that you somewhere made error in xml files.

In that case check all your xml, maybe you lost some resource or something.