Android Public void กับ Private void มันต่างกัน ยังไงเนี่ย

package promlert.sudoku;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.content.Intent;
import android.view.View;
import android.view.View.OnClickListener;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.util.Log;

public class SudokuActivity extends ActionBarActivity implements OnClickListener {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        // about button ซูโดกุ
        View btnAbout = findViewById(R.id.about_button);
        btnAbout.setOnClickListener(this);
        // new game
        View btnNewGame = findViewById(R.id.new_game_button);
        btnNewGame.setOnClickListener(this);
        // continue
        View btnContinue = findViewById(R.id.continue_button);
        btnContinue.setOnClickListener(this);
        // exit
        View btnExit = findViewById(R.id.exit_button);
        btnExit.setOnClickListener(this);
    }
    
    public void onClick(View v) {
    	switch (v.getId()) {
    	case R.id.about_button:
    		Intent i = new Intent(this, About.class);
    		startActivity(i);
    		break;
    case R.id.new_game_button:
    		openNewGameDialog();
    		break;
    		
    	}
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
    	super.onCreateOptionsMenu(menu);
    	MenuInflater inflater =getMenuInflater();
    	inflater.inflate(R.menu.menu,  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.
    	switch (item.getItemId()) {
    	case R.id.settings:
    		Intent i = new Intent(this, SettingsActivity.class);
    		startActivity(i);
    		return true;
    	}
        return false;
    }
}

private static final String TAG = "Sudoku";

private void openNewGameDialog() {
    			// 	สร้าง Alert Dialog
    			AlertDialog.Builder dialog = new AlertDialog.Builder(this);
    			
    			// กำหนด Title ของ Alert Dialog
    			dialog.setTitle(R.string.new_game_title);
    			
    			dialog.setItems(R.array.difficulty,
    					new DialogInterface.OnClickListener() {
    					@Override
    					public void onClick(DialogInterface dialog, int i){
    						startGame(i);
    					}
    			});
    			
    			dialog.show();
    		}

private void startGame(int i) {
    			Log.d(TAG, "ผู้ใช้คลิกเลือก " + i);
    			// new game
    		}