Java练习：是GBK格式文本吗？

import java.io.*;

public class Main {

    public static void main(String[] args) throws IOException {

        String filePath = "D:/temp/temp.txt";
        String str = "AAAAA\n我的成功之路\n66666\n一路畅通。";
        PrintWriter writer = new PrintWriter(filePath, "UTF-8");
        writer.println(str);
        writer.flush();
        writer.close();
        if (isGBK(filePath) == true) {
            System.out.println(filePath + ": GBK格式。");
        } else {
            System.out.println(filePath + ": 不是GBK格式。");
        }

        writer = new PrintWriter(filePath, "GBK");
        writer.println(str);
        writer.flush();
        writer.close();
        if (isGBK(filePath) == true) {
            System.out.println(filePath + ": GBK格式。");
        } else {
            System.out.println(filePath + ": 不是GBK格式。");
        }
    }


    public static boolean isGBK(String path){
        String str = null;
        int counter = 0;
        int error = 0;
        try (BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(path), "GBK"))) {
            while ((str = br.readLine()) != null) {
                counter ++;
                if (java.nio.charset.Charset.forName("GBK").newEncoder().canEncode(str) == false)  {
                    error ++;
                }
            }
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
        }
        if (counter < 1 || error < 1) {
            return true;
        } else {
            return false;
        }
    }
}