import cn.hutool.core.io.FileUtil;
import cn.hutool.core.util.RandomUtil;
import cn.hutool.core.util.RuntimeUtil;
import java.io.File;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Test {
    static List<String> collect = Arrays.stream("cc,cn,co".split(",")).map(v -> v).collect(Collectors.toList());
    static  File file = new File("bydoman.txt");
    public static String getUrl(String random){
        List<String> list = new ArrayList<>();
        for(String s: collect){
            String url =  random + "." + s;
            String ok = RuntimeUtil.execForStr("nslookup ", url);
            if(ok.contains("找不到")){
                list.add(url);
                System.out.println(url);
            }
        }
        FileUtil.writeLines(list, file,"UTF-8",true);
        return null;
    }
    public static void main(String[] args) {
       while (true){
           String s = RandomUtil.randomString(3);
           getUrl(s);
       }
    }
}

需要引用程序包

        <dependency>
            <groupId>cn.hutool</groupId>
            <artifactId>hutool-all</artifactId>
            <version>5.3.10</version>
        </dependency>