Task: Append a record to the end of a text file
C: #include <stdio.h>
#include <string.h>

typedef const char *STRING;
typedef struct{STRING fullname, office, extension, homephone, email; } gecos_t;
typedef struct{STRING account, password; int uid, gid; gecos_t gecos; STRING directory, shell; } passwd_t;

#define GECOS_FMT "%s,%s,%s,%s,%s"
#define PASSWD_FMT "%s:%s:%d:%d:"GECOS_FMT":%s:%s"

passwd_t passwd_list[]={
  {"jsmith", "x", 1001, 1000, 
    {"Joe Smith", "Room 1007", "(234)555-8917", "(234)555-0077", "jsmith@rosettacode.org"},
    "/home/jsmith", "/bin/bash"},
  {"jdoe", "x", 1002, 1000,
    {"Jane Doe", "Room 1004", "(234)555-8914", "(234)555-0044", "jdoe@rosettacode.org"},
    "/home/jdoe", "/bin/bash"}
};

main(){

  FILE *passwd_text=fopen("passwd.txt", "w");
  int rec_num;
  for(rec_num=0; rec_num < sizeof passwd_list/sizeof(passwd_t); rec_num++)
    fprintf(passwd_text, PASSWD_FMT"\n", passwd_list[rec_num]);
  fclose(passwd_text);


  passwd_text=fopen("passwd.txt", "a+");
  char passwd_buf[BUFSIZ]; 
  passwd_t new_rec =
      {"xyz", "x", 1003, 1000, 
          {"X Yz", "Room 1003", "(234)555-8913", "(234)555-0033", "xyz@rosettacode.org"},
          "/home/xyz", "/bin/bash"};
  sprintf(passwd_buf, PASSWD_FMT"\n", new_rec);

  write(fileno(passwd_text), passwd_buf, strlen(passwd_buf));
  close(passwd_text);


  passwd_text=fopen("passwd.txt", "r");
  while(!feof(passwd_text))
    fscanf(passwd_text, "%[^\n]\n", passwd_buf, "\n");
  if(strstr(passwd_buf, "xyz"))
    printf("Appended record: %s\n", passwd_buf);
}

Java: import static java.util.Objects.requireNonNull;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class RecordAppender {
    static class Record {
        private final String account;
        private final String password;
        private final int uid;
        private final int gid;
        private final List<String> gecos;
        private final String directory;
        private final String shell;

        public Record(String account, String password, int uid, int gid, List<String> gecos, String directory, String shell) {
            this.account = requireNonNull(account);
            this.password = requireNonNull(password);
            this.uid = uid;
            this.gid = gid;
            this.gecos = requireNonNull(gecos);
            this.directory = requireNonNull(directory);
            this.shell = requireNonNull(shell);
        }

        @Override
        public String toString() {
            return account + ':' + password + ':' + uid + ':' + gid + ':' + String.join(",", gecos) + ':' + directory + ':' + shell;
        }

        public static Record parse(String text) {
            String[] tokens = text.split(":");
            return new Record(
                    tokens[0],
                    tokens[1],
                    Integer.parseInt(tokens[2]),
                    Integer.parseInt(tokens[3]),
                    Arrays.asList(tokens[4].split(",")),
                    tokens[5],
                    tokens[6]);
        }
    }

    public static void main(String[] args) throws IOException {
        List<String> rawData = Arrays.asList(
                "jsmith:x:1001:1000:Joe Smith,Room 1007,(234)555-8917,(234)555-0077,[email protected]:/home/jsmith:/bin/bash",
                "jdoe:x:1002:1000:Jane Doe,Room 1004,(234)555-8914,(234)555-0044,[email protected]:/home/jdoe:/bin/bash",
                "xyz:x:1003:1000:X Yz,Room 1003,(234)555-8913,(234)555-0033,[email protected]:/home/xyz:/bin/bash"
        );

        List<Record> records = rawData.stream().map(Record::parse).collect(Collectors.toList());

        Path tmp = Paths.get("_rosetta", ".passwd");
        Files.createDirectories(tmp.getParent());
        Files.write(tmp, (Iterable<String>) records.stream().limit(2).map(Record::toString)::iterator);

        Files.write(tmp, Collections.singletonList(records.get(2).toString()), StandardOpenOption.APPEND);

        try (Stream<String> lines = Files.lines(tmp)) {
            lines.map(Record::parse).forEach(System.out::println);
        }
    }
}