Core Java. Lecture #5

Regular expressions

String input = "11:59 am";
Pattern pattern =
  Pattern.compile("([1-9]|1[0-2])\\s*:\\s*([0-5][0-9])\\s*([ap]m)");
Matcher matcher = pattern.matcher(input);

//matches() requires full match for the whole string,
//find() -- finds the first match
if (matcher.matches()) {
    String hours = matcher.group(1); //11
    String minutes = matcher.group(2); //59
    String period = matcher.group(3); //am
}

Find/replace in a loop

Map<String, String> replacements = new HashMap<String, String>();

replacements.put("${env1}", "1");
replacements.put("${env2}", "2");
replacements.put("${env3}", "3");


String line ="${env1}xxxx${env2}${env3}";

Find/replace in a loop

//Before Java9 StringBuffer was used
StringBuilder sb = new StringBuilder();

Pattern p = Pattern.compile("(\\$\\{[^}]+\\})");
Matcher m = p.matcher(line);
while (m.find()) {
    String repString = replacements.get(m.group(1));
    if (repString != null)
        m.appendReplacement(sb, repString);
}
m.appendTail(sb);

System.out.println(sb.toString()); //1xxxx23

Regular expressions

State machines

  Name,Goods and Services
  LLC "Monkey Business","stuff, things"
  "LLC ""Sea Shore Shop""",sea shells

Definite State Machine!

Implementation

enum State {START, QUOTED_BODY, QUOTE_READ, UNQUOTED_BODY}
List<String> parse(String line) {
  State state = State.START;
  List<String> result = new ArrayList<>();
  StringBuilder fieldValue = new StringBuilder();
  for (char c : line.toCharArray()) {
    switch (state) {
        case START -> { . . .  }
        case QUOTED_BODY -> { . . .  }
        case QUOTE_READ -> { . . . }
        case UNQUOTED_BODY -> { . . . }
    }
  }
  result.add(fieldValue.toString());
  return result;
}

Started to read the field

case START -> {
    fieldValue = new StringBuilder();
    if (c == '"')
        state = State.QUOTED_BODY;
    else {
        fieldValue.append(c);
        state = State.UNQUOTED_BODY;
    }}

Quoted value

case QUOTED_BODY -> {
    if (c == '"')
        state = State.QUOTE_READ;
    else {
        fieldValue.append(c);
    }}

Came across a quote mark

case QUOTE_READ -> {
    if (c == '"') {        fieldValue.append(c);
                           state = State.QUOTED_BODY;
    } else if (c == ',') { result.add(fieldValue.toString());
                           state = State.START;
    } else {               //invalid input format
                           throw new IllegalStateException();
    }}

Unquoted field

case UNQUOTED_BODY -> {
    if (c == ',') { result.add(fieldValue.toString());
                    state = State.START;
    } else {        fieldValue.append(c);
    }}

Conclusion

Wrapper Types

Wrapper-types are similar to strings

Object pool (caching)

Integer five1 = Integer.valueOf(5); Integer five2 = Integer.valueOf(5); /* DO NOT DO THIS!!*/ Integer five3 = new Integer(5);

What is cached

Autoboxing & unboxing

ArrayList<Integer> list = new ArrayList<>();

//autoboxing :-)
list.add(3);
//without autoboxing :-(
list.add(Integer.valueOf(3));

//unboxing :-)
int n = list.get(i);
//with no automatic unboxing :-(
int n = list.get(i).intValue();

Autoboxing is not always that obvious

Integer n = 3;
//unbox - increment - box.
//not so effective!!
n++;

Integer a = 1000;
Integer b = 1000;
if (a == b) . . . //NO! THIS WILL FAIL!

Convert a number from and to a string

BigInteger class

BigInteger i = new BigInteger("123456789012345678901234567890");
BigInteger j = new BigInteger("123456789012345678901234567892");

BigInteger sum = i.add(j);
BigInteger difference = i.subtract(j);
BigInteger quotient = i.divide(j);
BigInteger product = i.multiply(j);

BigDecimal class

public static BigDecimal calculateTotalAmount(
      BigDecimal quantity,     BigDecimal unitPrice,
      BigDecimal discountRate, BigDecimal taxRate) {
    BigDecimal amount = quantity.multiply(unitPrice);
    BigDecimal discount = amount.multiply(discountRate);
    BigDecimal discountedAmount = amount.subtract(discount);
    BigDecimal tax = discountedAmount.multiply(taxRate);
    BigDecimal total = discountedAmount.add(tax);

    // round to 2 decimal places using HALF_EVEN
    BigDecimal roundedTotal = total.setScale(2, RoundingMode.HALF_EVEN);
    return roundedTotal;
}

Input/Output

A complete zoo of I/O classes

Classes for text input/output

Basic I/O streams

Combining streams

Path path = Paths.get("path/to/file.dat");

try (DataInputStream dis =
         new DataInputStream(
             new BufferedInputStream(
                 Files.newInputStream(path)))) {
    double doubleVal = dis.readDouble();
    long longVal = dis.readLong();
}

Combining Streams: Reading a Zip File

Path path = Paths.get("path/to/file.zip");

try(ZipInputStream zis =  new ZipInputStream(
    Files.newInputStream(path));
    DataInputStream dis = new DataInputStream(zis)) {
    . . .
}

Read/write text data

Path path = Paths.get("path/to/file.txt");

//Lazy reading
Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);

//Reading of all the contents into memory
List<String> strings =
  Files.readAllLines(path, StandardCharsets.UTF_8);

Using BufferedReader

try (BufferedReader br =
     Files.newBufferedReader(path, StandardCharsets.UTF_8)){
    String line;
    while ((line = br.readLine())!=null){
           . . .
    }
}

Save prepared data to text

Path path = Paths.get("path/to/file.txt");
List<String> lines = new ArrayList<>();
lines.add(...)

//If everything is already in memory
Files.write(path, lines, StandardCharsets.UTF_8);

Saving the text data incrementally

try (PrintWriter printWriter = new PrintWriter(
                Files.newBufferedWriter(
                  path, StandardCharsets.UTF_8))) {
            printWriter.println(...);
            printWriter.printf(...);
        }

Working with the file system

Working with the file system

//Path does not necessarily specify
//an existing file or directory!!

Path absolute = Paths.get("/home", "harry");
Path relative = Paths.get("myprog", "conf", "user.properties");

Path subPath = absolute.resolve("file.txt");

subPath.getParent(); //parent
subPath.getFileName(); //the last component of the path

Working with the file system

//all components except the last component must exist
Files.createDirectory(path);
creates all directories in path
Files.createDirectories(path);
creates an empty file
Files.createFile(path);

//creates a temporary file in an OS-specific location
Files.createTempFile(prefix, suffix);

File verification

//it makes sense to check before you open a stream for reading
Files.exists(path)
Files.isReadable(path)

Files.size(path)

Getting the list of files and depth-first directory traversal

//the contents of the folder (without going to subfolders)
Stream<Path> list = Files.list(path);

//traverse the folder tree
Stream<Path> Files.walk(Path start,
   int maxDepth, FileVisitOption... options)

Copy, move and delete files

Files.copy(fromPath, toPath)

Files.move(fromPath, toPath)

Files.delete(path)

Clocks and Datetime API

System clock

When to use what?

DateTime API

Instant

//Current instant
Instant.now();
//explicitly specified Instant
Instant.ofEpochSecond(3);
//What year is it today?
int year = Instant.now().get(ChronoField.YEAR);

LocalDate, LocalTime and LocalDateTime

LocalDate date = LocalDate.of(2014, 3, 18); // March 18, 2014

int year      = date.getYear(); //2014
Month month   = date.getMonth();  //MARCH
int day       = date.getDayOfMonth(); //18
DayOfWeek dow = date.getDayOfWeek(); //TUESDAY

LocalTime and LocalDateTime

LocalTime time = LocalTime.of(13, 45, 20); //13:45:20

int hour   = time.getHour();
int minute = time.getMinute();
int second = time.getSecond();

LocalDateTime dt1 = LocalDateTime.of(2014, Month.MARCH, 18, 13, 45, 20);
LocalDateTime dt1 = LocalDateTime.of(date, time);
LocalDateTime dt2 = date.atTime(time);
LocalDateTime dt3 = time.atDate(date);

dt1.toLocalDate(); //2014-03-18
dt1.toLocalTime(); //13:45:20

Duration and Period

//LocalTime, LocalDateTime, and Instant are suitable as arguments
Duration d = Duration.between(time1, time2);

//Only LocalDate is suitable as an argument
Period tenDays = Period.between(
    LocalDate.of(2014, 3, 8),
    LocalDate.of(2014, 3, 18));

Constructing new objects from old ones

Instant i = Instant.now();
Duration d = Duration.ofSeconds(3);

//For example:
i.plus(dd.multipliedBy(10))
 .minus(dd.minus(Duration.ofMillis(1)));

//the same can be done with LocalDateTime

Format and parse date and time

DateTimeFormatter formatter1 =
  DateTimeFormatter.ISO_LOCAL_DATE; // YYY-MM-DD
DateTimeFormatter formatter2 =
  DateTimeFormatter.ofPattern("dd/MM/yyyy");
DateTimeFormatter formatter3 = new DateTimeFormatterBuilder()
  .appendText(ChronoField.DAY_OF_MONTH)
  .appendLiteral(". ")
  .appendText(ChronoField.MONTH_OF_YEAR)
  .appendLiteral(" ")
  .appendText(ChronoField.YEAR)
  .parseCaseInsensitive()
  .toFormatter(Locale.ITALIAN);

To string and from string

LocalDate date = LocalDate.of(2014, 3, 18);
String s = date.format(
  DateTimeFormatter.ISO_LOCAL_DATE); // 2014-03-18
LocalDate date2 = LocalDate.parse(
  s, DateTimeFormatter.ISO_LOCAL_DATE);

Time zones

ZoneId moscowZone = ZoneId.of("Europe/Moscow");
ZoneId romeZone = ZoneId.of("Europe/Rome");
LocalDateTime dateTime = LocalDateTime.now();

ZonedDateTime moscowTime = dateTime.atZone(moscowZone);
System.out.println(moscowTime);
//2019-09-30T18:15:52.670+03:00[Europe/Moscow]

ZonedDateTime romeTime =
  moscowTime.withZoneSameInstant(romeZone);
System.out.println(romeTime);
//2019-09-30T17:15:52.670+02:00[Europe/Rome]