Core Java. Lecture #5

Regular expressions. Wrapper classes. IO classes. DateTime API

ponomarev@corchestra.ru

@inponomarev

Ivan Ponomarev, Synthesized.io/MIPT

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

An example from StackOverflow: we want "1xxxx23" as an output:

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

  • Advantages

    • Can validate string format

    • Tolerable to optional separators, 'garbage' etc.

    • Groups are powerful device for data extraction

  • The price we pay

    • You have to learn (there are thick books on regular expressions)

    • You have to learn to use them correctly (you can easily shatter the performance)

    • They look horribly and perplexing

State machines

How can we parse the following string?

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

Definite State Machine!

dfa

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

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

Quoted value

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

Came across a quote mark

quote read
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

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

Conclusion

  • Mathematical fact: Every FSM can be converted into a regular expression, and vice versa.

  • When it comes to handling strings, regular expressions and DFAs are the bread and butter.

Wrapper Types

Primitive types are incompatible with objects. So there are Wrapper Classes:

byte, short, int, long

Byte, Short, Integer, Long

char

Character

float, double

Float, Double

boolean

Boolean

Wrapper-types are similar to strings

  • Objects are immutable. If a new one is needed, we make a new object, not modify the old one.

  • Equality check — with equals only!

  • Like strings in string pool, these objects are cached, so it they should be created with valueOf rather than new.

Object pool (caching)

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

What is cached

  • Integer, Short, Long — -128..127

  • 'Byte' — completely (-128..127)

  • Character — 0..127

  • 'Boolean' — 'Boolean.TRUE' and 'Boolean.FALSE'

  • 'Float', 'Double' — are not cached, although in the JavaDoc for valueOf the possible caching is mentioned.

  • Truth of Life: You’ll use autoboxing more often than valueOf.

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

Static methods:

  • Integer.parseInt(value, radix)

  • Integer.toString(value, radix)

  • Double.parseDouble(value)

  • Double.toString(value)

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

IBM2741

IBM2741 terminal, 1965 (source: Wikipedia)

A complete zoo of I/O classes

input output
Cay Horstmann. Core Java, vol. II

Classes for text input/output

reader writer
Cay Horstmann. Core Java, vol. II

Basic I/O streams

iostreams

Combining streams

combining
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

The modern, simple and correct method:

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

  • Obsolete File (can still be found, in particular, as an argument of the FileInputStream constructor).

  • More modern Path (Java 7+)

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

How many seconds are there in a day?

persistenceofmemory

System clock

  • 'long System.currentTimeMillis()' is the number of milliseconds that have elapsed since midnight on January 1, 1970 on the Greenwich Meridian minus the number of leap seconds, in terms of system clock.

    • Granularity can be greater than one millisecond.

    • "Freezes" when inserting a leap second.

    • Can "jump" in both directions when synchronizing via NTP!

  • 'long System.nanoTime()' — monotonous clock.

    • Nanosecond accuracy.

    • There is no reference to the "calendar" time.

    • Over long periods, inaccuracy of the clock may become noticeable.

When to use what?

  • 'long System.nanoTime()' — when measuring time periods between events occurred on a single machine.

    • But to measure the performance of Java programs, you should use JMH.

  • 'long System.currentTimeMillis()' — to generate time stamps (minding the constraints).

DateTime API

  • Obsolete, inconvenient, mutable, non-currently safe classes (do not use them):

    • Java 1.0 'Date' (it’s still with us because of JDBC),

    • Java 1.1 Calendar.

    • 'DateFormat' works with them.

  • Modern (Java 8+), immutable classes, learned from mistakes:

    • LocalDate, LocalTime, Instant, Duration, Period

    • ZoneId, ZonedDateTime.

    • 'DateTimeFormatter' works with them.

Instant

  • Point on time line

  • UNIX time

//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

  • Date and time in the "human" sense.

  • Without information about time zone → without reference to 'Instant'.

  • Often binding to an Instant is not needed: "weekly call at 9:00" bound to Instant will "creep" due to daylight saving time.

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

  • 'Duration': The amount of time between two points on a time line with nanosecond precision.

  • 'Period': the time difference between `LocalDate`s in days (e.g. 'how many days is the contract valid?'

  • Both implement the TemporalAmount interface.

//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]