Input/Output

Handling Data Formats

String class is handled in a special way for easy use. Literal String is an object.
4 ways how to concatenate an String - plus operator, or StringBuilder (StringBuffer for concurrency), String.concat()
String is immutable - It saves memory and use String pool, security reason (file can’t be corrupted), it can be safely shared between threads, String is final class (except of hash - effectively immutable) and can be used as key in HashMap (hashcode is saved -> fast)
Hashcode - Java calculates it only when it is needed and then cash the value for later use.
String and password - It is better to use char[]. The string is held in memory for a long time, it is immutable and therefore can be read later (i.e. memory dump). On other hand char[] can be erased, no risk to print it in logs.
String pool lives on Heap

Class Pattern is used (java.util.regex)
Pattern has method asPredicate() (since Java 8) for lambda expression
? optional char, * zero or more char, + one or more char, . any single char
/d digit, /D non-digit
/w word, /W nonword
/s whitespace /S non-whitespace
/n new line char, /t tab char
{M, N} between M and N of previous char
[ ] any char in bracket, [^ ] any char not in bracket
( ) group of patterns
| alternative (OR)
^ start of string, $ end of string

Integers types are all signed (represent negative and positive numbers)
BigDecimal has exact way of representing numbers (floating point error) and it is slower than Double

File is a cornerstone of an original way to do file i/o and represents file or directory. It has lots of methods
Stream is the sequential stream of bytes from disk or other sources. Abstract subclasses are InputStream and OutputStream then there are more specific FileInputStream or FileOutputStreams. It is dealing with the lack of flexibility. Often it is combined with Reader or Writer classes that provide character stream level of interaction.
The Modern I/O
- brand new API for i/o called NIO.2, package - java.nio.file
- Files:
  - File.createFile(target, attributes)
  - File.delete(target)
  - File.copy(source, target)
  - File.move(source, target)
  - File.isDirectory(target)
  - File.isSymbolicLink(target)
  - File.readAllLines(target, cs)
  - File.readAllBytes(target)
- Paths - has to get() method for creating Path object. The path is abstract representing file location.
- ByteBuffer - sequence of bytes
- Chanels