Java Keywords are predefined identifiers reserved by Java for a specific purpose. You cannot use keywords as names for your variables, classes, methods, and so on.

There are total of 50 reserved keywords in Java and they are, 

  1. abstract
  2. assert
  3. boolean
  4. break
  5. byte
  6. case
  7. catch
  8. char
  9. class
  10. const
  11. continue
  12. default
  13. do
  14. double
  15. else
  16. enum
  17. extends
  18. final
  19. finally
  20. float
  21. for
  22. goto
  23. if
  24. implements
  25. import
  26. instanceof
  27. int
  28. interface
  29. long
  30. native
  31. new
  32. package
  33. private
  34. protected
  35. public
  36. return
  37. short
  38. static
  39. strictfp
  40. super
  41. switch
  42. synchronized
  43. this
  44. throw
  45. throws
  46. transient
  47. try
  48. void
  49. volatile
  50. while
Currently the const and goto are not used but it is reserved for future use, we cannot used both as a identifier.

true, false and null are not a keyword they are called litereals, we cannot use this as an identifier.

Java Literals,
Literals are tokens that do not change, they are constant in value,

The different types of literals in Java are,
  1. Integer Literals
  2. Floating-Point Literals
  3. Boolean Literals
  4. Character Literals
  5. String Literals

Integer Literals:
Integer literals come in different formats,

  1. decimals which has base 10, for example: 15,
  2. hexadecimal has base 16, for example: 0xC
  3. octal has base 8, for example: 014
Floating Point Literals
Represent decimals with fractional parts, example 3.74, which has two notation are, 

  1. Standard Notation with value 3.23
  2. Scientific Notation with value 3.23e2
Boolean Literals:
Boolean literals have only two values, 
  1. true
  2. false
Character Literals:
Character Literals represent single unicode characters, Unicode allows the inclusion of symbols and special characters from other language, a 16-bit character set that replaces the 8-bit ASCII character set. To use a character literals, enclose the character in single quote delimiter. For eg. 'a'.
Special characters such as newline character, a backslash is used followed by the character code. for example, '\n' for the newline character, '\r' for the carriage return and '\b' for backspace.

String Literals:

String literals represent multiple characters and are enclosed by double quotes. 
An example of a string literal is, “Hello World”.