1. Api
   1. java.io
      1. Interface
         1. Serializable
         2. Object-Input/Output
      2. Class
         1. BufferedInput(Out)Stream
         2. BufferReader(Writer)
   2. java.lang
      1. Interface
         1. Comparable
         2. Iterable
         3. Runnable
      2. Class
         1. Wrapper
         2. Enum
         3. Math
         4. String
            1. Buffer
            2. Builder
         5. Thread
   3. java.util
      1. Interface
         1. Collection
         2. Comparator
         3. Iterator
         4. Map
            1. SortedMap
            2. NavigableMap
         5. Set
            1. SortedSet
            2. NavigableSet
      2. Class
         1. Arrays
         2. ArrayList
         3. Collections
         4. Calendar
         5. StringTokenizer
         6. Scanner
2. Theory
   1. Declaration & Access Control
      1. Interface
      2. Parameter
      3. Constructor
      4. Variable
         1. Primitive
            1. Subtopic 1
         2. Reference Var
         3. Enums
   2. OOP
      1. Modifier
         1. Access modifier
            1. priavte
            2. default
            3. Subtopic 1
            4. protected
            5. public
         2. Non-access modifier
            1. final
            2. static
            3. strictfp
            4. abstract
            5. synchronized
            6. native
      2. Encapsulation
      3. Polymorphism
         1. Overriding
         2. Overloadding
            1. Subtopic 1
   3. Thread
   4. IO
   5. Collection
   6. Inner vs Nested
   7. Generics
   8. Exception
   9. Casting
      1. Primitive var
      2. Reference var
3. Practical
   1. Shortcut IDE
   2. Debug
   3. c