Java

Important

• Java is pass-by-value!
• volatile vs static variables
• what does serialization of an object mean?
• nested classes
• visibility and access of member
• Generics and Collections
• Streams
• Java Operator Precedence
• Projekt Lombok boilerplate code reduction

Coding

• how to implement singelton pattern in Java?

• Class Builder pattern

• Remove boilerplate code with Lombok

• Why use Stringbuilder

• JavaBean is just a standard

  • All properties private (use getters/setters)
  • A public no-argument constructor
  • Implements Serializable.

• Difference between HashMap, LinkedHashMap and TreeMap

• Infinite Streams

  • HashMap makes absolutely no guarantees about the iteration order. It can (and will) even change completely when new elements are added.
  • TreeMap will iterate according to the "natural ordering" of the keys according to their compareTo() method (or an externally supplied Comparator). Additionally, it implements the SortedMap interface, which contains methods that depend on this sort order.
  • LinkedHashMap will iterate in the order in which the entries were put into the map

• Adding duplicate to set:

  • In the case of HashMap, it replaces the old value with the new one.
  • In the case of HashSet, the item isn't inserted.

• Java Date (java.time vs. java.util.date)

• Automating hashCode and equals with apache

Static block vs. initializer block

static {
  System.out.println("static init block")
}

{
  System.out.println("init block")
}

• static block is called once when class is loaded
  • used to init static variables
• init block is always called on object creation
  • used for code that is shared among multiple constructors
  • is called before constructor
  • is copied by compiler into constructor