Kotlin constructors: primary and secondary

Question

Just have started with Kotlin, where you can have a primary constructor and secondary ones. The question

Answer 1

the kotlin primary constructor help you to write the compact code :

• you can write class without body, e.g:data class, for example:

  data class Data(val value:String)
  

• if you don't have any annotation on constructor, then the keyword constructor can be ommitted. a negative example:

  class Foo @Annotation constructor()
  

• it make the inherit simply, for example:

  open class Bar(val value: String);
  
  class Primary(value: String, other: String) : Bar(value)
  
  class Secondary : Bar {
      constructor(value: String, other: String) : super(value)
  }
  

• it can using delegations by keyword by, but secondary constructor can't uses.

  interface Rule {
      fun apply(value: String): Int
  }
  
  
  open class Policy(rule: Rule) : Rule by rule;