In which situation val/var is necessary in Kotlin constructor parameter?

Question

Right code:

class MainActHandler(val weakActivity: WeakReference<Activity>): Handler() {     override fun handleMessage(msg: Message?) {         val trueAct = weakActivity.get() ?: return         if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){             val sentence = msg.obj as String?             trueAct.conversation.text = sentence         }         super.handleMessage(msg)     } } 

cannot be resolved code:

class MainActHandler(weakActivity: WeakReference<Activity>): Handler() {     override fun handleMessage(msg: Message?) {         val trueAct = weakActivity.get() ?: return         if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){             val sentence = msg.obj as String?             trueAct.conversation.text = sentence         }         super.handleMessage(msg)     } } 

cannot be resolved code screenshot

The only difference is the "val" has been deleted and cannot be resolve.

Which might be important is that it's a inner class.

BUT

This one class without "val/var" in constructor parameter is working:

class BookInfo(convrMgr: ConversationMgr, id: String, queue: RequestQueue, queueTag:String) {  val TAG = "BookInfo" var title: String? = ""  init {     val url = "https://api.douban.com/v2/book/$id"     // Request a string response from the provided URL.     val stringRequest = StringRequest(Request.Method.GET, url,             Response.Listener<String> { response ->                 Log.d(TAG + " Response", response.substring(0))                 // Parse JSON from String value                 val parser = Parser()                 val jsonObj: JsonObject =                         parser.parse(StringBuilder(response.substring(0))) as JsonObject                 // Initial book title of book properties.                 title = jsonObj.string("title")                 Log.d(TAG + " Book title", title)                 convrMgr.addNewMsg(title)             },             Response.ErrorListener { error -> Log.e(TAG + " Error", error.toString()) })     // Set the tag on the request.     stringRequest.tag = queueTag     // Add the request to the RequestQueue.     queue.add(stringRequest) } 

}

And if I add var/val before "queue: RequestQueue", I'll get suggestion:

"Constructor parameter is never used as a property less. This inspection reports primary constructor parameters that can have 'val' or 'var' removed. Unnecessary usage of 'val' and 'var' in primary constructor consumes unnecessary memory."

I am just confused about it.

Answer 1

When you write val/var within the constructor, it declares a property inside the class. When you do not write it, it is simply a parameter passed to the primary constructor, where you can access the parameters within the init block or use it to initialize other properties. For example,

class User(val id: Long, email: String) {     val hasEmail = email.isNotBlank()    //email can be accessed here     init {         //email can be accessed here     }      fun getEmail(){         //email can't be accessed here     } } 

Constructor parameter is never used as a property

This suggestion is saying that you do not use this property in place apart from the initialization. So, it suggests you to remove this property from the class.

Answer 2

Constructor parameters must use var or val when they are used as a property elsewhere in the class. They do not need to be properties if they are only used for class initialization.

In the example below, the parameter must be a property (var or val) because it is used in a method:

class A(val number: Int) {     fun foo() = number } 

In this other example, the parameter is only used to initialize the class, so it does not need to be a property:

class B(number: Int): A(number) {     init {         System.out.println("number: $number")     } }