In which situation val/var is necessary in Kotlin

问题:

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.

回答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.

回答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 this case, 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 case 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")
    }
}