Kotlin lateinit属性，NPE危险吗？

Question

I am using lateinit properties in order to avoid continuous null checking with the ? operator. I have a lot of View properties that are assigned first time in the getViews() function. If that function weren't there, my application would crash with a NPE, from a Kotlin code.

In my opinion lateinit properties basically ruin the nice null safety features of the language. I know they are introduced in M13 because of better framework support, but does it worth it?

Or am I missing something here?

Here is the code:

package com.attilapalfi.exceptional.ui

import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.view.View
import android.widget.Button
import android.widget.ImageView
import android.widget.TextView
import com.attilapalfi.exceptional.R
import com.attilapalfi.exceptional.dependency_injection.Injector
import com.attilapalfi.exceptional.model.Exception
import com.attilapalfi.exceptional.model.ExceptionType
import com.attilapalfi.exceptional.model.Friend
import com.attilapalfi.exceptional.persistence.*
import com.attilapalfi.exceptional.rest.ExceptionRestConnector
import com.attilapalfi.exceptional.ui.helpers.ViewHelper
import com.attilapalfi.exceptional.ui.question_views.QuestionYesNoClickListener
import com.google.android.gms.maps.MapView
import java.math.BigInteger
import javax.inject.Inject

public class ShowNotificationActivity : AppCompatActivity(), QuestionChangeListener {
    @Inject
    lateinit val exceptionTypeStore: ExceptionTypeStore
    @Inject
    lateinit val friendStore: FriendStore
    @Inject
    lateinit val imageCache: ImageCache
    @Inject
    lateinit val metadataStore: MetadataStore
    @Inject
    lateinit val viewHelper: ViewHelper
    @Inject
    lateinit val exceptionInstanceStore: ExceptionInstanceStore
    @Inject
    lateinit val exceptionRestConnector: ExceptionRestConnector
    @Inject
    lateinit val questionStore: QuestionStore
    private lateinit var sender: Friend
    private lateinit var exception: Exception
    private lateinit var exceptionType: ExceptionType
    private lateinit var exceptionNameView: TextView
    private lateinit var exceptionDescView: TextView
    private lateinit var senderImageView: ImageView
    private lateinit var senderNameView: TextView
    private lateinit var sendDateView: TextView
    private lateinit var mapView: MapView
    private lateinit var questionText: TextView
    private lateinit var noButton: Button
    private lateinit var yesButton: Button

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_show_notification)
        Injector.INSTANCE.applicationComponent.inject(this)
        questionStore.addChangeListener(this)
        getModelFromBundle()
        getViews()
        loadViewsWithData()
    }

    override fun onDestroy() {
        super.onDestroy()
        questionStore.removeChangeListener(this)
    }

    private fun getModelFromBundle() {
        val bundle = intent.extras
        val instanceId = BigInteger(bundle.getString("instanceId"))
        exception = exceptionInstanceStore.findById(instanceId)
        sender = friendStore.findById(exception.fromWho)
        exceptionType = exceptionTypeStore.findById(exception.exceptionTypeId)
    }

    private fun getViews() {
        exceptionNameView = findViewById(R.id.notif_full_exc_name) as TextView
        exceptionDescView = findViewById(R.id.notif_exc_desc) as TextView
        senderImageView = findViewById(R.id.notif_sender_image) as ImageView
        senderNameView = findViewById(R.id.notif_sender_name) as TextView
        sendDateView = findViewById(R.id.notif_sent_date) as TextView
        mapView = findViewById(R.id.notif_map) as MapView
        questionText = findViewById(R.id.notif_question_text) as TextView
        noButton = findViewById(R.id.notif_question_no) as Button
        yesButton = findViewById(R.id.notif_question_yes) as Button
    }

    private fun loadViewsWithData() {
        exceptionNameView.text = exceptionType.prefix + "\n" + exceptionType.shortName
        exceptionDescView.text = exceptionType.description
        imageCache.setImageToView(sender, senderImageView)
        senderNameView.text = viewHelper.getNameAndCity(exception, sender)
        sendDateView.text = exception.date.toString()
        loadQuestionToViews()
    }

    private fun loadQuestionToViews() {
        if (exception.question.hasQuestion) {
            showQuestionViews()
        } else {
            hideQuestionViews()
        }
    }

    private fun showQuestionViews() {
        questionText.text = exception.question.text
        val listener = QuestionYesNoClickListener(exception, exceptionRestConnector, noButton, yesButton)
        noButton.setOnClickListener(listener)
        yesButton.setOnClickListener(listener)
    }

    private fun hideQuestionViews() {
        questionText.visibility = View.INVISIBLE
        noButton.visibility = View.INVISIBLE
        yesButton.visibility = View.INVISIBLE
    }

    override fun onQuestionsChanged() {
        onBackPressed()
    }
}

Answer 1

The same basic feature of lateinit was actually possible with Delegates.notNull prior to M13.

There's other features that also allow you to bypass the nullability enforcements. The !! operator will convert a nullable value into a non-null value.

The point is not to strictly require nullability constraints, but to make nullability a very explicit part of the language. Every time you use lateinit or !! you are making a conscious decision to leave the safety of the nullability constraints, hopefully with good reason.

As a rule of thumb it is best to avoid lateinit , !! , and even ? (nullable) as much as possible.

A lot of the time you can use a lazy delegate to avoid lateinit which can keep you in the realm of non-null vals (M14 now prohibits using vals with lateinit ).

The code you linked to includes a lot of lateinit views. You could keep these as non-null vals by doing something like this:

private val mapView: MapView by lazy { findViewById(R.id.notif_map) as MapView }

This will initialize the value the first time mapView is used and use the previously initialized value thereafter. The caveat is that this could break if you tried to use the mapView before the call to setContentView . However, that doesn't seem like that big of a deal and you've gained the benefit that your initialization is right next to your declaration.

This is what the kotterknife library used to achieve view injection.

Answer 2

Kotlin's lazy delegation will work for many cases although you will run into trouble when reloading Fragments that have been saved in FragmentManager. When the Android system rebuilds the fragment it will actually recreate the view causing a view?.findViewById(R.id.notif_map) to actually return an invalid View.

In these cases you will have to use a read-only property:

private val mapView: MapView
    get() = view?.findViewById(R.id.notif_map) as MapView