Kotlin recyclerview 给了我空指针异常

Question

i started learing Kotlin after using Java, curently trying to create recyclerview using Kotlin, but everytime reach this code:

recyclerview!!.layoutManager = LinearlayoutManager(context)

inside a fragment, it always returns null for me, and by converting existing Java Code using converter in android studio returns error.Can anyone help me why this is always happened? code below is my current fragment code:

import android.os.Bundle
import android.support.v4.app.Fragment
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import kotlinx.android.synthetic.main.fragment_notes.*
import java.sql.Timestamp

class NotesFragment : Fragment() {

    companion object {
        fun newInstance(): NotesFragment {
            return NotesFragment()
        }
    }

    override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val view = inflater?.inflate(R.layout.fragment_notes, container, false)
        val messageList : MutableList<message> = prepareMessage()

        recycler_view!!.layoutManager = LinearLayoutManager(context)
        recycler_view!!.adapter = NotesAdapter(context)   
        (recycler_view!!.adapter as NotesAdapter).setMessage(messageList)
        return view
    }

    private fun prepareMessage(): MutableList<message> {
        var messageList : MutableList<message> = ArrayList()
        for(i in 0..10){
            val timestamp: Long = System.currentTimeMillis()/1000L
            val message = message(0,
                    "Judul Catatan ke $i",
                    resources.getString(R.string.lipsum),
                    timestamp
                    )
            messageList.add(message)
        }
        return messageList
    }

}

Answer 1

Thats because the views are not created yet in onCreateView . Since you are using kotlinx.android.synthetic you need to access it inside onViewCreated .Like this

   override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
     val view = inflater?.inflate(R.layout.fragment_notes, container, false)
     return view
   }


  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    recycler_view = view.findViewById(R.id.your_recycler_id)
    recycler_view?.layoutManager = LinearLayoutManager(context)
    recycler_view?.adapter = NotesAdapter(context)

    val messageList : MutableList<message> = prepareMessage()
    (recycler_view?.adapter as NotesAdapter).setMessage(messageList)
}

Answer 2

Try this...

You should declare your recyclerview object, before accessing their properties.

.....

 private var recycler_view: RecyclerView? = null

.....

override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    val view = inflater?.inflate(R.layout.fragment_notes, container, false)

    recycler_view = view.findViewById(R.id.your_recycler_id)
    recycler_view?.layoutManager = LinearLayoutManager(context)
    recycler_view?.adapter = NotesAdapter(context)  

    val messageList : MutableList<message> = prepareMessage()
    (recycler_view?.adapter as NotesAdapter).setMessage(messageList)
    return view
}

Answer 3

你有没有像这样声明你的 recylerview

 polishrecyler = view .findViewById<View>(R.id.polish_recyler) as RecyclerView

Answer 4

I just now had a similar situation.

In my project, I have a RecyclerView working perfectly. In my case, it has five files involved: Foo.kt (the model), FooList.kt (the AppCompatActivity class), FooListAdapter.kt , foo_list.xml (the layout with the RecyclerView ) and foo_list_row.xml (my custom row for the ReyclerView ). Not sure if it's important or not, but the icon for Foo.kt is while the icon for the FooList.kt and FooListAdapter.kt is

I needed another RecyclerView, so I simply created five new files: Bar.kt , BarList.kit , BarListAdapter.kit , bar_list.xml and bar_list_row.xml . I copied/pasted the code from one to the other, and made the appropriate Foo->Bar changes in the code. This is when I got the same error as the OP. Looking in the import section, the import for RecyclerView was flagged as "superfluous". Odd.

Most of the suggested answers here is to declare the variable and use findViewById to reference it. But, my working RecyclerView doesn't do that, so why should it be necessary in the second RecyclerView ? It doesn't make sense.

I continued to search and found this solution:

1. Save the code found in two files I had created manually ( BarList.kit (the AppCompatActivity file) and the 'corresponding' bar_list.xml )
2. Delete those files.
3. Right-click on the package name and select New > Activity > Empty Activity . This re-created the files I had just deleted with exactly the same names.
4. Restore the code saved from step 1 to the appropriate files.

Now both my RecyclerView s work correctly.

I guess when you use the New > Activity > Empty Activity , Android Studio does some work in the background which is not done if you just create the files manually.