使用查询时出现 KotlinNullException

Question

我正在尝试构建一个应用程序，将 FirebaseDatabase 中的图像显示到 RecyclerView，类似于 tutorial 但使用 Kotlin，现在我有 kotlin.KotlinNullPointerException 并且它指向此代码行

var options = FirebaseRecyclerOptions.Builder<Blog>()
        .setQuery(query!!, Blog::class.java)
        .build()

这两天没解决，这是活动代码：

class MainActivity : AppCompatActivity() {
    private var mDatabase:DatabaseReference? = null
    private var mBlogList:RecyclerView?=null
    private var query:Query?=null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        navBar.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)
        mDatabase=FirebaseDatabase.getInstance().getReference().child("mall")
        mDatabase?.keepSynced(true)
        mBlogList = findViewById(R.id.recyclee)
        mBlogList?.setHasFixedSize(true)
        mBlogList?.layoutManager = LinearLayoutManager(this)
        query = mDatabase?.orderByKey()
    }
    var options = FirebaseRecyclerOptions.Builder<Blog>()
            .setQuery(query!!, Blog::class.java)
            .build()
    override fun onStart() {
        super.onStart()
        val mAdapter = object : FirebaseRecyclerAdapter<Blog, BlogViewHolder>(
                options) {
            override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BlogViewHolder {
                val view = LayoutInflater.from(parent.context)
                        .inflate(R.layout.cardview, parent, false)
                return BlogViewHolder(view)}

            override fun onBindViewHolder(holder: BlogViewHolder, position: Int, model: Blog) {
                holder.setTitle(model.title)
                holder.setDes(model.des)
                holder.setImage(applicationContext, model.image)
            }
        }
        mBlogList?.adapter = mAdapter
    }
    inner class BlogViewHolder : RecyclerView.ViewHolder{
        var mView:View?=null
        constructor(itemView: View?) : super(itemView) {
            this.mView = itemView
        }
        fun setTitle(title:String){
            var postTitle = mView?.findViewById<TextView>(R.id.post_title)
            postTitle?.text = title
        }
        fun setDes(des:String){
            var postDes = mView?.findViewById<TextView>(R.id.post_des)
            postDes?.text = des
        }
        fun setImage(ctx:Context, image:String){
            var postImage = mView?.findViewById<ImageView>(R.id.post_title)
            Picasso.get().load(image).into(postImage)
        }
    }
}

博客类：

class Blog(var title: String?, var des: String?, var image: String?)

我的数据库看起来像：

Answer 1

您正在尝试在 query 最初分配到 onCreate 之前使用它。只有在query 具有非空值之后，您才应该等待创建options。请注意，您的 options 位于班级成员级别，在 Android 调用您的 onCreate 之前会对其进行评估。