Android 在RecyclerView.Adapter中绑定视图时出现NullPointerException

为什么在我的

ViewHolder

的

bindItems（）

方法中会出现

NullPointerException

我突出显示了获取

NullPointerException

的那一行。正如您在XML中看到的那样，

blogpost\u作者的ID是存在的，那么这里有什么问题呢？
findViewById（R.id.blogpost\u author）
如何返回null

Adapter
和
ViewHolder
代码：

class BlogPostAdapter(val blogList: ArrayList<BlogPost>) : RecyclerView.Adapter<BlogPostAdapter.ViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) : BlogPostAdapter.ViewHolder {
        val v = LayoutInflater.from(parent.context).inflate(R.layout.blog_post_list, parent, false)
        return ViewHolder(v)
    }

    override fun getItemCount(): Int {
        return blogList.size
    }

    override fun onBindViewHolder(holder: BlogPostAdapter.ViewHolder, position: Int) {
        holder.bindItems(blogList[position])
    }

    class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        fun bindItems(blogPost: BlogPost) {
            val blogPostAuthor = itemView.findViewById<TextView>(R.id.blogpost_author) // THIS LINE - NULL POINTER EXCEPTION
            val blogPostTitle = itemView.findViewById<TextView>(R.id.blogpost_title)
            blogPostAuthor.text = blogPost.author
            blogPostTitle.text = blogPost.title
        }
    }
}

class BlogPostListActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.blog_post_list)

        // Get the RecyclerView from XML itself
        val recyclerView = findViewById<RecyclerView>(R.id.recyclerview)

        // Add a layout manager - What does a layout manager do?
        recyclerView.layoutManager = LinearLayoutManager(this, LinearLayout.VERTICAL, false)

        // Create an array list to store blogposts using the the data class blogPost
        val blogPosts = ArrayList<BlogPost>()

        // Add some dummy data to the list
        blogPosts.add(BlogPost(123, "First Blog Post", "John"))
        blogPosts.add(BlogPost(456, "Second Blog Post", "Bob"))
        blogPosts.add(BlogPost(789, "Third Blog Post", "Mary"))

        // Create an adapter
        val adapter = BlogPostAdapter(blogPosts)

        // Add the adapter to the recyclerview
        recyclerView.adapter = adapter
    }
}

Kotlin数据类：

data class BlogPost(val id: Int, val title: String, val author: String)

用于回收视图的XML：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.topzap.android.kotlinlistapptest.BlogPostListActivity">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyclerview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:layout_editor_absoluteX="8dp"
        tools:layout_editor_absoluteY="8dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"/>
</android.support.constraint.ConstraintLayout>

您可能在您的RecyclerView中膨胀了错误的布局

onCreateViewHolder方法中的此行：

val v = LayoutInflater.from(parent.context).inflate(R.layout.blog_post_list, parent, false)

您正在膨胀blog_post_list.xml，我假设它是错误的布局文件，因为您也在膨胀BlogPostListActivity中的布局：

setContentView(R.layout.blog_post_list)

因此，当调用此行时：

val blogPostAuthor = itemView.findViewById<TextView>(R.id.blogpost_author)

你试过val recyclerView=findViewById（R.id.recyclerView）作为RecyclerViewHah的可能副本吗，我不敢相信是这个！事实上，我指的是第二个布局xml，而不是recyclerview布局。一个更好的命名约定会帮助我发现这一点！谢谢，迪恩。
val blogPostAuthor = itemView.findViewById<TextView>(R.id.blogpost_author)

val v = LayoutInflater.from(parent.context).inflate(R.layout.your_card_layout, parent, false)