Question

为什么在BlogPostAdapter的内部类ViewHolder上调用bindItems时会出现nullpointerexception错误？

我突出显示了我在下面的（ViewHolder - ＆gt; bindItems）中获得空指针的行。您可以在XML中看到findviewById（r.id.blogpost_author）存在，那么这里的问题是什么？它怎么样？

Adapter / 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)    // OMITTED VIEWHOLDER
}

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
    }
}

}

活动代码：

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)

Recyclerview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>

CardView布局的XML：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <android.support.v7.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">

        <TextView
            android:id="@+id/blogpost_author"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:padding="5dp"
            android:text="AuthorPlaceHolder"
            android:textAppearance="@style/Base.TextAppearance.AppCompat.Large"
            />

        <TextView
            android:id="@+id/blogpost_title"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:padding="5dp"
            android:text="TitlePlaceHolder"
            android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"
            />
        </LinearLayout>
    </android.support.v7.widget.CardView>
</LinearLayout>

Answer 1

您可能在RecyclerView中夸大了错误的布局。

onCreateViewHolder方法中的这一行：

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

你正在夸大blog_post_list.xml，我假设这是错误的布局文件，因为你在这里也在你的BlogPostListActivity中夸大了这个布局：

setContentView(R.layout.blog_post_list)

所以当这一行被调用时：

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

正在寻找id＆＃39; blogpost_author＆＃39;在R.layout.blog_post_list中，您可以看到该布局中没有blogpost_author TextView，因此它返回null。

要进行排序，它应该是直截了当的，只需更改您在onCreateViewHolder方法中为每个ViewHolder分配的布局资源，并为您的CardView布局更正。

这意味着该行应该是这样的：

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

RecyclerView - .findViewById上的NullPointerException

1 个答案: