在RecyclerView.Adapter中绑定视图时发生NullPointerException [英] NullPointerException when binding Views in RecyclerView.Adapter
问题描述
为什么我在ViewHolder的bindItems()方法中得到一个NullPointerException?
Why do I get a NullPointerException in my ViewHolder's bindItems() method?
我突出显示了我获得NullPointerException的那一行.您可以在XML中看到blogpost_author ID，所以这是什么问题? findViewById<TextView>(R.id.blogpost_author)如何返回null?
I've highlighted the line where I get the NullPointerException. The blogpost_author ID exists, as you can see in the XML, so what's the problem here? How is findViewById<TextView>(R.id.blogpost_author) returning 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
}
}
}
Activity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:
Kotlin data class:
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>
推荐答案
您可能在RecyclerView中夸大了错误的布局.
You may be inflating the wrong layout within your RecyclerView.
onCreateViewHolder方法中的这一行:
This line within your onCreateViewHolder method:
val v = LayoutInflater.from(parent.context).inflate(R.layout.blog_post_list, parent, false)
您正在夸大blog_post_list.xml，我认为这是错误的布局文件，因为您还在这里在BlogPostListActivity中夸大了该布局:
You are inflating the blog_post_list.xml, which I'm assuming is the wrong layout file due to the fact you're also inflating that layout within your BlogPostListActivity here:
setContentView(R.layout.blog_post_list)
因此，当调用此行时:
val blogPostAuthor = itemView.findViewById<TextView>(R.id.blogpost_author)
它正在R.layout.blog_post_list中寻找id'blogpost_author'，并且您可以看到该布局中没有blogpost_author TextView，因此它返回null.
It is looking for the id 'blogpost_author' within R.layout.blog_post_list and as you can see there is no blogpost_author TextView within that layout so it returns null.
要对其进行整理，应该简单明了，只需使用为CardView布局正确的布局更改onCreateViewHolder方法中分配给每个ViewHolder的布局资源即可.
To sort it out, it should be straight forward and just change the layout resource that you're assigning to each ViewHolder within your onCreateViewHolder method with the correct layout for your CardView layout.
这意味着该行应显示如下内容:
Which means the line should read something like:
val v = LayoutInflater.from(parent.context).inflate(R.layout.your_card_layout, parent, false)
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