为什么在我的ViewHolder
的bindItems()
方法中会出现NullPointerException
?
我已经用高亮标记了出现NullPointerException
的行。正如您在XML中所看到的,blogpost_author
ID是存在的,那么问题出在哪里呢?为什么findViewById<TextView>(R.id.blogpost_author)
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
}
}
}
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:
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>