我希望以编程方式（不含XML包含方式）添加一个RelativeLayout，它是在从外部函数设置之后在外部XML中创建的。
错误如下：
FATAL EXCEPTION: main
Process: ucam, PID: 8950
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setId(int)' on a null object reference
at ucam.misc.MessageSetter.setMessageInfo(MessageSetter.java:51)
at ucam.activities.MainBoard$3$1.onSuccess(MainBoard.java:133)
at ucam.misc.RequestManager$1.onResponse(RequestManager.java:58)
at ucam.misc.RequestManager$1.onResponse(RequestManager.java:43)
at com.android.volley.toolbox.JsonRequest.deliverResponse(JsonRequest.java:65)
at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:99)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
这里是mainactivity.java中的代码：
private Message m;
private List<Message> ml;
private TextView textBox, dateBox, likesBox, textTitle;
private ImageButton likeButton;
private LinearLayout content, main;
private RelativeLayout news;
private MessageSetter msetter;
private RequestManager rmanager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main_board);
news = (RelativeLayout) findViewById(R.id.news_template);
（...）
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
switch (item.getItemId()) {
case R.id.navigation_recents:
rmanager.getRecentMessages(new VolleyCallbackOp() {
public void onSuccess(List<Message> ml) {
for (int a = 0; a < ml.size(); a++) {
Message j = ml.get(a);
msetter = new MessageSetter(MainBoard.this);
View v = msetter.setMessageInfo(j, a);
});
content.addView(v);
}
}
});
这是外部制定者类：
public class MessageSetter {
public Activity mainBoard;
private TextView textBox, dateBox, likesBox, textTitle;
private ImageButton likeButton;
private RelativeLayout news;
public MessageSetter(Activity mainBoard) {
this.mainBoard = mainBoard;
this.news = (RelativeLayout) this.mainBoard.findViewById(R.id.news_template);
this.textBox = (TextView) this.mainBoard.findViewById(R.id.text_box);
this.dateBox = (TextView) this.mainBoard.findViewById(R.id.date_box);
this.likesBox = (TextView) this.mainBoard.findViewById(R.id.likes_box);
this.textTitle = (TextView) this.mainBoard.findViewById(R.id.text_title);
}
public View setMessageInfo(Message j, int a) {
int idDB = a;
int idLB = a;
int idLBT = a;
textTitle.setId(a);
textTitle.setText(j.getTitle());
textBox.setText(j.getContent());
textBox.setId((j.getId()));
dateBox.setText(String.valueOf(j.getDate()));
dateBox.setId(idDB);
likesBox.setText(String.valueOf(j.getLikesCount()));
likesBox.setId(idLB);
return news;
}
这是主要活动布局：
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout
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:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:context=".activities.MainBoard"
tools:openDrawer="start">
<ScrollView
android:id="@+id/main_scroll"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<LinearLayout
android:id="@+id/main_linear"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<include layout="@layout/app_bar_main_board" />
<LinearLayout
android:id="@+id/main_content"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="wrap_content">
</LinearLayout>
</LinearLayout>
</ScrollView>
<android.support.design.widget.CoordinatorLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.design.widget.BottomNavigationView
android:id="@+id/navigation"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom"
android:background="?android:attr/windowBackground"
app:menu="@menu/navigation" />
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="72dp"
android:layout_gravity="center|end"
android:layout_margin="@dimen/fab_margin"
app:srcCompat="@drawable/ic_comment_black_24dp" />
</android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.NavigationView
android:id="@+id/nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
app:headerLayout="@layout/nav_header_main_board"
app:menu="@menu/activity_main_board_drawer" >
</android.support.design.widget.NavigationView>
</android.support.v4.widget.DrawerLayout>
最后，这是我想要设置并包含在活动中的XML外部布局文件：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_marginTop="3dp"
android:layout_marginRight="3dp"
android:layout_marginLeft="3dp"
android:id="@+id/news_template"
android:layout_width="match_parent"
android:layout_height="120dp"
android:background="#5A8FF2"
android:orientation="horizontal"
tools:context=".activities.MainBoard">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="2dp"
android:id="@+id/text_title"
android:textColor="#000000"
android:textStyle="normal"
android:textSize="14dp"
android:gravity="center"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<TextView
android:id="@+id/text_box"
android:textColor="#000000"
android:textStyle="normal"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="14dp"
android:padding="10dp"
android:layout_above="@+id/news_answers"
android:layout_below="@+id/text_title"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true">
</TextView>
<LinearLayout
android:id="@+id/news_answers"
android:layout_width="20dp"
android:layout_height="20dp"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
android:layout_marginBottom="15dp"
android:layout_marginLeft="25dp"
android:orientation="horizontal">
</LinearLayout>
<LinearLayout
android:id="@+id/news_likes"
android:layout_marginRight="8dp"
android:layout_width="120dp"
android:layout_height="match_parent"
android:layout_alignParentBottom="true"
android:layout_alignTop="@+id/imageView2"
android:gravity="right"
android:orientation="vertical"
android:weightSum="2"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true">
<TextView
android:layout_width="wrap_content"
android:layout_height="0dp"
android:layout_weight="1"
android:textSize="12dp"
android:text="Likes" />
<TextView
android:id="@+id/likes_box"
android:textSize="12dp"
android:layout_width="wrap_content"
android:layout_height="0dp"
android:layout_weight="1" />
</LinearLayout>
<ImageButton
android:id="@+id/like_button"
android:layout_width="26dp"
android:layout_height="26dp"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:background="@drawable/ic_mood_black_24dp"
android:text="Like" />
<LinearLayout
android:id="@+id/imageView2"
android:layout_marginLeft="8dp"
android:layout_width="120dp"
android:layout_height="match_parent"
android:layout_alignParentBottom="true"
android:layout_alignTop="@+id/news_answers"
android:orientation="vertical"
android:weightSum="2">
<TextView
android:layout_width="wrap_content"
android:textSize="12dp"
android:layout_height="0dp"
android:layout_weight="1"
android:text="Date" />
<TextView
android:id="@+id/date_box"
android:textSize="12dp"
android:layout_width="wrap_content"
android:layout_height="0dp"
android:layout_weight="1" />
</LinearLayout>
</RelativeLayout>
我不明白为什么setfunction会返回一个空引用。
答案 0 :(得分：1)
发生这种情况的原因是您以错误的方式使用findViewById()。
this.news = (RelativeLayout) this.mainBoard.findViewById(R.id.news_template);
this.textBox = (TextView) this.mainBoard.findViewById(R.id.text_box);
this.dateBox = (TextView) this.mainBoard.findViewById(R.id.date_box);
this.likesBox = (TextView) this.mainBoard.findViewById(R.id.likes_box);
this.textTitle = (TextView) this.mainBoard.findViewById(R.id.text_title);
所有这些方法都会调用返回null，因为这些视图不会位于您在此处设置的R.layout.activity_main_board活动布局中setContentView(R.layout.activity_main_board);
相反，你应该使用sepertly膨胀你的xml布局，然后从这个xml中找到视图。 查看此代码：
//get the inflater from your activity
LayoutInflater inflater = mainBoard.getLayoutInflater();
//get the root view, this is the view that should be the parent of the new inflated view
View root = this.mainBoard.findViewById(R.id.main_content);
//false indicates that the new inflated view wont be attatched to its parent. Use true if this is not your wanted behavior
this.news = (RelativeLayout) = inflater.inflate(R.layout.news_template, (ViewGroup) root,false);
//find the views from the inflated xml
this.textBox = (TextView) news.findViewById(R.id.text_box);
this.dateBox = (TextView) news.findViewById(R.id.date_box);
this.likesBox = (TextView) news.findViewById(R.id.likes_box);
this.textTitle = (TextView) news.findViewById(R.id.text_title);