Android：自定义列表视图无法正常工作

Posted 2023-04-19

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【中文标题】Android：自定义列表视图无法正常工作【英文标题】：Android: Custom List View is not working properly 【发布时间】：2019-10-24 21:01:39 【问题描述】：

很抱歉没有提供正确的源代码。

基本上我在列表视图 xml 布局中拥有的是：

listleave.xml leaveSummary.xml 离开分贝自定义适配器申请休假

我正在创建一个具有列表视图的应用程序，当我运行我的应用程序时，我在 Locgat 中遇到错误。

Caused by:java.lang.NullPointerException: Attempt to invoke virtual 
method 'void 
android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null 
object reference
at com.example.huzai.civilsoft1.ApplyLeave.loadDataInListView(ApplyLeave.java:258 )
 at com.example.huzai.civilsoft1.ApplyLeave.onCreate(ApplyLeave.java:84)

在 loadDataInListView 行中，当我不评论 l1.setAdapter(customAdapter); 时，应用程序崩溃，但当我评论时它可以正常工作。

不评论结果：l1.setAdapter(customAdapter);

ApplyLeave 活动

public class ApplyLeave extends AppCompatActivity 
LeaveService myDb;
//Declaration EditTexts
EditText editstartDate;
EditText   editendDate;
DatePickerDialog datePickerDialog;
CheckBox checkgoingAbroad;
Spinner spinnertypess;
Spinner editleavess;
EditText editremarks;
EditText editnoOfDays1;
TextView noOfDays;
TextView endd;
Button btnApply;
Button addfield;
Button deletefield;
LinearLayout parentlinearlayout;
ListView l1;
ArrayList<Leave> arrayList;
CustomAdapter customAdapter;
@RequiresApi(api = Build.VERSION_CODES.N)
@Override
 protected void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_apply_leave);
myDb = new LeaveService (this);
 checkgoingAbroad =(CheckBox) findViewById(R.id.GoingAbroad);
spinnertypess = (Spinner)findViewById(R.id.types);
editleavess = (Spinner)findViewById(R.id.Leaves);
editremarks = (EditText)findViewById(R.id.reason);
editnoOfDays1 = (EditText)findViewById(R.id.noOfDays);
parentlinearlayout = 
(LinearLayout)findViewById(R.id.parent_linear_layout);
addfield = (Button)findViewById(R.id.addField);
deletefield = (Button)findViewById(R.id.removeField);
endd = (TextView)findViewById(R.id.date);
l1 = (ListView)findViewById(R.id.leaveApplicationSummary);
btnApply = (Button)findViewById(R.id.apply);
AddData();
arrayList = new ArrayList<>();
loadDataInListView();

 //Created a method for loadDataInListView
private void loadDataInListView() 
    arrayList = myDb.getAllData();
    customAdapter = new CustomAdapter(this,arrayList);
    l1.setAdapter(customAdapter);
    customAdapter.notifyDataSetChanged();

private void AddData() 
    btnApply.setOnClickListener(new View.OnClickListener() 
        @Override
        public void onClick(View v) 
        boolean isInserted = myDb.insertData("", checkgoingAbroad.toString(), spinnertypess.toString(), editleavess.toString(), editnoOfDays1.getText().toString(), editstartDate.getText().toString(), editendDate.getText().toString(), editremarks.getText().toString());
        if (isInserted == true)

            Toast.makeText(ApplyLeave.this, "Data Inserted Successfully",Toast.LENGTH_LONG).show();
         else 
            Toast.makeText(ApplyLeave.this, "Data Not Inserted",Toast.LENGTH_LONG).show();
        
    
);

评论结果：//l1.setAdapter(customAdapter);

CustomAdapter 类

public class CustomAdapter extends BaseAdapter 

Context context;

ArrayList<Leave> arrayList;

public CustomAdapter(Context context, ArrayList<Leave>arraylist) 

this.context = context;

this.arrayList = arraylist;


@Override
public int getCount() 

return this.arrayList.size();


@Override
public Object getItem(int position) 

return arrayList.get(position);


@Override
public long getItemId(int position) 

return position;


@SuppressLint("ViewHolder")
@Override
public View getView(int position, View convertView, ViewGroup parent) 

    LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    convertView = inflater.inflate(R.layout.leavesummary, null);
    TextView types = (TextView)convertView.findViewById(R.id.textViewtypes);
    TextView startDate = (TextView)convertView.findViewById(R.id.textViewstartDate);
    TextView endDate = (TextView)convertView.findViewById(R.id.textViewendDate);
    TextView leavetype = (TextView)convertView.findViewById(R.id.textViewleaveType);
    TextView noOfDays = (TextView)convertView.findViewById(R.id.textViewnoOfDays);
    TextView reason = (TextView)convertView.findViewById(R.id.textViewreason);



    Leave leave = arrayList.get(position);
    types.setText(leave.getTypes());
    startDate.setText(leave.getStartDate());
    endDate.setText(leave.getEndDate());
    leavetype.setText(leave.getLeaves());
    noOfDays.setText(leave.getNoOfDays());
    reason.setText(leave.getReason());

return convertView;

listleave.xml 这里我使用的是 ConstraintLayout

 <android.support.constraint.ConstraintLayout

 xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_
 android:layout_
 android:background="@drawable/background">

   <ListView

android:layout_
android:layout_
android:id="@+id/leaveApplicationSummary">
</ListView>

</android.support.constraint.ConstraintLayout>

leaveSummary.xml

     <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_
       android:layout_
      android:background="@drawable/background">
    <TextView
    android:layout_
    android:layout_
    android:text="Leave Application Summary"
    android:textColor="#080808"
    android:textSize="20dp"/>
<TextView
android:id="@+id/textView1"
android:layout_
android:layout_
android:layout_marginStart="28dp"
android:layout_marginLeft="28dp"
android:layout_marginTop="72dp"
android:text="Applicant Name :"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textViewApplicantName"
android:layout_
android:layout_
android:layout_marginStart="188dp"
android:layout_marginLeft="188dp"
android:layout_marginTop="72dp"
android:text="Huzaifa  [001]"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textView2"
android:layout_
android:layout_
android:layout_marginStart="28dp"
android:layout_marginLeft="28dp"
android:layout_marginTop="116dp"
android:text="Type of Leave :"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textViewtypes"
android:layout_
android:layout_
android:layout_marginStart="180dp"
android:layout_marginLeft="180dp"
android:layout_marginTop="116dp"
android:text="TextView"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textView3"
android:layout_
android:layout_
android:layout_marginStart="24dp"
android:layout_marginLeft="24dp"
android:layout_marginTop="164dp"
android:text="Leave Start Date :"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textViewstartDate"
android:layout_
android:layout_
android:layout_marginStart="188dp"
android:layout_marginLeft="188dp"
android:layout_marginTop="164dp"
android:text="TextView"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textView4"
android:layout_
android:layout_
android:layout_marginStart="28dp"
android:layout_marginLeft="28dp"
android:layout_marginTop="208dp"
android:text="Rejoining Date :"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textViewendDate"
android:layout_
android:layout_
android:layout_marginStart="180dp"
android:layout_marginLeft="180dp"
android:layout_marginTop="208dp"
android:text="TextView"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textView5"
android:layout_
android:layout_
android:layout_marginStart="32dp"
android:layout_marginLeft="32dp"
android:layout_marginTop="248dp"
android:text="Leave Type :"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textViewleaveType"
android:layout_
android:layout_
android:layout_marginStart="180dp"
android:layout_marginLeft="180dp"
android:layout_marginTop="248dp"
android:text="TextView"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

<TextView
android:id="@+id/textView6"
android:layout_
android:layout_
android:layout_marginStart="24dp"
android:layout_marginLeft="24dp"
android:layout_marginTop="288dp"
android:text="Total Days Applied :"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

 <TextView
android:id="@+id/textViewnoOfDays"
android:layout_
android:layout_
android:layout_marginStart="216dp"
android:layout_marginLeft="216dp"
android:layout_marginTop="288dp"
android:text="TextView"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

 <TextView
android:id="@+id/textView7"
android:layout_
android:layout_
android:layout_marginStart="28dp"
android:layout_marginLeft="28dp"
android:layout_marginTop="332dp"
android:text="Remarks :"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

 <TextView
android:id="@+id/textViewreason"
android:layout_
android:layout_
android:layout_marginStart="144dp"
android:layout_marginLeft="144dp"
android:layout_marginTop="332dp"
android:text="TextView"
android:textColor="#080808"
android:textSize="20dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<LinearLayout
android:layout_
android:layout_
android:gravity="bottom"
android:layout_margin="15dp"
android:orientation="vertical">

 <LinearLayout
android:layout_
android:layout_>

    <Button
    android:id="@+id/delete"
    android:layout_
    android:layout_
    android:layout_weight="1"
    android:text="Delete" />

    <Button
    android:id="@+id/edit"
    android:layout_
    android:layout_
    android:layout_weight="1"
    android:text="Edit" />
   </LinearLayout>
     <LinearLayout
      android:layout_
      android:layout_>

        <Button
        android:id="@+id/back"
        android:layout_
        android:layout_
        android:layout_weight="1"
        android:text="Back" />
      </LinearLayout>
     </LinearLayout>
    </android.support.constraint.ConstraintLayout>

apply_leave.xml

<LinearLayout 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_
  android:layout_
     android:background="@drawable/background"
      tools:context=".ApplyLeave">

    <LinearLayout
    android:layout_
    android:layout_
    android:id="@+id/parent_linear_layout"
      android:layout_margin="15dp"
     android:orientation="vertical">

<LinearLayout
    android:layout_
    android:layout_>

    <CheckBox
        android:layout_
        android:layout_
        android:id="@+id/GoingAbroad"
        android:hint="Going Abroad"
        android:inputType="text"
        />
</LinearLayout>
<TextView
    android:layout_
    android:layout_
    android:text="Need Advance Payment?"/>
<LinearLayout
    android:layout_
    android:layout_>

    <Spinner
        android:id="@+id/types"
        android:layout_
        android:layout_
        android:entries="@array/types"
        />
</LinearLayout>
<TextView
    android:layout_
    android:layout_
    android:text="Leave Type"/>
<LinearLayout
    android:layout_
    android:layout_>
    <Spinner
        android:id="@+id/Leaves"
        android:layout_
        android:layout_
        android:entries="@array/leaves"
        />
</LinearLayout>
<LinearLayout
    android:layout_
    android:layout_>

    <EditText
        android:layout_
        android:layout_
        android:id="@+id/noOfDays"
        android:hint="No. of Days"
        android:inputType="number"/>

</LinearLayout>
<LinearLayout
    android:layout_
    android:layout_>

    <Button
        android:layout_
        android:layout_
        android:id="@+id/addField"
        android:text="+"/>
    <Button
        android:layout_
        android:layout_
        android:id="@+id/removeField"
        android:text="-" />
</LinearLayout>
<LinearLayout
    android:layout_
    android:layout_>
    <TextView
        android:layout_
        android:layout_
        android:layout_weight="1"
        android:text="Leave starts on" />

    <EditText
        android:id="@+id/startDate"
        android:layout_
        android:layout_
        android:layout_weight="1"
        android:clickable="true"
        android:editable="false"
        android:hint="Select" />
</LinearLayout>
<LinearLayout
    android:layout_
    android:layout_>
    <TextView
        android:layout_
        android:layout_
        android:layout_weight="1"
        android:text="Rejoining will be on" />

    <EditText
        android:id="@+id/endDate"
        android:layout_
        android:layout_
        android:layout_weight="1"
        android:clickable="true"
        android:editable="false"
        android:hint="Select" />

</LinearLayout>
<EditText
    android:id="@+id/reason"
    android:layout_
    android:layout_
    android:hint="Remarks" />
<TextView
    android:id="@+id/days"
    android:layout_
    android:layout_
    android:text="Total leave days = 0"/>
<TextView
    android:id="@+id/date"
    android:layout_
    android:layout_
    android:text="Leave ends on = "/>
<LinearLayout
    android:layout_
    android:layout_
    android:layout_margin="15dp"
    android:gravity="bottom">

    <Button
        android:id="@+id/cancelButton"
        android:layout_
        android:layout_
        android:layout_weight="1"
        android:text="Cancel" />

    <Button
        android:id="@+id/apply"
        android:layout_
        android:layout_
        android:layout_weight="1"
        android:text="Apply" />
       </LinearLayout>
       </LinearLayout>
       </LinearLayout>

请告诉我哪里出错了。

【问题讨论】：

l1 应该是 RecyclerView，而不是 ListView。尝试使用 RecyclerView 而不是 ListView 再试一次 RecyclerView 应该放在哪里？能否分享您的 XML 问题？发布activity_apply_leave.xml 当然我会发布 xml 布局 【参考方案1】：

您将得到 null l1。

在您的activity_apply_leave.xml 中确认您的ListView id。应该是leaveApplicationSummary。

编辑：

您的 ListView 在 listleave.xml 中，但您在 Activity 中使用 activity_apply_leave.xml。

将您的 ListView 从 listleave.xml 移动到 activity_apply_leave.xml。

【讨论】：

我为列表视图发布了我的 xml 布局，请检查好的，我在 apply_leave.xml 中移动了列表视图，它正在工作 @HuzaifaYusuf 现在这是一个新问题。

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