空指针异常在自定义视图的适配器中发生

huangapple
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英文:

NullPointerException in the Adapter for a Custom View

问题

以下是翻译好的部分：

我在以下异常中遇到了问题：

异常：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)'

我有一个Adapter类：

  1. public class SimpleAdapter extends BaseAdapter {
  2.     // ... 省略其他代码
  3. }

我的CustomView类：

  1. public class SimpleView extends View {
  2.     // ... 省略其他代码
  3. }

我的Activity类：

  1. public class SimpleActivity extends AppCompatActivity {
  2.     // ... 省略其他代码
  3. }

test.xml：

  1. <?xml version="1.0" encoding="utf-8"?>
  2. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  3.     xmlns:custom="http://schemas.android.com/apk/res-auto"
  4.     xmlns:tools="http://schemas.android.com/tools"
  5.     android:layout_width="100dp"
  6.     android:layout_height="220dp"
  7.     android:layout_gravity="center">
  9.     <ListView
  10.         android:id="@+id/simple_list"
  11.         android:layout_width="100dp"
  12.         android:layout_height="100dp"
  13.         android:background="@color/orange" />
  14. </LinearLayout>

activity_main.xml：

  1. <?xml version="1.0" encoding="utf-8"?>
  2. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  3.     xmlns:tools="http://schemas.android.com/tools"
  4.     android:id="@+id/custom_view"
  5.     android:layout_width="400dp"
  6.     android:layout_height="match_parent"
  7.     android:orientation="vertical">
  8. </LinearLayout>

如果我不在SimpleView类中添加Adapter，那么它可以正常工作。但如果我在SimpleView类中添加Adapter，我会收到以下异常：

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)'（位于...此处：listView.setAdapter(simpleAdapter);（simpleView = new SimpleView(this, linearLayout));）

英文:

I have a problem with the following exception:

  1. exception:java.lang.NullPointerException: Attempt to invoke virtual method &#39;void android.widget.ListView.setAdapter(android.widget.ListAdapter)&#39; on a null object reference

I have an Adapter class:

  1. public class SimpleAdapter extends BaseAdapter {
  2.     private LayoutInflater lInflater;
  3.     private String [] simpleValueList;
  5.     public SimpleAdapter(Context context, String[] simpleValueList) {
  6.         lInflater = LayoutInflater.from(context);
  7.         this.simpleValueList = simpleValueList;
  8.     }
  10.     @Override
  11.     public int getCount() {
  12.         return simpleValueList.length;
  13.     }
  15.     @Override
  16.     public Object getItem(int position) {
  17.         return simpleValueList[position];
  18.     }
  20.     @Override
  21.     public long getItemId(int position) {
  22.         return position;
  23.     }
  25.     @Override
  26.     public View getView(int position, View convertView, ViewGroup parent) {
  27.         View view = convertView;
  28.         if (view == null) {
  29.             view = lInflater.inflate(R.layout.test, parent, false);
  31.             TextView textSimple = view.findViewById(R.id.simple_list);
  32.             textHours.setText(simpleValueList[position]);
  33.         }
  34.         return view;
  35.     }
  36. }

my CustomView class:

  1. public class SimpleView extends View {
  3.     private String[] valueList = {&quot;aa&quot;, &quot;2&quot;, &quot;bb&quot;, &quot;3&quot;};
  4.     private ListView listView;
  5.     private SimpleAdapter simpleAdapter;
  7.     public SimpleView(Context context, ViewGroup mviewGroup) {
  8.         super(context);
  9.         inflate(context, R.layout.test, mviewGroup);
  11.         listView = findViewById(R.id.simple_list);
  12.         simpleAdapter = new SimpleAdapter(context, valueList);
  13.         listView.setAdapter(simpleAdapter);
  14.   }
  15. }

my Activity class:

  1. public class SimpleActivity extends AppCompatActivity {
  2.     private SimpleView simpleView;
  4.     protected void onCreate(@Nullable Bundle savedInstanceState) {
  5.         super.onCreate(savedInstanceState);
  6.         setContentView(R.layout.activity_main);
  8.         LinearLayout linearLayout = findViewById(R.id.custom_view);
  9.         simpleView = new SimpleView(this, linearLayout);
  10.         linearLayout.addView(simpleView);
  11.     }
  12. }

test. xml:

  1. &lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
  2. &lt;LinearLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
  3.     xmlns:custom=&quot;http://schemas.android.com/apk/res-auto&quot;
  4.     xmlns:tools=&quot;http://schemas.android.com/tools&quot;
  5.     android:layout_width=&quot;100dp&quot;
  6.     android:layout_height=&quot;220dp&quot;
  7.     android:layout_gravity=&quot;center&quot;&gt;
  10.     &lt;ListView
  11.         android:id=&quot;@+id/simple_list&quot;
  12.         android:layout_width=&quot;100dp&quot;
  13.         android:layout_height=&quot;100dp&quot;
  14.         android:background=&quot;@color/orange&quot; /&gt;
  15. &lt;/LinearLayout&gt;

my activity_main.xml

  1.     &lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
  2.     &lt;LinearLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
  3.         xmlns:tools=&quot;http://schemas.android.com/tools&quot;
  4.         android:id=&quot;@+id/custom_view&quot;
  5.         android:layout_width=&quot;400dp&quot;
  6.         android:layout_height=&quot;match_parent&quot;
  7.         android:orientation=&quot;vertical&quot;&gt;
  8.  &lt;/LinearLayout&gt;

If I do not add the Adapter in the SimpleView class then it works. But if I add the Adapter in the SimpleView class I get the exception:

  1. java.lang.NullPointerException: Attempt to invoke virtual method &#39;void android.widget.ListView.setAdapter(android.widget.ListAdapter)&#39; on a null object reference. (at ...here: listView.setAdapter(simpleAdapter); (simpleView = new SimpleView(this, linearLayout));

答案1

得分: 2

问题在于代码调用了SimpleView的findViewById版本，它使用了test.xml，但ListView位于SimpleActivity的activity_main.xml中。

  1. listView = findViewById(R.id.custom_view1);

这会返回null，因为在SimpleView内部没有ID为R.id.custom_view1的视图。

也许没有必要使用SimpleView类。以下代码行可以移到SimpleActivity的onCreate方法中：

  1. listView = findViewById(R.id.custom_view1);
  2. simpleAdapter = new SimpleAdapter(context, valueList);
  3. listView.setAdapter(simpleAdapter);

如果您确实想创建一个CustomView，那么您需要确保所有与之交互的视图都位于R.layout.test中。理论上，您可以将ListView移到那里，但我认为这可能是不必要的。我建议在Activity的onCreate中进行视图的膨胀、查找和设置适配器。

如果您确实希望有一个包含ListView的抽象层，我建议将其放入一个Fragment中。

如果必须使用自定义视图，请确保在test.xml中包含以下内容：

  1. <ListView
  2.     android:id="@+id/custom_view1"
  3.     android:layout_width="71dp"
  4.     android:layout_height="77dp"
  5.     android:orientation="vertical">

很重要的是使用这个ID android:id="@+id/custom_view1"，以使其与Java代码中的 listView = findViewById(R.id.custom_view1); 相匹配。

为了避免您目前遇到的崩溃：

  1. public SimpleView(Context context, ViewGroup mviewGroup) {
  2.     super(context);
  3.     View layout = inflate(context, R.layout.test, mviewGroup);
  4.     listView = layout.findViewById(R.id.simple_list);
  5.     simpleAdapter = new SimpleAdapter(context, valueList);
  6.     listView.setAdapter(simpleAdapter);
  7. }

确保对从inflate返回的值进行查找操作。

此外，您可以清理布局。我从您的代码粘贴中注意到了一些“额外”的LinearLayout，但它们实际上并不是必要的。

但这段代码在我的设备上显示了4个列表项。

英文:

The problem is that the code is calling the SimpleView&#39;s version of findViewById which is using test.xml, but the ListView is located in activity_main.xml in the SimpleActivity.

  1. listView = findViewById(R.id.custom_view1);

This will return null because there is no view with the ID R.id.custom_view1 inside SimpleView.

There probably is not any need for the SimpleView class. The following lines of code could be moved into onCreate in SimpleActivity:

  1.  listView = findViewById(R.id.custom_view1);
  2.  simpleAdapter = new SimpleAdapter(context, valueList);
  3.  listView.setAdapter(simpleAdapter);

If you do want to create a CustomView on the other hand, all the views you are working with need to be in R.layout.test. You could in theory, move the ListView in there, but I think it is probably unnecessary. I would just inflate, find it and set the adapter in onCreate of the Activity.

If you do want an abstraction to contain the ListView, I would investigate putting it into a Fragment.

EDIT:
If it has to be done with a custom view then make sure the following is in test.xml:

  1. &lt;ListView
  2.     android:id=&quot;@+id/custom_view1&quot;
  3.     android:layout_width=&quot;71dp&quot;
  4.     android:layout_height=&quot;77dp&quot;
  5.     android:orientation=&quot;vertical&quot;&gt;

It is important to use this ID android:id=&quot;@+id/custom_view1 so that it matches up with listView = findViewById(R.id.custom_view1); in the Java code.

To void the crash you are getting now:

  1. public SimpleView(Context context, ViewGroup mviewGroup) {
  2.     super(context);
  3.     View layout = inflate(context, R.layout.test, mviewGroup);
  4.     listView = layout.findViewById(R.id.simple_list);
  5.     simpleAdapter = new SimpleAdapter(context, valueList);
  6.     listView.setAdapter(simpleAdapter);
  7. }

Make sure you do the find on the value being returned from inflate.

=== Code Dump ===:

activity_main.xml

  1. &lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
  2. &lt;LinearLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
  3.     xmlns:custom=&quot;http://schemas.android.com/apk/res-auto&quot;
  4.     xmlns:tools=&quot;http://schemas.android.com/tools&quot;
  5.     android:id=&quot;@+id/custom_view_container&quot;
  6.     android:orientation=&quot;vertical&quot;
  7.     android:layout_width=&quot;match_parent&quot;
  8.     android:layout_height=&quot;match_parent&quot;
  9.     android:layout_gravity=&quot;center&quot;/&gt;

test.xml:

  1. &lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
  2. &lt;LinearLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
  3.     xmlns:custom=&quot;http://schemas.android.com/apk/res-auto&quot;
  4.     xmlns:tools=&quot;http://schemas.android.com/tools&quot;
  5.     android:layout_width=&quot;match_parent&quot;
  6.     android:layout_height=&quot;match_parent&quot;
  7.     android:layout_gravity=&quot;center&quot;&gt;
  9.     &lt;ListView
  10.         android:id=&quot;@+id/simple_list&quot;
  11.         android:layout_width=&quot;match_parent&quot;
  12.         android:layout_height=&quot;match_parent&quot;/&gt;
  13. &lt;/LinearLayout&gt;

row.xml

  1. &lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
  2. &lt;LinearLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
  3.     xmlns:custom=&quot;http://schemas.android.com/apk/res-auto&quot;
  4.     xmlns:tools=&quot;http://schemas.android.com/tools&quot;
  5.     android:layout_width=&quot;100dp&quot;
  6.     android:layout_height=&quot;220dp&quot;
  7.     android:layout_gravity=&quot;center&quot;&gt;
  9.     &lt;TextView
  10.         android:id=&quot;@+id/txt&quot;
  11.         android:layout_width=&quot;100dp&quot;
  12.         android:layout_height=&quot;100dp&quot;/&gt;
  13. &lt;/LinearLayout&gt;

ActivityMain.java:

  1. package com.example.myapplication2000;
  3. import androidx.appcompat.app.AppCompatActivity;
  5. import android.os.Bundle;
  6. import android.widget.LinearLayout;
  8. public class MainActivity extends AppCompatActivity {
  10.     @Override
  11.     protected void onCreate(Bundle savedInstanceState) {
  12.         super.onCreate(savedInstanceState);
  13.         setContentView(R.layout.activity_main);
  15.         LinearLayout linearLayout = findViewById(R.id.custom_view_container);
  16.         SimpleView simpleView = new SimpleView(this, linearLayout);
  17.         linearLayout.addView(simpleView);
  18.     }
  19. }

SimpleView.java:

  1. package com.example.myapplication2000;
  3. import android.content.Context;
  4. import android.view.LayoutInflater;
  5. import android.view.View;
  6. import android.view.ViewGroup;
  7. import android.widget.BaseAdapter;
  8. import android.widget.ListView;
  9. import android.widget.TextView;
  11. public class SimpleView extends View {
  13.     private String[] valueList = {&quot;aa&quot;, &quot;2&quot;, &quot;bb&quot;, &quot;3&quot;};
  14.     private ListView listView;
  15.     private SimpleAdapter simpleAdapter;
  17.     public SimpleView(Context context, ViewGroup mviewGroup) {
  18.         super(context);
  19.         View layout = inflate(context, R.layout.test, mviewGroup);
  20.         listView = layout.findViewById(R.id.simple_list);
  21.         simpleAdapter = new SimpleAdapter(context, valueList);
  22.         listView.setAdapter(simpleAdapter);
  23.     }
  25.     public class SimpleAdapter extends BaseAdapter {
  26.         private LayoutInflater lInflater;
  27.         private String [] simpleValueList;
  29.         public SimpleAdapter(Context context, String[] simpleValueList) {
  30.             lInflater = LayoutInflater.from(context);
  31.             this.simpleValueList = simpleValueList;
  32.         }
  34.         @Override
  35.         public int getCount() {
  36.             return simpleValueList.length;
  37.         }
  39.         @Override
  40.         public Object getItem(int position) {
  41.             return simpleValueList[position];
  42.         }
  44.         @Override
  45.         public long getItemId(int position) {
  46.             return position;
  47.         }
  49.         @Override
  50.         public View getView(int position, View convertView, ViewGroup parent) {
  51.             View view = convertView;
  52.             if (view == null) {
  53.                 view = lInflater.inflate(R.layout.row, parent, false);
  55.                 TextView textSimple = view.findViewById(R.id.txt);
  56.                 textSimple.setText(simpleValueList[position]);
  57.             }
  58.             return view;
  59.         }
  60.     }
  61. }

Sorry, I might have changed one or two of the names. I hope it does not cause any confusion.

Also, you can clean-up the layout. There are a few 'extra' LinearLayouts that I picked up from pasting in your code, but they are not really needed.

But this code is displaying 4 list items on my device.

huangapple
  • 本文由 发表于 2020年7月29日 03:00:30
  • 转载请务必保留本文链接：https://go.coder-hub.com/63141068.html
  • android
  • android-custom-view
  • java
  • listview
  • nullpointerexception
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