[英]NullPointerException in the Adapter for a Custom View
我遇到以下異常問題:
exception:java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
我有一個Adapter
class:
public class SimpleAdapter extends BaseAdapter {
private LayoutInflater lInflater;
private String [] simpleValueList;
public SimpleAdapter(Context context, String[] simpleValueList) {
lInflater = LayoutInflater.from(context);
this.simpleValueList = simpleValueList;
}
@Override
public int getCount() {
return simpleValueList.length;
}
@Override
public Object getItem(int position) {
return simpleValueList[position];
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View view = convertView;
if (view == null) {
view = lInflater.inflate(R.layout.test, parent, false);
TextView textSimple = view.findViewById(R.id.simple_list);
textHours.setText(simpleValueList[position]);
}
return view;
}
}
我的自定義視圖 class:
public class SimpleView extends View {
private String[] valueList = {"aa", "2", "bb", "3"};
private ListView listView;
private SimpleAdapter simpleAdapter;
public SimpleView(Context context, ViewGroup mviewGroup) {
super(context);
inflate(context, R.layout.test, mviewGroup);
listView = findViewById(R.id.simple_list);
simpleAdapter = new SimpleAdapter(context, valueList);
listView.setAdapter(simpleAdapter);
}
}
我的活動 class:
public class SimpleActivity extends AppCompatActivity {
private SimpleView simpleView;
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
LinearLayout linearLayout = findViewById(R.id.custom_view);
simpleView = new SimpleView(this, linearLayout);
linearLayout.addView(simpleView);
}
}
測試。 xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:custom="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="100dp"
android:layout_height="220dp"
android:layout_gravity="center">
<ListView
android:id="@+id/simple_list"
android:layout_width="100dp"
android:layout_height="100dp"
android:background="@color/orange" />
</LinearLayout>
我的 activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/custom_view"
android:layout_width="400dp"
android:layout_height="match_parent"
android:orientation="vertical">
</LinearLayout>
如果我不在SimpleView
class 中添加Adapter
,那么它就可以工作。 但是,如果我在SimpleView
class 中添加Adapter
,則會出現異常:
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference. (at ...here: listView.setAdapter(simpleAdapter); (simpleView = new SimpleView(this, linearLayout));
問題是代碼正在調用使用 test.xml 的SimpleView's
版本的findViewById
,但ListView
位於SimpleActivity
中的 activity_main.xml 中。
listView = findViewById(R.id.custom_view1);
這將返回 null,因為在 SimpleView 中沒有 ID 為 R.id.custom_view1 的視圖。
可能不需要SimpleView
class。可以將以下代碼行移至SimpleActivity
的onCreate
中:
listView = findViewById(R.id.custom_view1);
simpleAdapter = new SimpleAdapter(context, valueList);
listView.setAdapter(simpleAdapter);
另一方面,如果您確實想創建 CustomView,則您正在使用的所有視圖都需要位於 R.layout.test 中。 理論上您可以將ListView
移到那里,但我認為這可能是不必要的。 我會膨脹,找到它並在Activity
的onCreate
中設置適配器。
如果您確實希望抽象包含ListView
,我會研究將其放入Fragment
中。
編輯:如果必須使用自定義視圖完成,請確保以下內容在 test.xml 中:
<ListView
android:id="@+id/custom_view1"
android:layout_width="71dp"
android:layout_height="77dp"
android:orientation="vertical">
使用此 ID android:id="@+id/custom_view1
很重要,以便它與listView = findViewById(R.id.custom_view1);
在 Java 代碼中匹配。
要避免您現在遇到的崩潰:
public SimpleView(Context context, ViewGroup mviewGroup) {
super(context);
View layout = inflate(context, R.layout.test, mviewGroup);
listView = layout.findViewById(R.id.simple_list);
simpleAdapter = new SimpleAdapter(context, valueList);
listView.setAdapter(simpleAdapter);
}
確保對從 inflate 返回的值進行查找。
===代碼轉儲===:
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:custom="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/custom_view_container"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_gravity="center"/>
測試.xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:custom="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_gravity="center">
<ListView
android:id="@+id/simple_list"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
</LinearLayout>
行.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:custom="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="100dp"
android:layout_height="220dp"
android:layout_gravity="center">
<TextView
android:id="@+id/txt"
android:layout_width="100dp"
android:layout_height="100dp"/>
</LinearLayout>
活動主要.java:
package com.example.myapplication2000;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.LinearLayout;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
LinearLayout linearLayout = findViewById(R.id.custom_view_container);
SimpleView simpleView = new SimpleView(this, linearLayout);
linearLayout.addView(simpleView);
}
}
簡單視圖.java:
package com.example.myapplication2000;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.TextView;
public class SimpleView extends View {
private String[] valueList = {"aa", "2", "bb", "3"};
private ListView listView;
private SimpleAdapter simpleAdapter;
public SimpleView(Context context, ViewGroup mviewGroup) {
super(context);
View layout = inflate(context, R.layout.test, mviewGroup);
listView = layout.findViewById(R.id.simple_list);
simpleAdapter = new SimpleAdapter(context, valueList);
listView.setAdapter(simpleAdapter);
}
public class SimpleAdapter extends BaseAdapter {
private LayoutInflater lInflater;
private String [] simpleValueList;
public SimpleAdapter(Context context, String[] simpleValueList) {
lInflater = LayoutInflater.from(context);
this.simpleValueList = simpleValueList;
}
@Override
public int getCount() {
return simpleValueList.length;
}
@Override
public Object getItem(int position) {
return simpleValueList[position];
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View view = convertView;
if (view == null) {
view = lInflater.inflate(R.layout.row, parent, false);
TextView textSimple = view.findViewById(R.id.txt);
textSimple.setText(simpleValueList[position]);
}
return view;
}
}
}
抱歉,我可能更改了其中一兩個名稱。 我希望它不會造成任何混亂。
此外,您還可以清理布局。 我從您的代碼中粘貼了一些“額外”的 LinearLayouts,但它們並不是真正需要的。
但是這段代碼在我的設備上顯示了 4 個列表項。
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