我有个问题。请帮我。我收到此异常: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Adapter类:
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CustomView类:
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);
}
}
我的活动课:
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);
}
}
测试。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类中添加适配器,则它可以工作。但是,如果我在SimpleView类中添加适配器,则会出现异常: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)'。(在...这里:listView.setAdapter(simpleAdapter);(simpleView = new SimpleView(this,linearLayout)));
问题在于该代码正在调用使用test.xml 的SimpleView's
版本findViewById
,但是该代码ListView
位于中的activity_main.xml中SimpleActivity
。
listView = findViewById(R.id.custom_view1);
这将返回null,因为在SimpleView中没有ID为R.id.custom_view1的视图。
SimpleView
上课可能没有任何必要。下面的代码行可以移到onCreate
在SimpleActivity
:
listView = findViewById(R.id.custom_view1);
simpleAdapter = new SimpleAdapter(context, valueList);
listView.setAdapter(simpleAdapter);
另一方面,如果您确实想创建CustomView,则要使用的所有视图都必须位于R.layout.test中。从理论上讲,您可以将其移到ListView
其中,但我认为这可能是不必要的。我只需要充气,找到它并在onCreate
中设置适配器即可Activity
。
如果您确实希望抽象包含ListView
,我会研究将其放入Fragment
。
编辑:如果必须使用自定义视图来完成,那么请确保以下内容在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
以便与listView = findViewById(R.id.custom_view1);
Java代码匹配非常重要。
要使崩溃无效,您现在可以:
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);
}
确保对从inflate返回的值进行查找。
===代码转储===:
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"/>
test.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>
row.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>
ActivityMain.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);
}
}
SimpleView.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,我可能已经更改了其中一个或两个名称。我希望它不会引起任何混乱。
另外,您可以清理布局。我从粘贴代码中获得了一些“额外的” LinearLayout,但并不是真正需要它们。
但是这段代码在我的设备上显示了4个列表项。
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