自定义视图适配器中的 NullPointerException

Question

我遇到以下异常问题：

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

我有一个 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;
    }
}

我的自定义视图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Activityclass:

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 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 我会得到异常：

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));

Answer 1

问题是代码正在调用使用 test.xml 的 findViewById 的 SimpleView's 版本，但 ListView 位于 activity_main.xml在 SimpleActivity.

listView = findViewById(R.id.custom_view1);

这将 return 为空，因为在 SimpleView.

中没有 ID 为 R.id.custom_view1 的视图

可能不需要 SimpleView class。以下代码行可以移动到 onCreate in SimpleActivity:

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

另一方面，如果您确实想创建一个 CustomView，则您正在使用的所有视图都需要在 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。我只是充气，找到它并在 Activity 的 onCreate 中设置适配器。

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完成，请确保以下内容位于 test.xml:

<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，这样它才能与 Java 代码中的 listView = findViewById(R.id.custom_view1); 匹配。

要避免您现在遇到的崩溃：

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);
}

确保您对 return 从 inflate 编辑的值进行查找。

===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，我可能更改了其中一两个名称。我希望它不会引起任何混淆。

此外，您还可以清理布局。我从您的代码中粘贴了一些 'extra' LinearLayouts，但实际上并不需要它们。

但是这段代码在我的设备上显示了 4 个列表项。

自定义视图适配器中的 NullPointerException

NullPointerException in the Adapter for a Custom View

java

android

listview

nullpointerexception

android-custom-view