java - 自定义 View 的适配器中的 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 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 Activity 类(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);
    }
}

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 Adapter ，那么它就可以工作。但是，如果我在 SimpleView 类中添加 Adapter ，则会出现异常:

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 SimpleView's findViewById 的版本它使用 test.xml，但是 ListView位于SimpleActivity中的activity_main.xml中.

listView = findViewById(R.id.custom_view1);

这将返回 null，因为 SimpleView 中没有 ID 为 R.id.custom_view1 的 View 。

可能不需要 SimpleView类(class)。以下代码行可以移至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，则您正在使用的所有 View 都需要位于 R.layout.test 中。理论上，您可以移动 ListView在那里，但我认为这可能是不必要的。我只需充气，找到它并将适配器设置为 onCreate Activity的.

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 View 来完成，请确保 test.xml 中包含以下内容:

<ListView
    android:id="@+id/custom_view1"
    android:layout_width="71dp"
    android:layout_height="77dp"
    android:orientation="vertical">

使用此ID很重要android:id="@+id/custom_view1使其与 listView = findViewById(R.id.custom_view1); 匹配在 Java 代码中。

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"/>

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="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>

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字。我希望它不会引起任何困惑。

此外，您还可以清理布局。我从粘贴到您的代码中选取了一些“额外”的 LinearLayout，但它们并不是真正需要的。

但是此代码在我的设备上显示 4 个列表项。