Android - 具有外部类的 Listview

Question

布局->

activity_main.xml
content_main.xml

逻辑->

ActivityMain.java
MainLogic.java

2 个问题：（第二个问题是实际问题）

1) java.lang.RuntimeException: 无法启动活动 ComponentInfo{com.test.app./com.test.app.MainActivity}: java.lang.NullPointerException: 尝试调用虚拟方法'android.view.View android.view.Window.findViewById(int)' 在 null 对象上引用

2) 原因：java.lang.NullPointerException：尝试在 android.app.Activity 的空对象引用上调用虚拟方法 'android.view.View android.view.Window.findViewById(int)' .findViewById(Activity.java:2124) 在 com.test.app.MainLogic.CreateListView(MainLogic.java:51)

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="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:fitsSystemWindows="true"
    tools:context=".MainActivity">

<android.support.design.widget.AppBarLayout android:layout_height="wrap_content"
    android:layout_width="match_parent" android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar android:id="@+id/toolbar"
        android:layout_width="match_parent" android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary" app:popupTheme="@style/AppTheme.PopupOverlay" />

</android.support.design.widget.AppBarLayout>

<include layout="@layout/content_main" />


<android.support.design.widget.FloatingActionButton android:id="@+id/fab"
    android:layout_width="wrap_content" android:layout_height="wrap_content"
    android:layout_gravity="bottom|end"     android:layout_margin="@dimen/fab_margin"
    android:src="@android:drawable/ic_dialog_email" />

</android.support.design.widget.CoordinatorLayout>

content_main.xml

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context=".MainActivity"
    tools:showIn="@layout/activity_main"
    android:id="@+id/main_layout">

<LinearLayout
    android:id="@+id/linLayout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_centerHorizontal="true">
    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/origin_string"
        android:textSize="20sp"/>
    <TextView
        android:id="@+id/textView2"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="@string/random_string_sample"
        android:textSize="20sp"
        />
</LinearLayout>
    <Button
        android:id="@+id/btnClick1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/linLayout"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="61dp"
        android:text="@string/app_name" />
<ListView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/listview1"
    android:layout_below="@+id/btnClick1">
</ListView>

</RelativeLayout>

ActivityMain.java->

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setIcon(R.drawable.logo);
        start();
    }
    void start(){
        ListView listView = (ListView) findViewById(R.id.listview1); //ADDED
        MainLogic ml = new MainLogic();
        ml.CreateListView(>ADDED: listview<);
    }
}

MainLogic.java-> 编辑：在下面添加了类行删除了扩展活动

public class MainLogic {

public void CreateListView() {
    final ListView list_View1 = (ListView) findViewById(R.id.listview1);
    final String[] listViewStrings = new String[]{"first_item", "second_item", "third_item", "fouth_item", "fifth_item", "sixth_item", "seventh_item", "eigth_item"};
    ArrayAdapter list_adapter = new ArrayAdapter<String>(`<s>`getApplicationContext()`</s>`, android.R.layout.simple_list_item_1,listViewStrings);
    //list_adapter.addAll(>REMOVED: listViewStrings.toString()<);
    list_View1.setAdapter(list_adapter); // Use data, pass the adapter with the strings, numbers, etc.
    }
}

如果你能解释我为什么做错事并可能告诉我如何修复代码，谁会回答这个问题，那就太好了。我已经阅读了这些链接：

http://www.vogella.com/tutorials/AndroidListView/article.html

http://developer.android.com/guide/topics/ui/layout/listview.html

http://www.tutorialspoint.com/android/android_list_view.htm

除了我在外部类中实现逻辑然后创建一个对象来调用该方法之外，我没有看到我的代码中的固有缺陷。

解决方案->

ActivityMain.java->

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setIcon(R.drawable.logo);
        start();
    }

    public void start(){
        MainLogic ml = new MainLogic();
        ml.CreateListView(this);
    }

}

MainLogic.java->

public class MainLogic {

public void CreateListView(final MainActivity newActivity) {

    final ListView list_View1 = (ListView) newActivity.findViewById(R.id.listview1);
        final String[] listViewStrings = new String[]{"first_item", "second_item", "third_item", "fouth_item", "fifth_item", "sixth_item", "seventh_item", "eigth_item"};
//      This will adapt array that listview can work with i.e. generate all rows
        ArrayAdapter list_adapter = new ArrayAdapter<>(newActivity.getApplicationContext(), android.R.layout.simple_list_item_1, listViewStrings);
        list_adapter.addAll();
        list_View1.setAdapter(list_adapter); // Use data, pass the adapter with the strings, numbers, etc.

        list_View1.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapter, View view, int position_in_array, long id) {
                // Logic for what happens when clicked
                // Context => http://stackoverflow.com/questions/3572463/what-is-context-in-android
                int position = position_in_array;
                String item_value = (String) adapter.getItemAtPosition(position);
                String item = "Item: " + listViewStrings.toString() + " / value " + item_value + " ." ;
                Toast.makeText(newActivity.getApplicationContext(), item, Toast.LENGTH_SHORT).show();
            }
        });  

} }

Answer 1

错误Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference表示找不到ListView。

您呼叫的findViewById 属于MainLogic，而不是MainActivity。由于您的ListView 在MainActivity 中，这就是它找不到ListView 的原因。尝试将MainActivity.this 从MainActivity 传递到MainLogic。另外，我认为没有必要使用list_adapter.addAll，因为你已经在ArrayAdapter的构造函数中传递了数组。

下面是一个例子：

主活动：

ml.CreateListView(this);

主逻辑：

public void CreateListView(Activity activity) {
    ListView list_View1 = (ListView) activity.findViewById(R.id.listview1);
    final String[] listViewStrings = new String[]{"first_item", "second_item", "third_item", "fouth_item", "fifth_item", "sixth_item", "seventh_item", "eigth_item"};
    ArrayAdapter list_adapter = new ArrayAdapter<String>(getApplicationContext(), android.R.layout.simple_list_item_1,listViewStrings);
    list_View1.setAdapter(list_adapter); // Use data, pass the adapter with the strings, numbers, etc.
}