ListView 所需的指针答案

【问题标题】：Pointers needed on ListViewListView 所需的指针
【发布时间】：2012-04-22 11:40:02
【问题描述】：

我使用ArrayAdapter 来显示通过ListView 保存在数组中的数据，我的代码编译得很好，但是当我调用下面的类时会导致进程以Sorry application has stopped 异常结束。

   public class Timetable extends Activity {

    private ListView timeTableView;
    private ArrayList <Module> Modules;
    private ArrayAdapter <Module> adaptModule;

     @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.view);

            timeTableView = (ListView) findViewById(R.id.timeTableView);

            adaptModule = new ArrayAdapter<Module>(this, android.R.layout.simple_list_item_1, Modules);
            timeTableView.setAdapter(adaptModule);
     }
    }

这是我的 Add 类，它处理创建数组并将项目添加到数组：

public class Add extends Activity implements OnClickListener{

private EditText ModuleText;
private EditText RoomText;
private Spinner Day;
private Spinner Start;
private Spinner Duration;
private Spinner Session;
private ArrayList<Module> Modules;

/*
 * OnCreate method for add class
 * @see android.app.Activity#onCreate(android.os.Bundle)
 */

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.add);

    // spinner for days
    Spinner day = (Spinner) findViewById(R.id.spinnerDay);
    ArrayAdapter<CharSequence> adapterDay = ArrayAdapter.createFromResource(
            this, R.array.days_array, android.R.layout.simple_spinner_item);
    adapterDay.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    day.setAdapter(adapterDay);


    // spinner for start time
    Spinner startTime = (Spinner) findViewById(R.id.spinnerStart);
    ArrayAdapter<CharSequence> adapterstart = ArrayAdapter.createFromResource(
            this, R.array.start_array, android.R.layout.simple_spinner_item);
    adapterstart.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    startTime.setAdapter(adapterstart);

    // spinner for duration
    Spinner duration = (Spinner) findViewById(R.id.spinnerDuration);
    ArrayAdapter<CharSequence> adapterduration = ArrayAdapter.createFromResource(
            this, R.array.duration_array, android.R.layout.simple_spinner_item);
    adapterduration.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    duration.setAdapter(adapterduration);

    // spinner for class type
    Spinner session = (Spinner) findViewById(R.id.spinnerSession);
    ArrayAdapter<CharSequence> adapterSession = ArrayAdapter.createFromResource(
            this, R.array.session_array, android.R.layout.simple_spinner_item);
    adapterSession.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    session.setAdapter(adapterSession);

    //create array
    Modules = new ArrayList<Module>();

    // ref to UI Widgets
    ModuleText = (EditText) findViewById(R.id.ModuleText);
    RoomText = (EditText) findViewById(R.id.RoomText);
    Day = (Spinner) findViewById(R.id.spinnerDay);
    Start = (Spinner) findViewById(R.id.spinnerStart);
    Duration = (Spinner) findViewById(R.id.spinnerDuration);
    Session = (Spinner) findViewById(R.id.spinnerSession);


    // buttons for this activity also includes listeners
    View addEntryButton = findViewById(R.id.addEntry_button);
    addEntryButton.setOnClickListener(this);
    View cancelEntryButton = findViewById(R.id.cancelEntry_button);
    cancelEntryButton.setOnClickListener(this);


}

/*
 * on click listeners for buttons
 * @see android.view.View.OnClickListener#onClick(android.view.View)
 */
public void onClick(View v) {
    switch (v.getId()) {
    case R.id.cancelEntry_button:
        Intent i = new Intent(this, Home.class);
        startActivity(i);
        break;

    }
     switch (v.getId()) {
        case R.id.addEntry_button:
            Intent i = new Intent(this, Home.class);
            addModule();
            startActivity(i);
            break;
    }

    }

    /* 
     * Method for adding an entry into the table
     */
public void addModule() 
   {
    if(ModuleText !=null || RoomText !=null)
    {
    Modules.add(new Module(ModuleText.getText().toString(), RoomText.getText().toString(), Day.getSelectedItem().toString(), Start.getSelectedItem().toString(), Duration.getSelectedItem().toString(), Session.getSelectedItem().toString()));
    Toast.makeText(getApplicationContext(), "Timetable entry has been added succesfully", Toast.LENGTH_SHORT).show();
    }
    else
    {
        Toast.makeText(getApplicationContext(), "Error adding timetable entry, please check details and try again", Toast.LENGTH_SHORT).show();
    }

   }


}

这是我的logcat，但有例外：

04-22 11:52:46.290: E/AndroidRuntime(426): FATAL EXCEPTION: main
04-22 11:52:46.290: E/AndroidRuntime(426): java.lang.RuntimeException: Unable to start    activity ComponentInfo{org.assignment.timetable/org.assignment.timetable.Timetable}:   java.lang.NullPointerException
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
04-22 11:52:46.290: E/AndroidRuntime(426):  at     android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.os.Handler.dispatchMessage(Handler.java:99)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.os.Looper.loop(Looper.java:123)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.app.ActivityThread.main(ActivityThread.java:4627)
04-22 11:52:46.290: E/AndroidRuntime(426):  at java.lang.reflect.Method.invokeNative(Native Method)
04-22 11:52:46.290: E/AndroidRuntime(426):  at java.lang.reflect.Method.invoke(Method.java:521)
04-22 11:52:46.290: E/AndroidRuntime(426):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
04-22 11:52:46.290: E/AndroidRuntime(426):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
04-22 11:52:46.290: E/AndroidRuntime(426):  at dalvik.system.NativeStart.main(Native Method)
04-22 11:52:46.290: E/AndroidRuntime(426): Caused by: java.lang.NullPointerException
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:291)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.widget.ListView.setAdapter(ListView.java:436)
04-22 11:52:46.290: E/AndroidRuntime(426):  at org.assignment.timetable.Timetable.onCreate(Timetable.java:25)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
04-22 11:52:46.290: E/AndroidRuntime(426):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)

【问题讨论】：

既然您删除了问题，所有有相同问题的用户将无法找到他们的答案。因为你……非常感谢
对不起，这只是个人项目的代码，所以不希望人们复制它等等......
@mitchnufc - 将其发布在 Stack Overflow 上已经同意将其作为知识共享许可。预计问题将具有长期持久的价值。如果您不想展示真实代码，那么您始终可以制作一个最小但完整的示例，这无论如何都是一件好事，因为它使问题更清晰，并将其简化为问题的核心部分。

标签： android arraylist android-listview android-arrayadapter

【解决方案1】：

您永远不会在Timetable 类中初始化Modules 字段，当您在adaptModuleadapter 中使用它时，它会抛出您看到的NullPointerException。如果您在另一个活动中构建Modules 列表，然后尝试在Timetable 活动中显示它，那么您必须从该活动中传递Modules 列表（可能是Add 活动）并使用它在适配器中列出。

有多种方法可以将ArrayList 从一个活动传递到另一个活动，您可以在 Stackoverflow 上查看更多示例，例如：

Android: how to pass ArrayList<object> from an activity to another

添加的小例子：

扩展Application的类：

/**
 * The class that extends Application, and where we will store our ArrayList
 */
public class MyApplication extends Application {

    /**
     * This will hold your Modules list so we can access it from evry activity
     */
    private ArrayList<Module> mModules;

    /**
     * We will call this method when we want to store the list of modules(for
     * example when we finish adding elements to it(in the Add activity) and we
     * are switching to the Timetable activity)
     */
    public void setModulesList(ArrayList<Module> data) {
        this.mModules = data;
    }

    /**
     * Retrieve the list of modules and work with it(for example to display it
     * in the Timetable activity)
     */
    public ArrayList<Module> getModulesList() {
        return mModules;
    }
}

我们在Modules列表中添加元素的活动：

public class AddActivity extends Activity {

    // local list of modules on which, in this activity, we will add elements
    private ArrayList<Module> items = new ArrayList<Module>();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // simple layout with one Button that will get us to the Timetable
        // activity
        Button b = new Button(this);
        b.setText("I've put something in the Modules arrayList so go to the new activity");
        b.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // we are switching to the other activity so store the list of
                // modules in MyApplication
                ((MyApplication) getApplicationContext()).setModulesList(items);
                Intent i = new Intent(AddActivity.this, Timetable.class);
                startActivity(i);
            }
        });
        setContentView(b);
        // some dummy work, adding elements to the list of Modules.
        for (int i = 0; i < 12; i++) {
            items.add(new Module("Module " + i));
        }

    }
}

我们将在其中显示之前在AddActivity 中创建的Modules 列表的活动：

public class Timetable extends ListActivity {

    // private field, that will hold a copy of the list from MyApplication
    private ArrayList<Module> mModules;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // retrieve the list of Modules from MyApplication
        mModules = ((MyApplication) getApplicationContext()).getModulesList();
        // it is possible to not have elements in the list at this time so just
        // create an
        // empty list so we don't throw exceptions
        if (mModules == null) {
            mModules = new ArrayList<Module>();
        }
        // set the adapter
        setListAdapter(new ArrayAdapter<Module>(this,
                android.R.layout.simple_list_item_1, mModules));
    }
}

为了让我们的MyApplication 工作，我们必须在清单中注册它（参见应用程序元素，以及添加的android:name 标签）：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.luksprog.passit"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="8" />

    <application
        android:name=".MyApplication"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:name=".AddActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".Timetable"
            android:label="@string/app_name" >
        </activity>
    </application>

</manifest>

最后是Module 类：

/**
 * A small Module class. 
 *
 */
public class Module {

    String name;

    public Module(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        //This will show a nice string in the `ListView` 
        return "This is:" + name;
    }

}

【讨论】：

感谢您的建议是否有任何详细的解释？谢谢
@mitchnufc 我不知道有什么教程可以推荐，但我在答案中添加了一个小例子，使用 Application 类。
感谢您的帮助，这是否意味着我将拥有另一个名为 addactivity 的类？
@mitchnufc 不，在我的示例中，AddActivity 是您列出列表的Add 活动（我想将其命名为Add，但我忘记了）。您必须像我一样实现MyApplication 类并修改应用程序的清单。然后，在另一个活动中设置/检索Modules 的列表，就像我在示例中使用MyApplication 类中的两种方法一样：setModulesList/getModulesList。
是的，我的模块类已经看起来像上面的一个，带有 get 和 set 方法等以及一个 toString，只是有了所有这些类，我有点迷失了

【解决方案2】：

在您的代码模块中没有初始化任何地方。私有 ArrayList 模块；

初始化它...

【讨论】：

我在另一个类上创建了一个数组，我试图通过将适配器应用于列表视图来在此类中显示该数组