your content must have a listview whose id attribute is 'android.r.id.list'

见android-14，ListActivity源码

/* * Copyright (C) 2006 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package android.app;import android.os.Bundle;import android.os.Handler;import android.view.View;import android.widget.AdapterView;import android.widget.ListAdapter;import android.widget.ListView;/** * An activity that displays a list of items by binding to a data source such as * an array or Cursor, and exposes event handlers when the user selects an item. * <p> * ListActivity hosts a {@link android.widget.ListView ListView} object that can * be bound to different data sources, typically either an array or a Cursor * holding query results. Binding, screen layout, and row layout are discussed * in the following sections. * <p> * <strong>Screen Layout</strong> * </p> * <p> * ListActivity has a default layout that consists of a single, full-screen list * in the center of the screen. However, if you desire, you can customize the * screen layout by setting your own view layout with setContentView() in * onCreate(). To do this, your own view MUST contain a ListView object with the * id "@android:id/list" (or {@link android.R.id#list} if it's in code) * <p> * Optionally, your custom view can contain another view object of any type to * display when the list view is empty. This "empty list" notifier must have an * id "android:id/empty". Note that when an empty view is present, the list view * will be hidden when there is no data to display. * <p> * The following code demonstrates an (ugly) custom screen layout. It has a list * with a green background, and an alternate red "no data" message. * </p> * * <pre> * <?xml version="1.0" encoding="utf-8"?> * <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" *         android:orientation="vertical" *         android:layout_width="match_parent" *         android:layout_height="match_parent" *         android:paddingLeft="8dp" *         android:paddingRight="8dp"> * *     <ListView android:id="@android:id/list" *               android:layout_width="match_parent" *               android:layout_height="match_parent" *               android:background="#00FF00" *               android:layout_weight="1" *               android:drawSelectorOnTop="false"/> * *     <TextView android:id="@android:id/empty" *               android:layout_width="match_parent" *               android:layout_height="match_parent" *               android:background="#FF0000" *               android:text="No data"/> * </LinearLayout> * </pre> * * <p> * <strong>Row Layout</strong> * </p> * <p> * You can specify the layout of individual rows in the list. You do this by * specifying a layout resource in the ListAdapter object hosted by the activity * (the ListAdapter binds the ListView to the data; more on this later). * <p> * A ListAdapter constructor takes a parameter that specifies a layout resource * for each row. It also has two additional parameters that let you specify * which data field to associate with which object in the row layout resource. * These two parameters are typically parallel arrays. * </p> * <p> * Android provides some standard row layout resources. These are in the * {@link android.R.layout} class, and have names such as simple_list_item_1, * simple_list_item_2, and two_line_list_item. The following layout XML is the * source for the resource two_line_list_item, which displays two data * fields,one above the other, for each list row. * </p> * * <pre> * <?xml version="1.0" encoding="utf-8"?> * <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" *     android:layout_width="match_parent" *     android:layout_height="wrap_content" *     android:orientation="vertical"> * *     <TextView android:id="@+id/text1" *         android:textSize="16sp" *         android:textStyle="bold" *         android:layout_width="match_parent" *         android:layout_height="wrap_content"/> * *     <TextView android:id="@+id/text2" *         android:textSize="16sp" *         android:layout_width="match_parent" *         android:layout_height="wrap_content"/> * </LinearLayout> * </pre> * * <p> * You must identify the data bound to each TextView object in this layout. The * syntax for this is discussed in the next section. * </p> * <p> * <strong>Binding to Data</strong> * </p> * <p> * You bind the ListActivity's ListView object to data using a class that * implements the {@link android.widget.ListAdapter ListAdapter} interface. * Android provides two standard list adapters: * {@link android.widget.SimpleAdapter SimpleAdapter} for static data (Maps), * and {@link android.widget.SimpleCursorAdapter SimpleCursorAdapter} for Cursor * query results. * </p> * <p> * The following code from a custom ListActivity demonstrates querying the * Contacts provider for all contacts, then binding the Name and Company fields * to a two line row layout in the activity's ListView. * </p> * * <pre> * public class MyListAdapter extends ListActivity { * *     @Override *     protected void onCreate(Bundle savedInstanceState){ *         super.onCreate(savedInstanceState); * *         // We'll define a custom screen layout here (the one shown above), but *         // typically, you could just use the standard ListActivity layout. *         setContentView(R.layout.custom_list_activity_view); * *         // Query for all people contacts using the {@link android.provider.Contacts.People} convenience class. *         // Put a managed wrapper around the retrieved cursor so we don't have to worry about *         // requerying or closing it as the activity changes state. *         mCursor = this.getContentResolver().query(People.CONTENT_URI, null, null, null, null); *         startManagingCursor(mCursor); * *         // Now create a new list adapter bound to the cursor. *         // SimpleListAdapter is designed for binding to a Cursor. *         ListAdapter adapter = new SimpleCursorAdapter( *                 this, // Context. *                 android.R.layout.two_line_list_item,  // Specify the row template to use (here, two columns bound to the two retrieved cursor * rows). *                 mCursor,                                              // Pass in the cursor to bind to. *                 new String[] {People.NAME, People.COMPANY},           // Array of cursor columns to bind to. *                 new int[] {android.R.id.text1, android.R.id.text2});  // Parallel array of which template objects to bind to those columns. * *         // Bind to our new adapter. *         setListAdapter(adapter); *     } * } * </pre> * * @see #setListAdapter * @see android.widget.ListView */public class ListActivity extends Activity {    /**     * This field should be made private, so it is hidden from the SDK.     * {@hide}     */    protected ListAdapter mAdapter;    /**     * This field should be made private, so it is hidden from the SDK.     * {@hide}     */    protected ListView mList;    private Handler mHandler = new Handler();    private boolean mFinishedStart = false;    private Runnable mRequestFocus = new Runnable() {        public void run() {            mList.focusableViewAvailable(mList);        }    };    /**     * This method will be called when an item in the list is selected.     * Subclasses should override. Subclasses can call     * getListView().getItemAtPosition(position) if they need to access the     * data associated with the selected item.     *     * @param l The ListView where the click happened     * @param v The view that was clicked within the ListView     * @param position The position of the view in the list     * @param id The row id of the item that was clicked     */    protected void onListItemClick(ListView l, View v, int position, long id) {    }    /**     * Ensures the list view has been created before Activity restores all     * of the view states.     *     *@see Activity#onRestoreInstanceState(Bundle)     */    @Override    protected void onRestoreInstanceState(Bundle state) {        ensureList();        super.onRestoreInstanceState(state);    }    /**     * @see Activity#onDestroy()     */    @Override    protected void onDestroy() {        mHandler.removeCallbacks(mRequestFocus);        super.onDestroy();    }    /**     * Updates the screen state (current list and other views) when the     * content changes.     *     * @see Activity#onContentChanged()     */    @Override    public void onContentChanged() {        super.onContentChanged();        View emptyView = findViewById(com.android.internal.R.id.empty);        mList = (ListView)findViewById(com.android.internal.R.id.list);        if (mList == null) {            throw new RuntimeException(                    "Your content must have a ListView whose id attribute is " +                    "'android.R.id.list'");        }        if (emptyView != null) {            mList.setEmptyView(emptyView);        }        mList.setOnItemClickListener(mOnClickListener);        if (mFinishedStart) {            setListAdapter(mAdapter);        }        mHandler.post(mRequestFocus);        mFinishedStart = true;    }    /**     * Provide the cursor for the list view.     */    public void setListAdapter(ListAdapter adapter) {        synchronized (this) {            ensureList();            mAdapter = adapter;            mList.setAdapter(adapter);        }    }    /**     * Set the currently selected list item to the specified     * position with the adapter's data     *     * @param position     */    public void setSelection(int position) {        mList.setSelection(position);    }    /**     * Get the position of the currently selected list item.     */    public int getSelectedItemPosition() {        return mList.getSelectedItemPosition();    }    /**     * Get the cursor row ID of the currently selected list item.     */    public long getSelectedItemId() {        return mList.getSelectedItemId();    }    /**     * Get the activity's list view widget.     */    public ListView getListView() {        ensureList();        return mList;    }    /**     * Get the ListAdapter associated with this activity's ListView.     */    public ListAdapter getListAdapter() {        return mAdapter;    }    private void ensureList() {        if (mList != null) {            return;        }        setContentView(com.android.internal.R.layout.list_content_simple);    }    private AdapterView.OnItemClickListener mOnClickListener = new AdapterView.OnItemClickListener() {        public void onItemClick(AdapterView<?> parent, View v, int position, long id)        {            onListItemClick((ListView)parent, v, position, id);        }    };}