Intent intent = getPackageManager().getLaunchIntentForPackage("com.test.mydemo");
startActivity(intent);

②

Intent intent=new Intent();  

//包名 包名+类名（全路径）  

intent.setClassName("com.test.mydemo", "com.test.mydemo.TestActivity");  

startActivity(intent)

二：通过自定义的Action

Intent intent = new Intent();
intent.setAction("com.test.mydemo.main");
                MainActivity.this.startActivity(intent);
<intent-filter>
<action android:name="com.test.mydemo.main" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>

Android.intent.action.MAIN决定应用程序最先启动的Activity，android.intent.category.LAUNCHER决定应用程序是否显示在 
程序列表里。

三：通过Scheme

Intent intent = new Intent();
                intent.setData(Uri.parse("mydemo://"));
                startActivity(intent);
<intent-filter>
                <action android:name="android.intent.action.VIEW" />
                <category android:name="android.intent.category.DEFAULT" />
                <data android:scheme="mydemo"  />               
            </intent-filter>

附加内容：
微博 com.sina.weibo com.sina.weibo.SplashActivity 
知乎 com.zhihu.android com.zhihu.android.ui.activity.GuideActivity 
薄荷 com.app.one com.app.one.ui.SplashActivity 
打开系统应用：

浏览网页

Uri uri =Uri.parse("http://www.google.com");
Intent it = new Intent(Intent.ACTION_VIEW,uri);
startActivity(it);   

拨打电话

Uri uri =Uri.parse("tel:xxxxxx");
Intent it = new Intent(Intent.ACTION_DIAL,uri);
startActivity(it);  

发短信

//方法1：
Intent it = newIntent(Intent.ACTION_VIEW);
it.putExtra("sms_body", "TheSMS text");
it.setType("vnd.android-dir/mms-sms");
startActivity(it);  //方法2：
Uri uri =Uri.parse("smsto:0800000123");
Intent it = newIntent(Intent.ACTION_SENDTO, uri);
it.putExtra("sms_body", "TheSMS text");
startActivity(it);  //方法三：
String body="this is sms demo";
Intent mmsintent = newIntent(Intent.ACTION_SENDTO, Uri.fromParts("smsto", number, null));
mmsintent.putExtra(Messaging.KEY_ACTION_SENDTO_MESSAGE_BODY,body);
mmsintent.putExtra(Messaging.KEY_ACTION_SENDTO_COMPOSE_MODE,true);
mmsintent.putExtra(Messaging.KEY_ACTION_SENDTO_EXIT_ON_SENT,true);
startActivity(mmsintent);

发送彩信

Uri uri =Uri.parse("content://media/external/images/media/23");
Intent it = newIntent(Intent.ACTION_SEND);
it.putExtra("sms_body","some text");
it.putExtra(Intent.EXTRA_STREAM, uri);
it.setType("image/png");
startActivity(it);
StringBuilder sb = new StringBuilder();
sb.append("file://");
sb.append(fd.getAbsoluteFile());
Intent intent = newIntent(Intent.ACTION_SENDTO, Uri.fromParts("mmsto", number, null));
// Below extra datas are all optional.
intent.putExtra(Messaging.KEY_ACTION_SENDTO_MESSAGE_SUBJECT,subject);
intent.putExtra(Messaging.KEY_ACTION_SENDTO_MESSAGE_BODY,body);
intent.putExtra(Messaging.KEY_ACTION_SENDTO_CONTENT_URI,sb.toString());
intent.putExtra(Messaging.KEY_ACTION_SENDTO_COMPOSE_MODE,composeMode);
intent.putExtra(Messaging.KEY_ACTION_SENDTO_EXIT_ON_SENT,exitOnSent);
startActivity(intent);

发送Email

Uri uri =Uri.parse("mailto:xxx@abc.com");
Intent it = newIntent(Intent.ACTION_SENDTO, uri);
startActivity(it);  Intent it = new Intent(Intent.ACTION_SEND);
it.putExtra(Intent.EXTRA_EMAIL,"me@abc.com");
it.putExtra(Intent.EXTRA_TEXT, "Theemail body text");
it.setType("text/plain");
startActivity(Intent.createChooser(it,"Choose Email Client"));  Intent it=new Intent(Intent.ACTION_SEND);
String[] tos={"me@abc.com"};
String[]ccs={"you@abc.com"};
it.putExtra(Intent.EXTRA_EMAIL, tos);
it.putExtra(Intent.EXTRA_CC, ccs);
it.putExtra(Intent.EXTRA_TEXT, "Theemail body text");
it.putExtra(Intent.EXTRA_SUBJECT, "Theemail subject text");
it.setType("message/rfc822");
startActivity(Intent.createChooser(it,"Choose Email Client"));     Intent it = newIntent(Intent.ACTION_SEND);
it.putExtra(Intent.EXTRA_SUBJECT, "Theemail subject text");
it.putExtra(Intent.EXTRA_STREAM,"file:///sdcard/mysong.mp3");
sendIntent.setType("audio/mp3");
startActivity(Intent.createChooser(it,"Choose Email Client"));

播放多媒体

Intent it = new Intent(Intent.ACTION_VIEW);
Uri uri =Uri.parse("file:///sdcard/song.mp3");
it.setDataAndType(uri,"audio/mp3");
startActivity(it);
Uri uri =Uri.withAppendedPath(MediaStore.Audio.Media.INTERNAL_CONTENT_URI,"1");
Intent it = new Intent(Intent.ACTION_VIEW,uri);
startActivity(it);

卸载 apk

Uri uri =Uri.fromParts("package", strPackageName, null);
Intent it = newIntent(Intent.ACTION_DELETE, uri);
startActivity(it);

安装 apk

Uri installUri = Uri.fromParts("package","xxx", null);
returnIt = newIntent(Intent.ACTION_PACKAGE_ADDED, installUri);  Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setDataAndType(Uri.parse("file://" + filepath),"application/vnd.android.package-archive");
startActivity(intent);// 安装

打开照相机

//1
Intent intent = new Intent("android.media.action.STILL_IMAGE_CAMERA"); //调用照相机
startActivity(intent);
//2
Intent i = new Intent(Intent.ACTION_CAMERA_BUTTON, null);
this.sendBroadcast(i);
//3
long dateTaken = System.currentTimeMillis();
String name = createName(dateTaken) + ".jpg";
fileName = folder + name;
ContentValues values = new ContentValues();
values.put(Images.Media.TITLE, fileName);
values.put("_data", fileName);
values.put(Images.Media.PICASA_ID, fileName);
values.put(Images.Media.DISPLAY_NAME, fileName);
values.put(Images.Media.DESCRIPTION, fileName);
values.put(Images.ImageColumns.BUCKET_DISPLAY_NAME, fileName);
Uri photoUri = getContentResolver().insert(
MediaStore.Images.Media.EXTERNAL_CONTENT_URI,values);  Intent inttPhoto = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
inttPhoto.putExtra(MediaStore.EXTRA_OUTPUT, photoUri);
startActivityForResult(inttPhoto, 10);

从gallery选取图片

Intent i = new Intent();
i.setType("image/*");
i.setAction(Intent.ACTION_GET_CONTENT);
startActivityForResult(i, 11);

打开录音机

Intent mi = new Intent(Media.RECORD_SOUND_ACTION);
startActivity(mi);

显示应用详细列表

Uri uri =Uri.parse("market://details?id=app_id");
Intent it = new Intent(Intent.ACTION_VIEW,uri);
startActivity(it);
//where app_id is the application ID, findthe ID
//by clicking on your application on Markethome
//page, and notice the ID from the addressbar
//发现用package name也可以
//Uri uri =Uri.parse("market://details?id=<packagename>");

寻找应用

Uri uri =Uri.parse("market://search?q=pname:pkg_name");
Intent it = new Intent(Intent.ACTION_VIEW,uri);
startActivity(it);
//where pkg_name is the full package pathfor an application

打开联系人列表

//1
Intent i = new Intent();
i.setAction(Intent.ACTION_GET_CONTENT);
i.setType("vnd.android.cursor.item/phone");
startActivityForResult(i, REQUEST_TEXT);
//2
Uri uri = Uri.parse("content://contacts/people");
Intent it = new Intent(Intent.ACTION_PICK, uri);
startActivityForResult(it, REQUEST_TEXT);

调用系统编辑添加联系人

Intent intent = newIntent(Intent.ACTION_INSERT_OR_EDIT);
intent.setType(People.CONTENT_ITEM_TYPE);
intent.putExtra(Contacts.Intents.Insert.NAME, "My Name");
intent.putExtra(Contacts.Intents.Insert.PHONE, "+1234567890");
intent.putExtra(Contacts.Intents.Insert.PHONE_TYPE,Contacts.PhonesColumns.TYPE_MOBILE);
intent.putExtra(Contacts.Intents.Insert.EMAIL, "com@com.com");
intent.putExtra(Contacts.Intents.Insert.EMAIL_TYPE, Contacts.ContactMethodsColumns.TYPE_WORK);
startActivity(intent);