<application>
  <activity android:name=".MainActivity" >
    <intent-filter>
      <action android:name="android.intent.action.MAIN" />
      <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
  </activity>
  ...
</application>

String package_name="xx.xx.xx";
PackageManager packageManager = context.getPackageManager();
Intent it = packageManager.getLaunchIntentForPackage(package_name);
startActivity(it);

/**
* Returns a "good" intent to launch a front-door activity in a package.
* This is used, for example, to implement an "open" button when browsing
* through packages. The current implementation looks first for a main
* activity in the category {@link Intent#CATEGORY_INFO}, and next for a
* main activity in the category {@link Intent#CATEGORY_LAUNCHER}. Returns
* <code>null</code> if neither are found.
*
* @param packageName The name of the package to inspect.
*
* @return A fully-qualified {@link Intent} that can be used to launch the
* main activity in the package. Returns <code>null</code> if the package
* does not contain such an activity, or if <em>packageName</em> is not
* recognized.
*/
public abstract Intent getLaunchIntentForPackage(String packageName);

String package_name = "xx.xx.xx";
PackageManager packageManager = getPackageManager();
Intent it = packageManager.getLaunchIntentForPackage(package_name);
if (it != null){
  startActivity(it);
}else{
  //没有默认的入口 Activity
}

String package_name = "xx.xx.xx";
String activity_path = "xx.xx.xx.ab.xxActivity";
Intent intent = new Intent();
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);//可选
ComponentName comp = new ComponentName(package_name,activity_path);
intent.setComponent(comp);
startActivity(intent);

String package_name = "xx.xx.xx";
String activity_path = "xx.xx.xx.ab.xxActivity";
Intent intent = new Intent();
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);//可选
ComponentName cn = new ComponentName(package_name,activity_path);
intent.setComponent(cn);
if (intent.resolveActivity(getPackageManager()) != null) {
  startActivity(intent);
} else {
  //找不到指定的 Activity
}

public ComponentName resolveActivity(PackageManager pm) {
  if (mComponent != null) {
    return mComponent;
  }
  ResolveInfo info = pm.resolveActivity(this,
    PackageManager.MATCH_DEFAULT_ONLY);
  if (info != null) {
    return new ComponentName(
      info.activityInfo.applicationInfo.packageName,
      info.activityInfo.name);
  }
  return null;
}
public ActivityInfo resolveActivityInfo(PackageManager pm, int flags) {
  ActivityInfo ai = null;
  if (mComponent != null) {
    try {
      ai = pm.getActivityInfo(mComponent, flags);
    } catch (PackageManager.NameNotFoundException e) {
      // ignore
    }
  } else {
    ResolveInfo info = pm.resolveActivity(this,
      PackageManager.MATCH_DEFAULT_ONLY | flags);
    if (info != null) {
      ai = info.activityInfo;
    }
  }
  return ai;
}

String package_name = "xx.xx.xx";
String activity_path = "xx.xx.xx.ab.xxActivity";
Intent intent = new Intent();
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);//可选
ComponentName cn = new ComponentName(package_name,activity_path);
intent.setComponent(cn);
if (intent.resolveActivityInfo(getPackageManager(), PackageManager.MATCH_DEFAULT_ONLY) != null) {
  startActivity(intent);
} else {
  //找不到指定的 Activity
}

Intent intent = new Intent();
intent.setAction(action);
intent.addCategory(category);
intent.setDataAndType("abc://www.dfg.com","image/gif");
startActivity(intent);

Uri uri = Uri.parse("http://www.abc.xyz");
Intent intent = new Intent(Intent.ACTION_VIEW, uri);
startActivity(intent);

Uri uri = Uri.parse("http://www.abc.xyz");
Intent intent = new Intent(Intent.ACTION_VIEW, uri);
if (intent.resolveActivity(getPackageManager()) != null) {
  startActivity(intent);
} else {
  // 没有安装所需应用
}

public boolean isAvailable(Context context, Intent intent) {
  PackageManager packageManager = context.getPackageManager();
  List list = packageManager.queryIntentActivities(intent,
  PackageManager.MATCH_DEFAULT_ONLY);
  return list.size() > 0;
}