List<PackageInfo> packs = getPackageManager().getInstalledPackages(0);//获取安装程序的包名
for (int i = 0; i < packs.size(); i++) {
 PackageInfo p = packs.get(i);//某个包信息
 //打印：版本好，版本名，包名....
 Log.i("", "-------" + p.versionCode + "-------" + p.versionName + "--------" 
  + p.packageName + "-------" + p.applicationInfo);
}

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.testapk"
   android:versionCode="2" // p.versionCode 
   android:versionName="Version1" // p.versionName
>

int versionCode = 0;
try {
  versionCode = getPackageManager().getPackageInfo(this.getPackageName(), 0).versionCode;
} catch (NameNotFoundException e) {
  e.printStackTrace();
}

// 通过检测包名，判断APK是否安装
private boolean checkPackageExist(boolean getSysPackages) {
 boolean packageExist = false;
 int versionCode = 0;
 try {
  versionCode = getPackageManager().getPackageInfo(this.getPackageName(), 0).versionCode;
 } catch (NameNotFoundException e) {
  e.printStackTrace();
 }
 Log.i("", "-------" + this.getPackageName() + "-------" + versionCode);//获取当前包名
 List<PackageInfo> packs = getPackageManager().getInstalledPackages(0);
 for (int i = 0; i < packs.size(); i++) {
  PackageInfo p = packs.get(i);
  Log.i("", "-------" + p.versionCode + "-------" + p.versionName + "--------" 
   + p.packageName + "-------" + p.applicationInfo);
  if ((!getSysPackages) && (p.versionName == null)) {
  continue;
  }
  if (p.packageName.equalsIgnoreCase(PACKAGENAME)) {
  packageExist = true;
  break;
  }
 }
 return packageExist;
 }
 //安装APK
 private void installApk() {
 if (checkFileExist(fileRoot + fileName)) {
  Intent intent = new Intent();
  intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
  intent.setAction(android.content.Intent.ACTION_VIEW);
  String type = "application/vnd.android.package-archive";
  intent.setDataAndType(Uri.parse("file://" + fileRoot + fileName),
   type);
  startActivity(intent);
 } else {
  downloadapk();
 }
}