How to get the build/version number of your Android application?

Use:

PackageInfo pInfo = getPackageManager().getPackageInfo(getPackageName(), 0);
version = pInfo.versionName;

String versionName = context.getPackageManager()
    .getPackageInfo(context.getPackageName(), 0).versionName;

If you're using the Gradle plugin/Android Studio, as of version 0.7.0, version code and version name are available statically in BuildConfig:

int versionCode = BuildConfig.VERSION_CODE;
String versionName = BuildConfig.VERSION_NAME;