Best practice for getting datatype size(sizeof) in Java

See @Frank Kusters' answer, below!

(My original answer here was for Java versions < 8.)

Since Java 8, all wrapper classes of primitive types (except Boolean) have a BYTES field. So in your case:

int size = numDouble * Double.BYTES + numInt * Integer.BYTES;

Documentation: http://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html

Write your own method. In Java the datatypes are platform independent always the same size:

public static int sizeof(Class dataType)
{
    if (dataType == null) throw new NullPointerException();

    if (dataType == int.class    || dataType == Integer.class)   return 4;
    if (dataType == short.class  || dataType == Short.class)     return 2;
    if (dataType == byte.class   || dataType == Byte.class)      return 1;
    if (dataType == char.class   || dataType == Character.class) return 2;
    if (dataType == long.class   || dataType == Long.class)      return 8;
    if (dataType == float.class  || dataType == Float.class)     return 4;
    if (dataType == double.class || dataType == Double.class)    return 8;

    return 4; // 32-bit memory pointer... 
              // (I'm not sure how this works on a 64-bit OS)
}

Usage:

int size = numDouble * sizeof(double.class) + numInt * sizeof(int.class);