After knowing primitive data types and Java rules of Data Type Casting (Type Conversion), let us cast short to double as an example.
The short takes 2 byte of memory and double takes 8 bytes of memory. Assignment 2 bytes of memory to 8 byte of memory is done by JVM implicitly and is known as widening conversion.
See the data types order to know the brief rules of casting.
The left-side value can be assigned to any right-side value and is done implicitly. The reverse like int to short requires explicit casting.
Examples of implicit casting
short x = 10;
int y = x;byte x = 10;
short y = x;
Following program on short to double explains the Java syntax of implict casting where a short value is assigned to a double value.
public class Conversions
{
public static void main(String args[])
{
short s1 = 10; // 2 bytes
double d1 = s1; // 2 bytes assigned to 8 bytes
System.out.println("short value: " + s1); // prints 10
System.out.println("Converted double value: " + d1); // prints 10.0
}
}

Output screenshot of short to double Java
double d1 = s1;
The short s1 is assigned to double d1. This is done automatically by JVM.