Java + Operator

Question

I am unable to grasp the idea of either addition operator or short data-type.

It's said that;

short a = 1;
short b = 2;
short c = a + b;

which will not compile because addition operator always cast short, chart, byte data-types to int and I understand this. But this;

short c = 1 + 2;

works totally fine. So, if addition operator auto converts short to int and then apply the result (where result ofcourse will be an int), why this works fine?

Edit: This question is not duplicate of Primitive type 'short' - casting in Java since I understand the conversions process. Also, the question talks about conversions of data types where as my question relates to int literals.

Answer 1

1 + 2 is a constant expression while a + b is not.
It matters on the evaluation of them.
The first one will be done at compile time, the second one at runtime.

The JLS 8 states :

15.28. Constant Expressions

A constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following:

• Literals of primitive type and literals of type String (§3.10.1, §3.10.2, §3.10.3, §3.10.4, §3.10.5)

• Casts to primitive types and casts to type String (§15.16)

• The unary operators +, -, ~, and ! (but not ++ or --) (§15.15.3, §15.15.4, §15.15.5, §15.15.6)

• The multiplicative operators *, /, and % (§15.17)

• The additive operators + and - (§15.18)

........................

Here :

short c = 1 + 2;

1 + 2 is composed of two int literals and one additive operator.
So it is considered as a constant expression.
Constant expressions are evaluated at compile time.
So short c is evaluated as 3

Here is a sample class :

package stackoverflow;

public class EvaluationClass {

    public void foo(){
       short c = 1 + 2;
    }
}

Here is the disassembled code :

Compiled from "EvaluationClass.java"
public class stackoverflow.EvaluationClass {
  public stackoverflow.EvaluationClass();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()
       4: return

  public void foo();
    Code:
       0: iconst_3
       1: istore_1
       2: return
}

We can see the 0: iconst_3 instruction that loads the 3 int onto the stack.

---

Whereas here :

short a = 1;
short b = 2;
short c = a + b;

a + b is evaluated only at runtime as a and b are not constant values.
Their values may indeed change at anytime.
Note that the compiler doesn't try to be clever by reading each statement to guess if a and b effectively mutate.
It considers that it may and so evaluate a + b only at runtime.

Now in this case, why a + b doesn't produce a short but an int ?
Because the JLS 8 specifies that :

4.2.2. Integer Operations

If an integer operator other than a shift operator has at least one operand of type long, then the operation is carried out using 64-bit precision, and the result of the numerical operator is of type long. If the other operand is not long, it is first widened (§5.1.5) to type long by numeric promotion (§5.6).

Otherwise, the operation is carried out using 32-bit precision, and the result of the numerical operator is of type int. If either operand is not an int, it is first widened to type int by numeric promotion.

---

As a side note, if change your code to make a and b constants:

final short a = 1;
final short b = 2;
short c = a + b;

This will compile fine now as a + b would be evaluated as a constant expression (3).