Where is the source code for Java Type Promotion and how to debug it

Question

I basically want to debug the following code for Type Promotion to understand the temporary double variable created for x. How can I do this?

public class TypePromotion {
public static void main(String args[]){
    int x = 10;
    double y = 20.0;
    double z = x + y;

    System.out.println("value of Z is :: "+z); // outputs 30.0
    System.out.println("value of X is :: "+x); // outputs 10
}}

Answer 1

The answer to your question (as I understand it) is in the Java Language Specification,

https://docs.oracle.com/javase/specs/jls/se11/html/jls-5.html#jls-5.6.2 https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.18.2

When you use an operator (in this case +) on a pair of operands (x and y) the resulting numeric type is determined by these definitions.

To add a bit more detail, if you disassemble the classfile generated by your code, the relevant instructions are:

   0: bipush        10
   2: istore_1
   3: ldc2_w        #2                  // double 20.0d
   6: dstore_2
   7: iload_1
   8: i2d
   9: dload_2
  10: dadd
  11: dstore        4

As you can see, x is stored as an int, y is stored as a double. x is pushed onto the stack then converted to a double. After y is pushed onto the stack a dadd (double add) can be performed as both operands are doubles. The javac compiler generates the code that conforms to the Java language spec, as detailed above.