The document discusses Project Valhalla and its goal of supporting value types in Java. Value types would behave like primitive types like int where they are passed by value rather than by reference. Project Valhalla aims to support small, immutable value types that lack identity. It provides examples of potential value types like Complex and Decimal64 that would code like classes but work like primitive types by being passed as copies.

1. Value Types
Project Valhalla

2. Project Valhalla
• Specialisation of Generics
• Atomic VarHandles
• Value Types

3. Codes like a class,
works like an int.
http://cr.openjdk.java.net/~jrose/values/values-0.html
“…to support small immutable, identityless value types.”

4. Type Signature Java Type
1) Z boolean
2) B byte
3) C char
4) S short
5) I int
6) J long
7) F float
8) D double
9) L class ; class
10) [ type type[]
11) V void

5. Type Signature Java Type
1) Z boolean
2) B byte
3) C char
4) S short
5) I int
6) J long
7) F float
8) D double
9) L class ; class
10) [ type type[]
11) V void

6. Type Signature Java Type
1) Z boolean
2) B byte
3) C char
4) S short
5) I Values
int
6) J long
7) F float
8) D double
9) L class ; class
10) [ type References
type[]
11) V void

7. Type Signature Java Type
1) Z boolean
2) B byte
3) C char
4) S short
5) I Values
int
6) J long
7) F float
8) D double
9) Q class ; value
10) L class ; class
11) [ type References
type[]
12) V void

8. Codes like a class
public __ByValue class Complex {
double real;
double imaginary;
}

9. Codes like a class
public __ByValue class Slice {
byte[] array;
int offset;
int length;
}

10. Codes like a class
public __ByValue class Decimal64 {
long value;
long significand() {
return value & 0xFFFFFFFFFFFFFF;
}
long exponent() {
return value >> 56;
}
}

11. Codes like a class
public __ByValue class Price {
Decimal64 value;
}
public __ByValue class Quantity {
Decimal64 value;
}

12. Works like an int
• Immutable
• Passed as a copy (semantically)
• Lacks identity
• Equals??? (vcmp)

13. Why?
Because caches.

14. public class Order {
Price orderPrice;
Quantity orderQuantity;
}

17. java.util.Optional
This is a value-based class; use of identity-sensitive
operations (including reference equality (==), identity
hash code, or synchronization) on instances of
Optional may have unpredictable results and should
be avoided.
http://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html