5-jvm-type-system.md

JVM Type System

The types in the JVM are divided into primitive types and reference types:

Primitive Types	Reference Types
"value"	"object"
local instances on stack	instances on heap
copy is by value	copy is by reference
cannot be null	can be null

• The lifetime of local variables (on the stack) is equal to the execution time of the function to which they belong;
• The lifetime of objects on the heap is longer than the execution time of the function, and is the target of GC when the objects are unreachable, that is, there is no reference to them.

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Primitive Types (Value Types)

Java	Kotlin
int	Int
long	Long
float	Float
double	Double
char	Char
boolean	Boolean
byte	Byte
short	Short

Example:

fun bar() {
    val v: Int = 731
    val b = v
}

• The variables v and b are local to bar;
• The copy of v is made by value; b gets a copy of the value of v:

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Reference Types (Object Types or Wrapper Types)

Java	Kotlin
Integer	Int?
Long	Long?
Float	Float?
Double	Double?
Character	Char?
Boolean	Boolean?
Byte	Byte?
Short	Short?

Example:

class Foo {};

fun bar() {
    val v = Foo()
    val b = v
}

• The variables v and b are local to bar;
• The copy of v is made by reference; b points to the same object:

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Type Conversion

• TR -> TR: casting;

• TP -> TP: coercion;

• TP -> TR: boxing;

• TR -> TP: unboxing.

• RT -> RT: Casting: maintains the instance and changes the reference;

• PT -> PT: Coercion: changes the value of the instance;

• PT -> RT: Boxing: copy of the primitive value to the heap - <Wrapper>.valueOf(<primitive>);

• RT -> PT: Unboxing: copy of the heap value to the primitive value - <Wrapper>.<primitive>Value().

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes.

Two types of casting:

• Upcasting: generalization; "Child -> Parent";
• Downcasting: specificationF; "Child -> Child".

checkcast Instruction

• Checks if the reference at the top of the stack is compatible with the specified reference type;
• Compatibility means being of the same type or of a derived type;
• Used only in downcasting.

If the check is successful, the cast is performed, otherwise the ClassCastException exception is thrown.