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Casting

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Casting

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A cast converts the value of an original type to the equivalent value of a target type. An implicit cast infers the target type and automatically occurs during certain operations. An explicit cast specifies the target type and forcefully occurs as its own operation. Use the cast operator '()' to specify an explicit cast.

Refer to the cast table for a quick reference on all allowed casts.

Errors

If during a cast there exists no equivalent value for the target type.
If an implicit cast is given, but an explicit cast is required.

Grammar

cast: '(' TYPE ')' expression

Examples

Valid casts.

int i = (int)5L;         
Map m = new HashMap();   
HashMap hm = (HashMap)m;

	declare `int i`; explicit cast `long 5` to `int 5` → `int 5`; store `int 5` to `i`
	declare `Map m`; allocate `HashMap` instance → `HashMap reference`; implicit cast `HashMap reference` to `Map reference` → `Map reference`; store `Map reference` to `m`
	declare `HashMap hm`; load from `m` → `Map reference`; explicit cast `Map reference` to `HashMap reference` → `HashMap reference`; store `HashMap reference` to `hm`

Numeric Type Casting

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A numeric type cast converts the value of an original numeric type to the equivalent value of a target numeric type. A cast between two numeric type values results in data loss when the value of the original numeric type is larger than the target numeric type can accommodate. A cast between an integer type value and a floating point type value can result in precision loss.

The allowed casts for values of each numeric type are shown as a row in the following table:

	byte	short	char	int	long	float	double
byte		implicit	implicit	implicit	implicit	implicit	implicit
short	explicit		explicit	implicit	implicit	implicit	implicit
char	explicit	explicit		implicit	implicit	implicit	implicit
int	explicit	explicit	explicit		implicit	implicit	implicit
long	explicit	explicit	explicit	explicit		implicit	implicit
float	explicit	explicit	explicit	explicit	explicit		implicit
double	explicit	explicit	explicit	explicit	explicit	explicit

Examples

Valid numeric type casts.

int a = 1;            
long b = a;           
short c = (short)b;   
double e = (double)a;

	declare `int a`; store `int 1` to `a`
	declare `long b`; load from `a` → `int 1`; implicit cast `int 1` to `long 1` → `long 1`; store `long 1` to `b`
	declare `short c`; load from `b` → `long 1`; explicit cast `long 1` to `short 1` → `short 1`; store `short 1` value to `c`
	declare `double e`; load from `a` → `int 1`; explicit cast `int 1` to `double 1.0`; store `double 1.0` to `e`; (note the explicit cast is extraneous since an implicit cast is valid)

Invalid numeric type casts resulting in errors.

int a = 1.0; // error 
int b = 2;            
byte c = b;  // error

	declare `int i`; error → cannot implicit cast `double 1.0` to `int 1`; (note an explicit cast is valid)
	declare `int b`; store `int 2` to `b`
	declare byte `c`; load from `b` → `int 2`; error → cannot implicit cast `int 2` to `byte 2`; (note an explicit cast is valid)

Reference Type Casting

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A reference type cast converts the value of an original reference type to the equivalent value of a target reference type. An implicit cast between two reference type values is allowed when the original reference type is a descendant of the target type. An explicit cast between two reference type values is allowed when the original type is a descendant of the target type or the target type is a descendant of the original type.

Examples

Valid reference type casts.

List x;                        
ArrayList y = new ArrayList(); 
x = y;                         
y = (ArrayList)x;              
x = (List)y;

	declare `List x`; store default value `null` to `x`
	declare `ArrayList y`; allocate `ArrayList` instance → `ArrayList reference`; store `ArrayList reference` to `y`;
	load from `y` → `ArrayList reference`; implicit cast `ArrayList reference` to `List reference` → `List reference`; store `List reference` to `x`; (note `ArrayList` is a descendant of `List`)
	load from `x` → `List reference`; explicit cast `List reference` to `ArrayList reference` → `ArrayList reference`; store `ArrayList reference` to `y`;
	load from `y` → `ArrayList reference`; explicit cast `ArrayList reference` to `List reference` → `List reference`; store `List reference` to `x`; (note the explicit cast is extraneous, and an implicit cast is valid)

Invalid reference type casts resulting in errors.

List x = new ArrayList();          
ArrayList y = x;          // error 
Map m = (Map)x;           // error

	declare `List x`; allocate `ArrayList` instance → `ArrayList reference`; implicit cast `ArrayList reference` to `List reference` → `List reference`; store `List reference` to `x`
	declare `ArrayList y`; load from `x` → `List reference`; error → cannot implicit cast `List reference` to `ArrayList reference`; (note an explicit cast is valid since `ArrayList` is a descendant of `List`)
	declare `ArrayList y`; load from `x` → `List reference`; error → cannot explicit cast `List reference` to `Map reference`; (note no cast is valid since neither `List` nor `Map` is a descendant of the other)

Dynamic Type Casting

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A dynamic (def) type cast converts the value of an original def type to the equivalent value of any target type or converts the value of any original type to the equivalent value of a target def type.

An implicit cast from any original type value to a def type value is always allowed. An explicit cast from any original type value to a def type value is always allowed but never necessary.

An implicit or explicit cast from an original def type value to any target type value is allowed if and only if the cast is normally allowed based on the current type value the def type value represents.

Examples

Valid dynamic type casts with any original type to a target def type.

def d0 = 3;               
d0 = new ArrayList();     
Object o = new HashMap(); 
def d1 = o;               
int i = d1.size();

	declare `def d0`; implicit cast `int 3` to `def`; store `int 3` to `d0`
	allocate `ArrayList` instance → `ArrayList reference`; implicit cast `ArrayList reference` to `def` → `def`; store `def` to `d0`
	declare `Object o`; allocate `HashMap` instance → `HashMap reference`; implicit cast `HashMap reference` to `Object reference` → `Object reference`; store `Object reference` to `o`
	declare `def d1`; load from `o` → `Object reference`; implicit cast `Object reference` to `def` → `def`; store `def` to `d1`
	declare `int i`; load from `d1` → `def`; implicit cast `def` to `HashMap reference` → HashMap reference`; call `size` on `HashMap reference` → `int 0`; store `int 0` to `i`; (note `def` was implicit cast to `HashMap reference` since `HashMap` is the child-most descendant type value that the `def` type value represents)

Valid dynamic type casts with an original def type to any target type.

def d = 1.0;         
int i = (int)d;      
d = 1;               
float f = d;         
d = new ArrayList(); 
List l = d;

	declare `def d`; implicit cast `double 1.0` to `def` → `def`; store `def` to `d`
	declare `int i`; load from `d` → `def`; implicit cast `def` to `double 1.0` → `double 1.0`; explicit cast `double 1.0` to `int 1` → `int 1`; store `int 1` to `i`; (note the explicit cast is necessary since a `double` type value is not converted to an `int` type value implicitly)
	store `int 1` to `d`; (note the switch in the type `d` represents from `double` to `int`)
	declare `float i`; load from `d` → `def`; implicit cast `def` to `int 1` → `int 1`; implicit cast `int 1` to `float 1.0` → `float 1.0`; store `float 1.0` to `f`
	allocate `ArrayList` instance → `ArrayList reference`; store `ArrayList reference` to `d`; (note the switch in the type `d` represents from `int` to `ArrayList`)
	declare `List l`; load from `d` → `def`; implicit cast `def` to `ArrayList reference` → `ArrayList reference`; implicit cast `ArrayList reference` to `List reference` → `List reference`; store `List reference` to `l`

Invalid dynamic type casts resulting in errors.

def d = 1;                  
short s = d;       // error 
d = new HashMap();          
List l = d;        // error

	declare `def d`; implicit cast `int 1` to `def` → `def`; store `def` to `d`
	declare `short s`; load from `d` → `def`; implicit cast `def` to `int 1` → `int 1`; error → cannot implicit cast `int 1` to `short 1`; (note an explicit cast is valid)
	allocate `HashMap` instance → `HashMap reference`; implicit cast `HashMap reference` to `def` → `def`; store `def` to `d`
	declare `List l`; load from `d` → `def`; implicit cast `def` to `HashMap reference`; error → cannot implicit cast `HashMap reference` to `List reference`; (note no cast is valid since neither `HashMap` nor `List` is a descendant of the other)

String to Character Casting

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Use the cast operator to convert a String type value into a char type value.

Errors

If the String type value isn’t one character in length.
If the String type value is null.

Examples

Casting string literals into char type values.
```
char c = (char)"C"; 
c = (char)'c';      
```
declare char c; explicit cast String "C" to char C → char C; store char C to c

explicit cast String 'c' to char c → char c; store char c to c
Casting a String reference into a char type value.
```
String s = "s";   
char c = (char)s; 
```
declare String s; store String "s" to s;

declare char c load from s → String "s"; explicit cast String "s" to char s → char s; store char s to c

Character to String Casting

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Use the cast operator to convert a char type value into a String type value.

Examples

Casting a String reference into a char type value.
```
char c = 65;          
String s = (String)c; 
```
declare char c; store char 65 to c;

declare String s load from c → char A; explicit cast char A to String "A" → String "A"; store String "A" to s

Boxing and Unboxing

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Boxing is a special type of cast used to convert a primitive type to its corresponding reference type. Unboxing is the reverse used to convert a reference type to its corresponding primitive type.

Implicit boxing/unboxing occurs during the following operations:

Conversions between a def type and a primitive type are implicitly boxed/unboxed as necessary, though this is referred to as an implicit cast throughout the documentation.
Method/function call arguments are implicitly boxed/unboxed as necessary.
A primitive type value is implicitly boxed when a reference type method is called on it.

Explicit boxing/unboxing is not allowed. Use the reference type API to explicitly convert a primitive type value to its respective reference type value and vice versa.

Errors

If an explicit cast is made to box/unbox a primitive type.

Examples

Uses of implicit boxing/unboxing.

List l = new ArrayList();       
l.add(1);                       
Integer I = Integer.valueOf(0); 
int i = l.get(i);

	declare `List l`; allocate `ArrayList` instance → `ArrayList reference`; store `ArrayList reference` to `l`;
	load from `l` → `List reference`; implicit cast `int 1` to `def` → `def`; call `add` on `List reference` with arguments (`def`); (note internally `int 1` is boxed to `Integer 1` to store as a `def` type value)
	declare `Integer I`; call `valueOf` on `Integer` with arguments of (`int 0`) → `Integer 0`; store `Integer 0` to `I`;
	declare `int i`; load from `I` → `Integer 0`; unbox `Integer 0` → `int 0`; load from `l` → `List reference`; call `get` on `List reference` with arguments (`int 0`) → `def`; implicit cast `def` to `int 1` → `int 1`; store `int 1` to `i`; (note internally `int 1` is unboxed from `Integer 1` when loaded from a `def` type value)

Uses of invalid boxing/unboxing resulting in errors.

Integer x = 1;                   // error 
Integer y = (Integer)1;          // error 
int a = Integer.valueOf(1);      // error 
int b = (int)Integer.valueOf(1); // error

	declare `Integer x`; error → cannot implicit box `int 1` to `Integer 1` during assignment
	declare `Integer y`; error → cannot explicit box `int 1` to `Integer 1` during assignment
	declare `int a`; call `valueOf` on `Integer` with arguments of (`int 1`) → `Integer 1`; error → cannot implicit unbox `Integer 1` to `int 1` during assignment
	declare `int a`; call `valueOf` on `Integer` with arguments of (`int 1`) → `Integer 1`; error → cannot explicit unbox `Integer 1` to `int 1` during assignment

Promotion

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Promotion is when a single value is implicitly cast to a certain type or multiple values are implicitly cast to the same type as required for evaluation by certain operations. Each operation that requires promotion has a promotion table that shows all required implicit casts based on the type(s) of value(s). A value promoted to a def type at compile-time is promoted again at run-time based on the type the def value represents.

Errors

If a specific operation cannot find an allowed promotion type for the type(s) of value(s) given.

Examples

Uses of promotion.

double d = 2 + 2.0; 
def x = 1;          
float f = x + 2.0F;

	declare `double d`; promote `int 2` and `double 2.0 @0`: result `double`; implicit cast `int 2` to `double 2.0 @1` → `double 2.0 @1`; add `double 2.0 @1` and `double 2.0 @0` → `double 4.0`; store `double 4.0` to `d`
	declare `def x`; implicit cast `int 1` to `def` → `def`; store `def` to `x`;
	declare `float f`; load from `x` → `def`; implicit cast `def` to `int 1` → `int 1`; promote `int 1` and `float 2.0`: result `float`; implicit cast `int 1` to `float 1.0` → float `1.0; add `float 1.0` and `float 2.0` → `float 3.0`; store `float 3.0` to `f`; (note this example illustrates promotion done at run-time as promotion done at compile-time would have resolved to a `def` type value)

Allowed Casts

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The following tables show all allowed casts. Read the tables row by row, where the original type is shown in the first column, and each subsequent column indicates whether a cast to the specified target type is implicit (I), explicit (E), boxed/unboxed for methods only (A), a reference type cast (@), or is not allowed (-). See reference type casting for allowed reference type casts.

Primitive/Reference Types

def

Object ( O )

Number ( N )

String ( T )

boolean ( b )

byte ( y )

short ( s )

char ( c )

int ( i )

long ( j )

float ( f )

double ( d )

Boolean ( B )

Byte ( Y )

Short ( S )

Character ( C )

Integer ( I )

Long ( J )

Float ( F )

Double ( D )

Reference ( R )

def Type

def as String

def as boolean/Boolean

def as byte/Byte

def as short/Short

def as char/Character

def as int/Integer

def as long/Long

def as float/Float

def as double/Double

def as Reference

« Types Operators »

	declare `char c`; explicit cast `String "C"` to `char C` → `char C`; store `char C` to `c`
	explicit cast `String 'c'` to `char c` → `char c`; store `char c` to `c`

	declare `String s`; store `String "s"` to `s`;
	declare `char c` load from `s` → `String "s"`; explicit cast `String "s"` to `char s` → `char s`; store `char s` to `c`

	declare `char c`; store `char 65` to `c`;
	declare `String s` load from `c` → `char A`; explicit cast `char A` to `String "A"` → `String "A"`; store `String "A"` to `s`