Type Conversion
Syntax#
- TargetType target = (SourceType) source;
Non-numeric primitive casting
The boolean
type cannot be cast to/from any other primitive type.
A char
can be cast to/from any numeric type by using the code-point mappings specified by Unicode. A char
is represented in memory as an unsigned 16-bit integer value (2 bytes), so casting to byte
(1 byte) will drop 8 of those bits (this is safe for ASCII characters). The utility methods of the Character
class use int
(4 bytes) to transfer to/from code-point values, but a short
(2 bytes) would also suffice for storing a Unicode code-point.
int badInt = (int) true; // Compiler error: incompatible types
char char1 = (char) 65; // A
byte byte1 = (byte) 'A'; // 65
short short1 = (short) 'A'; // 65
int int1 = (int) 'A'; // 65
char char2 = (char) 8253; // ‽
byte byte2 = (byte) '‽'; // 61 (truncated code-point into the ASCII range)
short short2 = (short) '‽'; // 8253
int int2 = (int) '‽'; // 8253
Numeric primitive casting
Numeric primitives can be cast in two ways. Implicit casting happens when the source type has smaller range than the target type.
//Implicit casting
byte byteVar = 42;
short shortVar = byteVar;
int intVar = shortVar;
long longVar = intvar;
float floatVar = longVar;
double doubleVar = floatVar;
Explicit casting has to be done when the source type has larger range than the target type.
//Explicit casting
double doubleVar = 42.0d;
float floatVar = (float) doubleVar;
long longVar = (long) floatVar;
int intVar = (int) longVar;
short shortVar = (short) intVar;
byte byteVar = (byte) shortVar;
When casting floating point primitives (float
, double
) to whole number primitives, the number is rounded down.
Object casting
As with primitives, objects can be cast both explicitly and implicitly.
Implicit casting happens when the source type extends or implements the target type (casting to a superclass or interface).
Explicit casting has to be done when the source type is extended or implemented by the target type (casting to a subtype). This can produce a runtime exception (ClassCastException
) when the object being cast is not of the target type (or the target’s subtype).
Float floatVar = new Float(42.0f);
Number n = floatVar; //Implicit (Float implements Number)
Float floatVar2 = (Float) n; //Explicit
Double doubleVar = (Double) n; //Throws exception (the object is not Double)
Basic Numeric Promotion
Testing if an object can be cast using instanceof
Java provides the instanceof
operator to test if an object is of a certain type, or a subclass of that type. The program can then choose to cast or not cast that object accordingly.
Object obj = Calendar.getInstance();
long time = 0;
if(obj instanceof Calendar)
{
time = ((Calendar)obj).getTime();
}
if(obj instanceof Date)
{
time = ((Date)obj).getTime(); // This line will never be reached, obj is not a Date type.
}