Type conversion in Java

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

NOTEWhen casting floating point primitives (float, double) to whole number primitives, the number is rounded down.

Basic Numeric Promotion 

static void testNumericPromotion() {

char char1 = 1, char2 = 2;
short short1 = 1, short2 = 2;
int int1 = 1, int2 = 2;
float float1 = 1.0f, float2 = 2.0f;

// char1 = char1 + char2;      // Error: Cannot convert from int to char; 
// short1 = short1 + short2;   
// Error: Cannot convert from int to short; 
int1 = char1 + char2;             // char is promoted to int.
int1 = short1 + short2;           // short is promoted to int.
int1 = char1 + short2;            // both char and short promoted to int.
float1 = short1 + float2;        // short is promoted to float.
int1 = int1 + int2;                 // int is unchanged.


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 = (inttrue;        // Compiler error: incompatible types
char1 = (char65// A
byte byte1 = (byte'A'// 65
short short1 = (short'A'// 65
int int1 = (int'A'// 65
char char2 = (char8253// 
byte byte2 = (byte''// 61 (truncated code-point into the ASCII range)
short short2 = (short''// 8253
int int2 = (int''// 8253 

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)

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.