Java - Type Conversion and Casting

Type Conversion

Assigning variables of same data type is normal.

Assigning lower to higher can be known as widening conversion. It is correct. Here implicit casting(Automatic type conversion) will be made by java run time automatically.

Assigning higher to lower can be known as narrowing conversion. The error will be raised for these statements. Because lower type can’t have enough memory to accommodate the value from higher type. Here explicit casting must be needed.

Automatic type conversion

int c=100;

float a=c;//Automatic type conversion

     Here RHS data type range is lesser than LHS data type range. Now automatically conversion will be done by JRE.

Casting

float a=10.5f;

int c=a; // error

int c=(int)a; // explicit casting

How casting can be made

The following example illustrates one surprise fact.

int x=900;

byte y=(byte)x;

     This is legal. The compile time error will not be raised. But how 900 can be stored in y. The answer is simple.

     Here java first calculates 900%256. The remainder is 132. 132 can’t be stored in y. Because byte range is -128 to 127. So JRE stores -124 in y. How? The answer is following program.

     Run following prg, and analyze carefully.

class A

{

   public static void main(String args[])

   {

         for (int x=256;x<=1000;x++)

              {

                   byte y=(byte)x;

                   System.out.println("x="+x+"y="+y);

              }

   }

}

Automatic type promotion

In mixed expressions, lower data type will be converted to higher data type automatically if LHS is higher data type. If LHS is lower than RHS, explicit cast is needed.

Consider following example

byte b=b*2; // error because 2 will be treated as integer.

int x=b*50; // correct. Here b will be promoted to int.

long y=x*20; // correct

int z=y*2; // error because RHS is larger.

double p=y*z // correct

Type promotion Rules

·      All byte and short values are promoted to int.

·      If one expression is long, the whole expression is promoted to long.

·      If one expression is float, the whole expression is promoted to float.

·      If one expression is double, the whole expression is promoted to double.