Skip to main content

Type Casting

Type casting is the process of converting a value from one data type into another. Since Java is statically typed, the compiler needs to know exactly what type of data it's working with at all times, so converting between types has to be done explicitly, or in some cases, automatically.

wizard-casting

Widening (Implicit) Casting​

Widening casting happens automatically when converting a smaller data type into a larger one, since no data is lost in the process.

byte -> short -> int -> long -> float -> double
int numMotors = 4;
double numMotorsAsDouble = numMotors; // Automatically widened to a double

Since a double can represent every value an int can (and more), Java doesn't require you to do anything special here.

Narrowing (Explicit) Casting​

Narrowing casting is required when converting a larger data type into a smaller one, since data could potentially be lost in the process. Because of this risk, Java requires you to explicitly tell it that you understand the potential consequences.

square-peg-round-hole

Casting Formula​

The general formula for explicitly casting a value is:

([data type]) [value]
double velocity = 5.75;
int velocityAsInt = (int) velocity; // Explicitly narrowed to an int, equal to 5

Notice that narrowing a double to an int doesn't round the value, it simply truncates (cuts off) the decimal portion.

Overflow​

Narrowing casting can also cause overflow if the value doesn't fit within the range of the smaller data type. For example:

int bigNumber = 130;
byte smallNumber = (byte) bigNumber; // Overflows, equal to -126

Since a byte can only hold values from -128 to 127, casting 130 into a byte causes it to wrap around past the maximum value and come out the other side.

Casting with char​

The char data type is stored internally as a number (its Unicode code point), so it can be cast to and from numeric types as well.

char letter = 'A';
int letterAsInt = letter; // Widened to an int, equal to 65

int number = 66;
char numberAsChar = (char) number; // Narrowed to a char, equal to 'B'

Casting Reference Types​

Casting isn't limited to primitive data types, reference types can be cast as well. This is covered in more detail in the Object-Oriented Programming section, but unlike primitive casting, casting a reference type incorrectly doesn't cause overflow, it throws a ClassCastException at runtime instead.