1. Java’s Central Casting
Data Types in Java
Java’s Central Casting CS
Lecture 1-2 1

2. Are You Java’s Type?
Type: a set of values that are semantically similar
Java is a strongly typed language
Every variable and every expression has a type that is known at compile time.
Strong typing helps detect errors at compile time.

3. What’s the Role of Types?
Types limit the:
Values that a variable can hold or that an expression can produce
Limit the operations supported on those values
Determine the meaning of the operations.

4. Java Types Primitive types
boolean
numeric
Integral: byte, short, int, long, and char
Floating-point: float and double
Variables of primitive types hold the actual value

5. Inside a Primitive Type
Actual values for integral types:
byte: to 127
short: to 32767
int: to
long: to
char: from '\u0000' to '\uffff’ (from 0 to 65535)
Why use int instead of long?

6. Boolean Type
boolean type represents a logical quantity with two possible values, indicated by the literals true and false

7. Building a Boolean from a Number
Can’t say (why not?):
if (x) System.out.println(“Congratulations, it’s a Boole!”);
Convert an integer x (following the C language convention that any nonzero value is true):
if (x != 0) System.out.println(“Congratulations, it’s a Boole!”);

8. Building a Boolean from an Object
Object reference obj can be converted (any reference other than null is true): obj! = null

9. The Other Kind of Type Reference types
Variables of reference types don’t hold values, but references to values
Classes, interfaces and arrays are all reference types

10. A Graphical View 0010010 1110010 The data of the midway object
int counter
Airport midway
The data of the midway object

11. Array Types
If T is a primitive type, then a variable of type "array of T"can hold:
Null reference
Reference to any array of type "array of T"
If T is a reference type, then a variable of type "array of T" can hold:
Reference to any array of type "array of S" such that type S is assignable to type T

12. Object is the Root of All Java
Variable of type Object can hold:
Null reference
Reference to any object, whether class instance or array.

13. Class  Type Variables have types
Objects (and arrays) don’t have a type, but belong to a class
Usually we’ll consider them the same

14. Casting Against Type A value could be two different types
Is 12 an int or a float?
Compiler isn’t smart, so it’s conservative (signals an error)
Override the compiler with a cast
Cast says: Treat this variable as the type I say
To cast in Java, write:
(newType) variable

15. Examples of casting
average = (double) total / counter;

16. Can’t Always Cast Can’t do this:
if ((boolean) x) System.out.println(“Congratulations, it’s a Boole!”);
Sometimes casts are automatic, and are called conversions

17. One of Two Ways
Create an expression in a context where the type of the expression is not appropriate and either:
Error at compile time (if statement has any type other than boolean)
May be able to accept a type that is related to the type of the expression

18. Automatic Conversion
For convenience, Java performs an implicit conversion
From the type of the expression to a type acceptable for its surrounding context
Kinds of conversions:
Identity, Widening primitive, Narrowing primitive, Widening reference, Narrowing reference, String

19. Funky Conversions What does this print? class Test {
public static void main(String[] args) {
int big = ;
float approx = big;
System.out.println(big -(int)approx); }

20. Coming Attractions
Arrays: Grouping related values together