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Data Types in Java
Java’s Central Casting
CS 102
CS102
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.
CS102
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.
CS102
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
CS102
Inside a Primitive Type
• Actual values for integral types:
byte:
-128 to 127
short: -32768 to 32767
int: -2147483648 to 2147483647
long: -9223372036854775808 to
9223372036854775807
char: from '\u0000' to '\uffff’
(from 0 to 65535)
• Why use int instead of long?
CS102
Boolean Type
• boolean type represents a logical
quantity with two possible values,
indicated by the literals true and
false
CS102
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!”);
CS102
Building a Boolean from an Object
• Object reference obj can be converted
(any reference other than null is true):
obj! = null
CS102
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
CS102
A Graphical View
int counter
Airport midway
0010010
1110010
The data of the
midway object
CS102
Classes, Interfaces and Arrays, oh my!
• Classes we’ve already seen
• Interfaces are programming contracts
– An interface is a set of constants and
methods
– In Java, a class implements an interface
CS102
Implementing an Interface
• An example from the Comparison
applet:
public class Comparison extends Applet
implements ActionListener
• Comparison promises to do everything
an ActionListener does
CS102
Comparison Does What It Has To
• What does an ActionListener have to do?
actionPerformed
public abstract void
actionPerformed(ActionEvent e)
• Provides an implementation for the
interface:
public void actionPerformed(ActionEvent e) {
number1 = Integer.parseInt(input1.getText() );
number2 = Integer.parseInt(input2.getText() );
repaint();
}
CS102
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:
– Null reference
– Reference to any array of type "array of S"
such that type S is assignable to type T
CS102
Object is the Root of All Java
• Variable of type Object can hold:
– Null reference
– Reference to any object, whether class
instance or array.
CS102
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
CS102
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
CS102
Examples of casting
// Casting a float literal to a type int.
// Without the cast operator, this would be a
// compile-time error, because it’s a narrowing
// conversion:
int i = (int)12.5f;
// From class average applet (Figure 2.9)
if ( counter != 0 ) {
average = (double) total / counter;
System.out.println( "Class average is " +
average );
} else ...
CS102
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
CS102
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
CS102
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
CS102
Coming Attractions
• Arrays: Grouping related values
together
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