Data Types in Java

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Data Types in Java
Javas Central Casting

• CS 102

2
Are You Javas 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
Whats 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.

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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

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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?

6
Boolean Type

• boolean type represents a logical quantity with
  two possible values, indicated by the literals
  true and false

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Building a Boolean from a Number

• Cant say (why not?)
• if (x) System.out.println(Congratulations, its
  a Boole!)
• Convert an integer x (following the C language
  convention that any nonzero value is true)
• if (x ! 0) System.out.println(Congratulations,
  its a Boole!)

8
Building a Boolean from an Object

• Object reference obj can be converted (any
  reference other than null is true) obj! null

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The Other Kind of Type

• Reference types
• Variables of reference types dont hold values,
  but references to values
• Classes, interfaces and arrays are all reference
  types

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A Graphical View
0010010
int counter
1110010
Airport midway
The data of the midway object
11
Classes, Interfaces and Arrays, oh my!

• Classes weve already seen
• Interfaces are programming contracts
• An interface is a set of constants and methods
• In Java, a class implements an interface

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Implementing an Interface

• An example from the Comparison applet
• public class Comparison extends Applet implements
  ActionListener
• Comparison promises to do everything an
  ActionListener does

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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()

14
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

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Object is the Root of All Java

• Variable of type Object can hold
• Null reference
• Reference to any object, whether class instance
  or array.

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Class ? Type

• Variables have types
• Objects (and arrays) dont have a type, but
  belong to a class
• Usually well consider them the same

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Casting Against Type

• A value could be two different types
• Is 12 an int or a float?
• Compiler isnt smart, so its 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

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Examples of casting

• // Casting a float literal to a type int.
• // Without the cast operator, this would be a
• // compile-time error, because its 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 ...

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Cant Always Cast

• Cant do this
• if ((boolean) x) System.out.println(Congratulatio
  ns, its a Boole!)
• Sometimes casts are automatic, and are called
  conversions

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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

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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

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Coming Attractions

• Arrays Grouping related values together