Download Kinds of Data Primitive Data Numeric Primitive Data

CS257 Computer Science II
Kevin Sahr, PhD
Lecture 3:
Kinds of Data
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Kinds of Data
✴ In Java there are two distinct kinds of data:
1. primitive data: any data value or variable that has as its data
type one of the predefined keyword Java data types
2. objects: data values or variables that are derived from an
object class
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Primitive Data
✴ There are eight primitive data types in Java
✴ Four of them represent integers:
•
byte, short, int, long
✴ Two of them represent floating point numbers:
•
float, double
✴ One of them represents characters:
•
char
✴ And one of them represents boolean values:
•
boolean
Numeric Primitive Data
✴ The difference between the various numeric primitive
types is their size, which determines the range of
values they can represent:
Type
Storage
Min Value
Max Value
byte
short
int
long
8 bits
16 bits
32 bits
64 bits
-128
-32,768
-2,147,483,648
< -9 x 1018
127
32,767
2,147,483,647
> 9 x 1018
float
double
32 bits
64 bits
+/- 3.4 x 1038 with 7 significant digits
+/- 1.7 x 10308 with 15 significant digits
Objects
✴ objects consist of behavior and attributes
✴ each object is an instance of a particular class
• the class is a template or blueprint for creating objects
• the class defines the behavior and attributes of instances of
that class
✦ attributes are defined by instance variables
✦ behavior is defined by instance methods
✴ primitive data variables are created by declaring
them, but objects require a two-step creation
process:
• declaration of the object variable
• instantiation of the object using the new operator
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Application Programming Interface
✴ the capabilities that a class provides are
documented in the class Application Programming
Interface (API)
✴ the API is external documentation for human use
✴ the API includes publicly visible:
•
•
•
•
class/static variables (usually constants only)
class/static methods
constructors
instance methods
✴ for each service method (method with public
visibility) in the class the API contains all the
information you need to invoke that method:
• the method header
• a brief description of what the method does
Special Classes
✴ Java provides some classes with special abilities
that are not available to the classes you create
✴ EX: Java provides the String class with these
abilities:
• abbreviated instantiation syntax (no new operator
required)
• concatenation using the + operator
Wrapper Classes
• Primitive data variables have limited functionality
compared to objects
§ in some OOP languages all variables are objects
• It would be useful if they had more object-like
functionality
• To solve this problem, Java provides wrapper classes
that allow primitive data values to be represented as
objects with additional functionality
Wrapper Classes
✴ The java.lang package contains wrapper
classes that correspond to each primitive type:
Primitive Type
byte
Wrapper Class
Byte
short
Short
int
Integer
long
Long
float
Float
double
Double
char
Character
boolean
Boolean
void
Void
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Wrapper Classes
✴ The API for each wrapper class lists the services
that that class provides
✴see the link to the Java Standard Class Library
class API’s on the website
✴ A wrapper class object can always be created
from a corresponding primitive data value (and
vice versa)
✴ EX: The following statement creates an Integer
object that represents the integer 40 as an object
Integer age = new Integer(40);
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Wrapper Classes
• An object of a wrapper class can be used in any
situation where a primitive value will not suffice
• For example, many Java data structures (EX:
ArrayList) can only store objects
§ Primitive data values can not be stored in such
data structures, but wrapper objects can be
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Wrapper Classes
✴ Wrapper classes contain static methods that help
manage the associated type
• EX: the Integer class contains a method to convert an integer
stored in a String to an int value:
int num = Integer.parseInt(str);
✴ Wrapper classes often contain useful constants as well
• EX: the Integer class contains Integer.MIN_VALUE and
Integer.MAX_VALUE which hold the smallest and largest int
values respectively
Autoboxing
✴ To make using wrapper classes more convenient, Java
provides special instantiation syntax for wrapper objects
• like the String class, no new operator is required
✴ Autoboxing is the automatic conversion of a primitive
value to a corresponding wrapper object, EX:
Integer age = 40;
• The assignment creates the appropriate Integer object
✴ The reverse conversion (called unboxing) also occurs
automatically as needed, EX:
int older = age + 5;
• age is converted to an int before the addition is
performed
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Command Line Programs
• Our first few programs in this class will be command
line programs
§ command line programs are those that use only text input
from the keyboard and produce only text output
• Later we will write programs that use a Graphical User
Interface (GUI)
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System I/O Objects
✴ The System class contains two public class/static
objects that can be used for command line I/O:
• The System.out object is of class PrintStream and
represents a command line destination to which we can send
output (the Monitor)
• The System.in object is of class InputStream and
represents a command line source we can use for text input
(the Keyboard)
✴ The System, PrintStream, and InputStream
classes are all in the java.lang package
✴ available to your programs without an import statement
Command Line Output
✴ The PrintStream instance method println can
be used to send text output to an output destination
✴ the println method automatically appends a new
line character (‘\n’) to the output
System.out.println ("Whatever you are, be a good one.");
PrintStream method
object
name
single String argument
The print Method
✴ The PrintStream instance method print can also
be used to send text output to an output destination
✴ The print method is similar to the println method,
except that it does not append a newline character to
the output
• does not advance to the next line
• anything printed after a print statement will appear on the
same line
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The Scanner Class
• The Scanner class provides convenient methods for
reading input values of various types from an
InputStream object
• The Scanner class is part of the java.util
package, and therefore must be imported
• The following line creates a Scanner object that reads
from the keyboard:
§
Scanner scan = new Scanner (System.in);
• Once created, the Scanner object can be used to
invoke various input methods, such as:
§
String answer = scan.nextLine();
Input Tokens
✴ Unless specified otherwise, white space is used to
separate the elements (called tokens) of the input
✴ White space includes space characters, tabs, new line
characters
✴ The next method of the Scanner class reads the
next input token and returns it as a String
✴ Methods such as nextInt and nextDouble read the
next token and convert it to a particular type
• if token is the entire line, the ‘\n’ character remains (see
ScanProblem.java and ScanProblem2.java)
• Invalid input will throw an exception
References
✴ Note that a primitive variable contains the value itself,
but an object variable contains the address of the
object
✴ An object reference can be thought of as a pointer to
the location of the object
✴ Rather than dealing with arbitrary addresses, we often
depict a reference graphically
num1
name1
38
"Steve Jobs"
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Assignment Revisited
✴ The act of assignment takes a copy of a value and
stores it in a variable
✴ For primitive types:
Before:
num1
38
num2
96
num2 = num1;
After:
num1
38
num2
38
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Reference Assignment
✴ For object references, assignment copies the address:
Before:
name1
"Steve Jobs"
name2
"Steve Wozniak"
name2 = name1;
After:
name1
"Steve Jobs"
name2
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Aliases
✴ Two or more references that refer to the same object
are called aliases of each other
✴ That creates an interesting situation: one object can
be accessed using multiple reference variables
✴ Aliases can be useful, but should be managed
carefully
✴ Changing an object through one reference changes it
for all of its aliases, because there is really only one
object
Garbage Collection
✴ When an object no longer has any valid references to
it, it can no longer be accessed by the program
✴ The object is useless, and therefore is called garbage
✴ Java performs automatic garbage collection
periodically, returning an object's memory to the
system for future use
✴ In some other languages, the programmer is
responsible for performing garbage collection
Lecture 3 Vocabulary
primitive data
Application Programming Interface (API)
service method
wrapper classes
autoboxing
unboxing
command line programs
references
aliases
garbage collection
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