Transcript Using Pre-Defined classes - Illinois Institute of Technology

CS 115
OBJECT ORIENTED PROGRAMMING I
LECTURE 6
GEORGE KOUTSOGIANNAKIS
Copyright: FALL 2015 Illinois Institute of TechnologyGeorge Koutsogiannakis
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NEW TOPICS
• Null Objects.
• Using Java Pre Defined Classes (Library
Classes).
• String Class.
• Formatting Numbers.
• Math class.
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NEW TOPICS
• Using the Scanner class to read user input
from the keyboard.
• Wrapper classes.
• Autoboxing / Unboxing.
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Java Predefined Classes (Library
Classes)
• The Java SDK provides more than 2,000 classes that can be used to add
functionality to our programs.
• The Java Documentation provides specific information on each of these
library classes.
– For each class it lists the:
• Fields of the class
• Constructors of the class
• Methods of the class.
• The documentation is referred to as the “Application
Programming Interface” abbreviated as API.
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Using Library Classes
• These Library classes are already coded, ready
to be used by a programmer.
• They offer certain functionality which you
don’t have to recreate in your program.
• When you want to use a library class use the
API to find out what methods are available in
that class by going online:
http://java.sun.com/javase/6/docs/api/
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Why Use Library Classes
• No reason to reinvent the wheel!!
• They provide certain functionality thus freeing
the program from duplicating the code to
create himself/herself the functionality.
• The java runtime system can use the
functionality provided by the library class you
are using.
• You need, however, to notify the runtime
system (JVM) with an import statement.
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Using Library Classes
• Library classes belong in packages.
– A package can have many library classes.
• Some library classes can be used with any of
your programs without any special effort i.e.
– The library classes belonging to the package
java.lang are an example.
• Math class is an example of a class that belongs to the
package java.lang.
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Using Library Classes
• Other library classes are not automatically
used. They need to be imported.
– Imported means that the compiler needs to be
notified so that it can load these library classes
that we want to use.
– An example of such a class is the classes in
package:
java.util
The Scanner class is a class in that package.
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Examples
public class MyCalculator
{
public static void main(String[] args)
{
double a=3.5;
double b=6.5;
double c=5.2;
double z=a*Math.pow(b,c);
System.out.println(“The result is:”+z);
}
}
The Math library classes calls its method pow that calculates the value of b raised to
the c power.
No import statement is used because Math class is one of those classes that are
imported automatically with every program (also the Math class is like that).
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Examples
import java.util.Scanner;
public class MyCalculator
{
public static void main(String[] args)
{
//code here
Scanner scan=new Scanner();
// other code here
}
}
Since we want to use the Scanner library class an import statement is needed.
The scanner class will allow us to receive input from the user of the program.
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Some Java Library Packages (or Pre
Defined Classes as sometimes are called)
• Classes are grouped in packages according to functionality
Package
Categories of Classes
java.lang
Basic functionality common to many programs, such as
the String class and Math class
java.awt
Graphics classes for drawing and using colors
javax.swing User-interface components
java.text
Classes for formatting numeric output
java.util
The Scanner class, the Random class and other
miscellaneous classes
java.io
Classes for reading from and writing to files
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Using a Class From a Package
• Classes in java.lang are automatically available to use.
• Classes in other packages need to be "imported" using this
syntax:
import package.ClassName;
or
import package.*;
(imports all required classed from package)
• Example
import java.text.DecimalFormat;
or
import java.text.*;
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Import Statements
•
•
An import statement has to be written above the line that names your class.
For example:
import java.util.*;
public class MyClass
{
}
The package name is java.util
The .* means that we want to use all the classes
in that package.
import is a keyword.
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The String Class
• The String class allows creation and
manipulation of Strings data types.
• String is a library class in java.lang package,
therefore every java program gets access to
this class without the need for an import
statement.
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The String Class
Represents a sequence of characters
String Constructors
String( String str )
allocates a String object with the value of
str, which is a String object or a String literal
String( )
allocates an empty String object
Examples:
String greeting = new String( "Hello" );
String empty = new String( );
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The String Class
• Remember that you can also create a String via a
declaration and assignment of a value i.e.
String lastName=“Jones”
• + appends a String to another String. At
least one operand must be a String.
+ in this case is called the concatenation operator
Example:
String s1 = new String( "Hello " );
String s2 = "there. ";
String s3 = s1 + s2;
// s3 is: Hello there.
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The String Class
• For instance in the statement:
int x;
System.out.println(“The value of the number is:”+x);
The concatenation operator in front of the int data type x converts x from
an int type to a String type.
• The String Library class provides many different methods that can help us
for instance:
–
–
–
–
Identify the number of characters in a String (the length of the String.)
Isolate a substring from within the String.
Identify the position of a specific character within a String
Others.
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Sample Methods from String Class
(from Java API-Documentation)
•
•
•
•
•
•
char charAt(int index)
Returns the char value at the specified index. Int
codePointAt(int index)
Returns the character (Unicode code point) at the specified index.
int codePointBefore(int index)
Returns the character (Unicode code point) before the specified
index..
int compareTo(String anotherString)
Compares two strings lexicographically. The comparison is based on
the Unicode value of each character in the strings. The returned integer is
zero if the two Strings are equal or a positive number otherwise
int compareToIgnoreCase(String str)
Compares two strings lexicographically, ignoring case differences.
String concat(String str)
Concatenates the specified string to the end of this string.
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How to use the methods
• You need the string object first. i.e.
String str=new String(“My name”);
(keep in mind that this is the same as String str=“My Name”;)
int x=str.length();
x now indicates the number of characters in the String.
• Another example:
String str1=“My Name is John”;
String str2=“Your name is Helen”;
str1.concat(str2);
str1 is now “My Name is John Your name is Helen”
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The indexOf Methods
Return
type
int
int
Method name and argument list
indexOf( String searchString )
returns the index of the first occurrence of searchString, or 1 if not found
indexOf( char searchChar )
returns the index of the first occurrence of searchChar, or 1 if not found
The index of the first character of a String is 0.
Example:
String hello = "Hello";
int index = hello.indexOf( 'e' );
The value of index is 1.
NOTICE THAT: The first position (first character) has index 0.
The substring Method
Return type
Method name and argument list
String
substring( int startIndex, int endIndex )
returns a substring of the String object beginning at the
character at index startIndex and ending at the character at index
end Index – 1
Example:
String hello = "Hello";
String endOfHello = hello.substring( 3,
hello.length( ) );
The value of endOfHello is “lo”
because we asking for the part of the String that starts with character at index zero and ends with the last
character in the String.
H has index 0
e has index 1
l has index 2
l has index 3
0 has index 4 (which is the same as hello.length() index minus 1)
See Example 3.6 StringDemo.java
FORMATTING NUMBERS
The DecimalFormat LIBRARY class and the
NumberFormat LIBRARY class allow you to
specify the number of digits for printing and
add dollar signs and percent signs to your
output
• Both classes are in the java.text package
• We will cover the NumberFormat class later.
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FORMATTING NUMBERS
• The DecimalFormat Class :
DecimalFormat Constructor:
DecimalFormat( String pattern )
instantiates a DecimalFormat object with the format specified
by pattern IN THE ARGUMENT LIST OF THE CONSTRUCTOR.
Pattern characters:
0
required digit
#
optional digit, suppress if 0
.
decimal point
,
comma separator
%
multiply by 100 and display a percent sign
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FORMATTING NUMBERS
• The DecimalFormat format method
String format( double number )
• returns a formatted String representation of
number passed as argument.
• See example on next slide:
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FORMATTING NUMBERS
import java.text.NumberFormat;
Import java.text.DecimalFormat;
public class Formatting
{
public static void main(String[] args)
{
double d=23.456289;
DecimalFormat twodigits=new DecimalFormat("0.00");
NumberFormat moneyFormat=NumberFormat.getCurrencyInstance();
String b=twodigits.format(d);
System.out.println(b);
String money=moneyFormat.format(d);
System.out.println(money);
}
}
//The first output is 23.46
//Notice the rounding of the decimal points
//second output is $23.46
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null Objects
• An object reference is term used to denote the nam ewe give to an object.
• We can declare an object without using the new operator to actually give
initial values to the instance variables of the class that represents the
object.
• An object reference can point to no object. In that case, the object
reference has the value null.
• Object references have the value null when they have been declared, but
have not been used to instantiate an object.
i.e Student myself;
myself is an object reference of class Student in this case.
• Attempting to use a null object reference causes a NullPointerException at
run time.
– This kind of a message from the compiler could mean that you forgot to
instantiate (create using new operator) the object.
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null Objects
• Thus, the following declaration causes a null
reference:
Student stud1;
stud1 points to nothing until it is instantiated
using the new operator.
stud1=new Student();
now std1 points to an area in memory where
the initialized values of the instance variables
of the class Student are stored..
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Input Using the Scanner Class
• We need to be able to capture input from the
user.
• One way of providing input to the program is via
the keyboard
• The Scanner library class provides the
functionality to allow us to capture keyboard
input (and also read input from a file—we will
cover the file reading later)
• Scanner class is part of the Library package
java.util
– Therefore we have to import the Scanner class
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Receiving input from the user.
• Remember that we have used the argument
list of the main method to receive input from
the user when the user types the commend to
interpret on a DOS pane:
i.e. >java Myprogram George 25
(where George and 25 are two inputs provided
by the user of the program).
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Receiving input from the user.
The class MyProgram has a main method. The identifier
args in the list of arguments of the main method
captures the inputs typed by the user.
public static void main(String[] args)
{
String name=args[0]; //captures the input: George
String mynumber=args[1]; //captures the next input : 25
//Notice that even though the number 25 was typed, it has been
captured as a String data type. If we need the number in an arithmetic
expression we will have to convert it from a String to an int data type. We
will learn how to do that later.
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Receiving Input from the User
Using the Scanner library class.
• Sometimes we need to prompt the user during the execution
of the program so that input is provided after a specific set of
instructions has been executed.
• The previous technique of making the input part of the
command to start the execution WILL NOT work then.
• We can provide this functionality by using the Scanner library
class. The user can be prompted to enter data, after the
program starts execution, via the keyboard. The Scanner
object will read the characters typed on the keyboard.
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Input Using the Scanner Class
• Provides methods for reading byte, short, int,
long, float, double, and String data types from the
Java console (keyboard) and other sources (like a
file).
• Scanner parses (separates) input into sequences
of characters called tokens.
• By default, tokens are separated by standard
white space characters (tab, space, newline, etc.)
• i.e in the String “My name is Joe” there are 4
tokens. The first token is the substring “My” and
so on.
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A Scanner Constructor
Scanner( InputStream source )
creates a Scanner object for reading from source. If source is System.in,
this instantiates a Scanner object for reading from the Java console (keyboard).
(System.in is the keyboard)
Example:
Scanner scan = new Scanner( System.in );
scan then is the object that we are going to use to
read characters from the keyboard if System.in has
been passed to the constructor of the Scanner class.
The Scanner class has methods that help us capture the
user ‘s input (as an example from the keyboard or a
file).
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Scanner class methods
• Some of the methods are listed
in the next slide.
• Connect to the link for the java API
http://java.sun.com/javase/6/docs/api/
and study the methods of Scanner class.
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Scanner next… Methods
Return type
Method name and argument list
dataType
nextDataType( )
returns the next token in the input stream as a
dataType. dataType can be byte, int, short, long,
float, double, or boolean i.e nextDouble() or
nextInt()
next( )
returns the next token in the input stream as a
String
nextLine( )
returns the remainder of the line as a String
String
String
Prompting the User
• The next… methods do not prompt the user for an input value
• Use System.out.print to print the prompt, then call the next…
method
Example:
1.
2.
3.
Scanner scan = new Scanner( System.in );
System.out.print( "Enter your age > " );
int age = scan.nextInt( );
In this example the user program stops execution after
statement 2 is executed. The program waits for the user to
type something (a number in this case)
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Prompting the User
On the DOS window we will see:
C:\Users\George>java MyProgram
Please enter your age > _
After the user enters a number (let us say that the user typed the number 19)
and presses the Enter key, the program resumes execution and line 3 is
executed.
Line 3 reads what the user typed (19) as a String and at the same time
converts it into an int data type. It assigns that data type (stores in
memory) to the identifier age in this example.
From there on the program continues with the execution of other
instructions. It is possible that we may want to stop the program further
down again and ask the user to enter another input.
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SOFTWARE ENGINEERING TIP
Provide the user with clear prompts for input.
Prompts should use words the user understands
and should describe the data requested and any
restrictions on valid input values.
Example:
Enter your first and last name
or
Enter an integer between 0 and 10
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String Input
• Use the next method to read a String
– scan.next(); captures a String typed on the keyboard.
(Remember that anything you type on the keyboard is
captured as a String first by the Scanner. It can be
converted after that).
• The next method will read only one word of String
input because the space is a whitespace character.
• To read a whole line, including spaces, use the nextLine
method.
Example:
System.out.print( “Enter a sentence > “ );
String sentence = scan.nextLine( );
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Capture an entire line of Input as a
one String
• i.e.
>java Myprogram
Please enter a short story:_
• The user could enter (type via the keyboard)
for example
My name is Peter and I am an Electrical Engineering student
• What will be stored in the memory location labeled sentence
(see previous example code) is the entire phrase as a String
data type:
“My name is Peter and I am an Electrical Engineering student”
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Scanner Example
• Suppose that we want to read a String that a user of our program enters
on the keyboard:
import java.util.Scanner;
class UsingScanner
{
public static void main(String[] args)
{
Scanner scan =new Scanner(System.in);
System.out.println("Enter a decimal number");
String a=scan.next();
double d=Double.parseDouble(a);
System.out.println(d);
}
}
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Scanner Example
• Interpreting this program has the following
outcome:
C:\CS115\Myexamples\ScannerExamples>java UsingScanner
Enter a decimal number
12.34
The number you entered is:12.34
•
•
•
Notice that after the statement “Enter a decimal number” is displayed, the
program waits for the user to enter something on the keyboard.
Notice that the next action is for the user to type 12.34 on the keyboard.
The program then prints the last statement “The number you entered
is:12.34”
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Scanner Example
• In the program the line:
String a=scan.next();
captures whatever the user typed as a String
data type (even if the user typed a number!)
• Our program converts the data type String
represented by the identifier a to a double
data type via the line:
double d=Double.parseDouble(a);
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Wrapper Classes
• We can always convert a String to a primitive data
type using classes called “Wrapper classes”.
In this example the class Double is the Wrapper
class (notice that it is written with a capital D to
differentiate it from the primitive data type
double written with a lower case d).
• Its job is to convert a String to a double data type
by invoking the method parseDouble that takes a
primitive data type double as argument.
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The Wrapper Classes
• “Wrap” the value of a primitive data type into
an object
• Useful when methods require an object
argument
• Also useful for converting Strings to an int or
double
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Wrapper Classes
Primitive Data Type
double
float
long
int
short
byte
char
boolean
Wrapper Class
Double
Float
Long
Integer
Short
Byte
Character
Boolean
Integer and Double Methods
static Integer Methods
Return value
Method Name and argument list
int
parseInt( String s )
returns the String s as an int
Integer
valueOf( String s )
returns the String s as an Integer object
static Double Methods
Return value
Method Name and argument list
double
parseDouble( String s )
Double
returns the String s as a double
valueOf( String s )
returns the String s as a Double object
Back to Scanner
• We can capture the input from the keyboard
directly as a numeric primitive data type
instead of capturing it as a String first and
then converting it (parsing it) to the particular
numeric data type:
i.e. Scanner scan =new Scanner(System.in);
System.out.println("Enter a decimal number");
double a=scan.nextDouble();
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Using the nextDouble method with
Scanner object
import java.util.Scanner;
class UsingScannerDouble
{
public static void main(String[] args)
{
Scanner scan =new Scanner(System.in);
System.out.println("Enter a decimal number");
double d=scan.nextDouble();
System.out.println("The number you entered is:"+d);
}
}
Output: Enter a decimal numbner
13.456
The number you entered is:13.456
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Autoboxing and Unboxing
• Autoboxing (Converting a primitive data type to an
equivalent object):
Automatic conversion between a primitive type and a wrapper
object when a primitive type is used where an object is
expected
Integer intObject = 42;
• Unboxing (Converting an Object to a primitive data
type)
Automatic conversion between a wrapper object and a primitive
data type when a wrapper object is used where a primitive data
type is expected
int fortyTwo = intObject;
To String() method
• We can add a new method in a template class
that will allow us to see the values of the
attributes of the class all at once without
asking the accessor method for each one to
give us its value and then displaying that
value.
• We will call this method toString method
public String toString() { code goes here }
Notice that it returns a String.
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Example of toString in the template
class
• Suppose we have the class Person with attributes :
String firstName, String lastName, int age.
After all the other methods (i.e. constructors , accessor, mutator methods)
we will add the following method at the bottom of the Person template
class:
public String toString()
{
String output=“The First Name is:”+ “ “+firstName+” ‘’+”The Last Name
is:”+” “+” “+lastName+” “+ “The Age is :”+” “+age ;
return output;
}
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Using the toString in the client class
• Now assume that we have a class PersonClient
that uses the Person class.
• In the client class’ main method we instantiate an
object of Person and use it to set the attributes .
i.e. Person p=new Person();
p.setFirstName(“George”); etc.
• We can now get the attributes’ values and
display them by including the following line of
code:
System.out.println(p.toString());
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Study Guide
• Study Chapter 3
• Read chapter 3
– Sections 3.6, 3.7, 3.8, 3.9
– Sections: 3.10, 3.11, 3.12, 3.15
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