Understanding Java

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Understanding Java

• Discussion Section 08/28/2008
• 22C21 Computer Science II Data Structures

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Sample Programs

• In this class we will be discussing programs
• ReadNumbers.java. This shows a simple example of
  Java input statements.
• TwoDPoint.java. This shows how to define a class
  (called TwoDPoint) and TwoDPointTest.java shows
  how to define and use TwoDPoint objects.
• (Distribute printed copies of the programs to the
  students)

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ReadNumbers.java illustrates

• (a) the overall structure of a java program
• (b) how to import packages/classes
• (c) while-statements
• (d) the try-catch stuff
• (e) Defining a BufferedReader
• (f) the readLine method
• (g) the Integer class and the parseInt method

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Structure
Class
Import packages
Variable declaration
Main method declaration
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import

• only tells the compiler where to look for other
  classes.
• has purpose to allow a class to use the
  facilities of another class

import java.util. import java.io.
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Java.util.

• The java.util package defines a number of useful
  classes, primarily collections classes that are
  useful for working with groups of objects.
• (Java in a Nutshell, Orielly)

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Java.io.

• Provides for system input and output through data
  streams, serialization and the file system.

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Class
In the real world, you'll often find many
individual objects all of the same kind. There
may be thousands of other bicycles in existence,
all of the same make and model. bicycle was
built from the same set of blueprints and
therefore contains the same components. In
object-oriented terms, we say that your bicycle
is an instance of the class of objects known as
bicycles. A class is the blueprint from which
individual objects are created. (www.java.sun.com
)
class ReadNumbers
Name of the class
Keyword class
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Declaration
the field is accessible only within its own
class.
keyword used to define a variable as a class
variable
we read the user's input
private static BufferedReader stdin new
BufferedReader( new InputStreamReader( System.in
) )
java.io.BufferedReader is used to read
character Strings
convert byte streams to character streams
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The object created
stdin
Class BufferedReader http//java.sun.com/j2se/1.5
.0/docs/api/java/io/BufferedReader.html
Class InputStreamReader http//java.sun.com/j2se
/1.4.2/docs/api/java/io/InputStreamReader.html
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Main method
public static void main(String args)
the method or variable can be accessed by
elements residing in other classes
command-line argument
The main method is similar to the main function
in C and C it's the entry point for your
application and will subsequently invoke all the
other methods required by your program.
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While Loop
while(true) // statements // end of
while-true
Statements inside a while (indicated by ) are
executed repeatedly, till the condition of the
while (the parameter In brackets ()) is false. In
this case, the loop will be executed forever
till something happens inside the loop to
stop the execution.
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Getting out of the Loop
if((number lt 0) (number gt 1000)) break
If the given condition is not met, the break
statement is called. This break will take us
out of the while loop!
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Try and Catch
try //statements catch(the exception to
catch)) // statements
In the try block, you enter the code that might
cause an exception. At the end of this portion
of code, you close it with a catch block. The
catch block will tell your program just what to
do in case the exception actually does occur. If
no exception occurs, then the catch block is
ignored, and your program moves along smoothly.
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Try and Catch

• In this program we are executing in the try
  block, the statement
• which may throw an exception
• which will take us to the catch block.

stdin.readLine()
java.io.IOException
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readLine() method
// Read a line of text from the user. String
input stdin.readLine()
The BufferedReader class gives us the readLine()
method, and applies buffering to the input
character input stream. Here, the String input
will hold the user input.
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Integer Class
// converts a String into an int value int number
Integer.parseInt( input )
The Integer class wraps a value of the primitive
type int in an object. In addition, this class
provides several methods
Parses the string argument as a signed decimal
integer.
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Questions?
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TwoDPoint.java, TwoDPointTest.java

• how to define a class,
• the notion of a default constructor
• class methods
• the notion of this, etc.

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TwoDPoint.java
TwoDPointTest.java
Creating TwoDPoint Objects here
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Defining Class

• class TwoDPoint
• /// variables, methods

The name of the Java file is the same as that of
the name of the class
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Constructors

• Constructors have one purpose in life to create
  an instance of a class. This can also be called
  creating an object, as in

TwoDPoint p p new TwoDPoint()
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Constructors

• A java constructor has the same name as the name
  of the class to which it belongs. Constructor's
  syntax does not include a return type, since
  constructors never return a value.
• Java provides a default constructor which takes
  no arguments and performs no special actions or
  initializations, when no explicit constructors
  are provided.

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Class Method

• A java method is a series of statements that
  perform some repeated task.
• It is like a C/C function
• The methods in TwoDPoint.java are
• String getAsString()
• void setX(double value)
• void setY(double value)
• double getX()
• double getY()

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this

• A method uses this to refer to the instance of
  the class that is executing the method
• Instance of the class the object of the class
  in question
• Instance variable the variables declared in
  the class outside the methods

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Why do we need this?

• The keyword helps us to avoid name conflicts.
• In some case we may declare the name of instance
  variable and local variables same.
• Now to avoid the confliction between them we use
  this keyword.

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Object
// Declare two TwoDPoint objects, p and
q TwoDPoint p TwoDPoint q
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Method Call
// Call the set methods to assign the two //
coordinates of p p.setX(1.5) p.setY(2.5)
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Questions?