Workshop on Java Programming

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Workshop on Java Programming

• September 29, October 13, October 20
• YANPING CHEN
• ychen_at_site.uottawa.ca

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Introduction to Object-Oriented Programming
Concepts
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Software Life Cycle

• Steps in software development
• Problem specification
• Algorithm development
• Coding
• Testing
• Debugging
• Maintenance
• Documenting is an activity that occurs throughout
  the cycle

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Problem Specification

• The goal use computer to solve problems
• Problems are usually stated in natural language
• Software designer extracts the exact problem
  specification from the informal problem --
  requirements analysis

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Understand the Problem

• Clarify
• What data is given (available for use when the
  problem-solving takes place)
• What results are required
• What assumptions are safe to make
• What constraints must be obeyed

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Example 1

• Informal statement
• John wants to know the total and average cost of
  the three books he just bought.
• Givens descriptions and names of the values that
  are know
• A, B, C numbers representing the cost of each of
  Johns books
• Results descriptions and names of the values to
  be computed from the givens
• SUM, the sum of A, B and C
• AVG, the average of A, B, C

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Example 2

• Informal statement
• Write an algorithm that takes four scores (each
  out of 25) and computes their average (out of
  100)
• Givens descriptions and names of the values that
  are know
• A, B, C, D numbers representing the four scores
• Results descriptions and names of the values to
  be computed from the givens
• AVG, the average of A, B, C, D out of 100

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Algorithm

• An algorithm is a solution for a problem
• An algorithms HEADER specifies the name of the
  algorithm, an order for the givens and an order
  for the results
• An algorithms BODY is a sequence of instructions
  which, when executed, computes the desired
  results from the givens.
• Algorithms are written in pseudocode. A precise
  notation convenient for people but not executable
  by computers.

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Intermediate Variables

• Often it is useful within an algorithm to use a
  variable that is neither a given nor a result to
  temporarily hold a value.
• Intermediate variables values are not returned
  to the calling statement, nor are they remembered
  from one call to the next.

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Algorithm for Example1

• GIVENS numbers A, B, C
• RESULTS
• SUM, the sum of A, B and C
• AVG, the average of A, B and C
• HEADER
• Alg1(A, B, C)
• Return (Sum, Avg)
• BODY
• Sum ABC
• Avg Sum /3

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Algorithm for Example2

• GIVENS numbers A, B, C, D
• RESULTS
• AVG, the average of A, B, C and D out of 100
• HEADER
• Alg2(A, B, C, D)
• Return (Avg)
• BODY
• Sum A B C D //an Intermediate Variable
• Avg Sum /4/25100

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Tracing an Algorithm

• To TRACE an algorithm is to execute it by hand,
  one statement at a time, keeping track of the
  value of each variable.
• The aim is either to see what results the
  algorithm produces or to locate bugs in the
  algorithm.

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Tracing Example

• Example 1
• Avg Alg1 (18, 25, 20)
• Example 2
• Avg Alg2 (18, 25, 20, 19)

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Invoking an Algorithm

• To invoke an algorithm you use a call statement
  which is identical to the header except for the
  names of the givens and results.
• (X,Y) Alg1(10, 7, -2)
• invokes algorithm Alg1 with givens A10, B7,
  C-2 and returns the results in X(Sum) and Y(Avg)
• Information is passed between the call statement
  and the algorithm based on the ORDER of the
  givens and results, not their names.

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Coding

• Coding translating an algorithm into a
  particular programming language so it can be
  executed on a computer
• Be sure your algorithm is correct before coding
  it by tracing it on test data.
• Coding is largely a mechanical process, not a
  creative one.
• Both algorithm development and coding require
  very careful attention to detail

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Translating to Code

• To program is to first develop and test
  algorithms in pseudocode and then TRANSLATE them
  into code in a programming language
• Translating algorithms into code is very much a
  mechanical process, involving only a few
  decisions.
• For each pseudocode block we will see one
  foolproof way of translating it to code.
  Algorithms are then translated block by block.

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Using Algorithms

• When developing an algorithm, it is a good idea
  to make as much use as possible of existing
  algorithms (ones you have written or that are
  available in a library)
• You can put a CALL statement to any existing
  algorithm wherever it is needed in the algorithm
  you are developing. Be sure you get the ORDER of
  the givens and results correctly.
• To call an algorithm you need to know its header
  but not how it works you must just trust that
  it works correctly.

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Information Passing

• When calling an algorithm the call statement and
  the header must be identical except for the names
  of the givens and results. These are matched
  one-to-one in the order they appear.
• CALL (T, AvgOutOf25) Alg1(X,Y,Z)
• HEADER (Sum, Avg) Alg1(A,B,C)
• The arrow show how information is passed.

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Testing, Debugging and Maintenance

• Testing looking for errors (bugs) in an
  algorithm or program by executing it on test data
  (givens) and checking the correctness of the
  results.
• Debugging locating and correcting an error in
  an algorithm or program
• Maintenance changing an algorithm or program
  that is in use.

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What is an Object?

• An object is a software bundle of variables and
  related methods
• Software objects are used to model real-world
  objects
• Real-world objects share two characteristics
  They all have state and behavior.
• Software objects also have state and behavior. A
  software object maintains its state in one or
  more variables. And implements its behavior with
  methods.
• A variable is an item of data named by an
  identifier.
• A method is a function (subroutine) associated
  with an object

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Examples of Object

• Bicycles
• States current gear, current pedal cadence, two
  wheels, number of gears
• Behaviors braking, accelerating, slowing down,
  changing gears
• Dogs
• States name, color, breed, hungry
• Behaviors barking, fetching, wagging tail

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What is a Message?

• Software objects interact and communicate with
  each other using messages
• A single object alone is generally not very
  useful. Instead, an object usually appears as a
  component of a larger program or application that
  contains many other objects.
• Through the interaction of these objects,
  programmers achieve higher-order functionality
  and more complex behavior.

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What is a Message? (2)

• Messages provide two important benefits.
• An object's behavior is expressed through its
  methods, so message passing supports all possible
  interactions between objects.
• Objects don't need to be in the same process or
  even on the same machine to send and receive
  messages back and forth to each other.

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Components of Message (1)

• When object A wants object B to perform one of
  B's methods, object A sends a message to object B
• Sometimes, the receiving object needs more
  information so that it knows exactly what to do
• when you want to change gears on your bicycle,
  you have to indicate which gear you want. This
  information is passed along with the message as
  parameters

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Components of Message (2)

• Three components that comprise a message
• The object to which the message is addressed
  (YourBicycle)
• The name of the method to perform (changeGears)
• Any parameters needed by the method (lowerGear)

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What is a Class? (1)

• A class is a blueprint, or prototype, that
  defines the variables and the methods common to
  all objects of a certain kind.
• In the real world, you often have many objects of
  the same kind. For example, your bicycle is just
  one of many bicycles in the world.
• Bicycles have some state and behavior in common.
  However, each bicycle's state is independent of
  and can be different from that of other bicycles.
  
• Using o-o terminology, we say that your bicycle
  object is an instance of the class of objects
  known as bicycles.

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What is a Class? (2)

• In object-oriented software, it's also possible
  to have many objects of the same kind that share
  characteristics rectangles, employee records,
  video clips, and so on. Objects of the same kind
  are similar and you can create a blueprint for
  those objects.

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Class and Instance (1)

• After create the bicycle class, you can create
  any number of bicycle objects from the class.
• When create an instance of a class, the system
  allocates enough memory for the object and all
  its instance variables.
• Each instance gets its own copy of all the
  instance variables defined in the class.

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Class and Instance (2)

• Variables
• Class variable contains information that is
  shared by all instances of the class
• Instance variable contains information for one
  specific instance
• Methods
• Class methods invoked directly from the class.
• Instance methods invoked on a particular instance

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Example for Class Variables

• Suppose that all bicycles had the same number of
  gears.
• Defining an instance variable to hold the number
  of gears is inefficient each instance would have
  its own copy of the variable, but the value would
  be the same for every instance.
• Define a class variable that contains the number
  of gears. All instances share this variable. If
  one object changes the variable, it changes for
  all other objects of that type.

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Objects vs. Classes

• The difference between classes and objects is
  confusing.
• In software, the term "object" is sometimes used
  to refer to both classes and instances.
• Class is not shaded. It represents a blueprint of
  an object rather than an object itself. A
  blueprint of a bicycle is not a bicycle.
• An object is shaded, indicating that the object
  exists and that you can use it.

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What is Inheritance?

• A class inherits state and behavior from its
  superclass.
• Inheritance provides a powerful and natural
  mechanism for organizing and structuring software
  programs.
• You know a lot about an object by knowing its
  class. Even if you don't know what a
  penny-farthing is, if I told you it was a
  bicycle, you would know that it had two wheels,
  handle bars, and pedals.
• Object-oriented systems allow classes to be
  defined in terms of other classes.

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Example for Inheritance

• Mountain bikes, racing bikes, and tandems are all
  kinds of bicycles. In object-oriented
  terminology, they are all subclasses of the
  bicycle class. Similarly, the bicycle class is
  the superclass

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How does Inheritance work? (1)

• Each subclass inherits state (in the form of
  variable declarations) and methods from the
  superclass.
• e.g. Mountain bikes, racing bikes, and tandems
  share some states cadence, speed, and some
  behaviors braking and changing pedaling speed
• Subclasses are not limited to the state and
  behaviors provided to them by their superclass.
  Subclasses can add variables and methods to the
  ones they inherit from the superclass.
• e.g. Some mountain bikes have an extra set of
  gears with a lower gear ratio.

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How does Inheritance work? (2)

• Subclasses can also override inherited methods
  and provide specialized implementations for those
  methods.
• e.g You had a mountain bike with an extra set of
  gears, you would override the "change gears"
  method so that the rider could use those new
  gears.
• Not just one layer of inheritance the
  inheritance tree, or class hierarchy, can be as
  deep as needed. -- The farther down in the
  hierarchy a class appears, the more specialized
  its behavior.

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How does Inheritance work? (3)

• In JAVA, the Object class is at the top of class
  hierarchy, and each class is its descendant
  (directly or indirectly). Object provides
  behaviors that are required of all objects
  running in the Java Virtual Machine.
• e.g. All classes inherit Object's toString
  method, which returns a string representation of
  the object.

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Benefits of Inheritance

• Reuse the code in the superclass Subclasses
  provide specialized behaviors from the basis of
  common elements provided by the superclass.
• Programmers can implement superclasses called
  abstract classes that define "generic"
  behaviors. The abstract superclass defines and
  may partially implement the behavior, but much of
  the class is undefined and unimplemented. Other
  programmers fill in the details with specialized
  subclasses.

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Why Abstract Class

• Abstract class The methods can not be specified
  at that moment.
• e.g. TextMessage, VoiceMessage, FaxMessage all
  have a method called play(), how can you
  implement play() in the superclass Message?

Message
TextMessage
VoiceMessage
FaxMessage
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What is an Interface? (1)

• An interface is a contract in the form of a
  collection of method and constant declarations.
• An interface is probably most analogous to a
  protocol (an agreed on behavior).
• When a class implements an interface, it promises
  to implement all of the methods declared in that
  interface

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What is an Interface? (2)

• Interface defines a protocol of behavior that can
  be implemented by any class anywhere in the class
  hierarchy.
• Capturing similarities among unrelated classes
  without artificially forcing a class
  relationship.
• Declaring methods that one or more classes are
  expected to implement.
• Revealing an object's programming interface
  without revealing its class.

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Example of Interface

• An inventory program for bicycle doesn't care
  what class of items it manages as long as each
  item provides certain information, such as price
  and tracking number.
• The inventory program sets up a protocol of
  communication. This protocol comes in the form of
  a set of constant and method definitions
  contained within an interface.
• The inventory interface would define, but not
  implement, methods that set and get the retail
  price, assign a tracking number, and so on.

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Summary

• Class is a prototype for objects
• Objects are created from classes
• An object's class is its type
• How to create an object from a class
• What constructors are
• What class variables and methods are
• What instance variables and methods are
• How to find out what a class's superclass is
• Interface is a protocol of behavior

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Exercises

• Define classes based on the following scenario
  (make reasonable assumption)
• Your use an bank card to do some transaction from
  a bank machine.
• There are three type of transaction withdraw,
  deposit and pay bill.
• You need input a 4 digits PIN before you can do
  any transaction
• Bank machine will give you a record after each
  transaction

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First Cup of JAVA
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Concise History of Java

• 1991 Group of Sun engineers design a small
  portable computer language for consumer devices
• 1992 First product delivered by the group
• 1995 Hotjava a WWW browser written in Java
  with a built-in interpreter of intermediate
  bytecodes starts Java hype
• 1996 First official version of Java is released
• 1999 Professional-looking applications start to
  appear using the new Swing GUI API

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About Java

• Java technology is both a programming language
  and a platform
• The Java programming language is a high-level
  language
• Simple
• Architecture neutral
• Object oriented
• Portable
• Distributed
• High performance
• Interpreted
• Multithreaded
• Robust
• Dynamic
• Secure

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Overview of Java (1)

• Object-Orientation (OO)
• Programs describe interactions between a
  collection of objects which are designed to model
  some aspect of the real world. For example, an
  airline reservation system would involve
  Airplanes, Seats, Passengers, Tickets and so on.
• Syntax is borrowed from C, while object model
  from Smalltalk.

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Overview of Java (2)

• Portability
• As in some version of Pascal, source code is
  first compiled into platform-independent
  intermediate code bytecode. Unlike Pascal, this
  intermediate compilation stage is explicit
• Once in the form of bytecodes, programs can be
  executed by platform-dependent virtual machines.
• Java is often advertised as an interpreter. In
  reality, Java source code is always compiled into
  bytecodes first. Bytecodes themselves are
  interpreted by Java virtual machine (Java VM)

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How Java works?

• The Java programming language is unusual both
  compiled and interpreted.
• With the compiler, first you translate a program
  into an intermediate language called Java
  bytecodes The platform-independent codes
  interpreted by the interpreter on the Java
  platform. The interpreter parses and runs each
  Java bytecode instruction on the computer.
• Compilation happens just once interpretation
  occurs each time the program is executed.

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Why Java Platform-independent

• Java bytecodes help make "write once, run
  anywhere" possible.
• You can compile your program into bytecodes on
  any platform that has a Java compiler. The
  bytecodes can then be run on any implementation
  of the Java VM.
• As long as a computer has a Java VM, the same
  program written in the Java programming language
  can run on Windows 2000, a Solaris workstation,
  or on an iMac.

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Java Programming Environment

• Official address for download http//java.sun.com
  /j2se/
• SDK JavaTM 2 Platform, Standard Edition v 1.2 or
  v1.3
• Document JavaTM 2 Platform, Standard Edition, v
  1.2 or v1.3 Documentation
• Java 2 Runtime Environment browser plug-in (not
  necessary)

54
Java Directory Tree

• \jdk
• docs library documentation in HTML format is
  here
• bin the compiler and tools
• demo look here for demo
• include files for native methods
• lib library files
• src various subdirectories for the library
  source

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Before using Java

• Update the PATH variable
• e.g. In Autoexec.bat
• SET PATHd\Jdk1.2.2\bin
• Set the CLASSPATH if necessary
• e.g. in Autoexec.bat
• SET CLASSPATH. e\myJava

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A Simple Java Program
Everything in Java lives in a class
Access modifier
Name of class

• Public class FirstSample
• public static void main(String args)
• //Do nothing
• System.out.println("Hello World!")

Entrance from command line
Command-line argument
Comments
Semicolon at the end of every statement
A block of code
Call the method println of System.out object
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Several Rules for Start (1)

• Naming rules for classes, methods, variables,
  etc.
• Begin with a letter A-Z, a-z, - or any
  Unicode character that denotes a letter in a
  language
• Symbols like or _at_ can not be used, nor can
  spaces
• All characters are case sensitive
• Following with any combination of letters and
  digits
• Unlimited in length
• Can not use Java reserved words

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Several Rules for Start (2)

• File name must be exactly the same with class
  name
• Must have a main method for execution
• Braces (opening brace and close brace) are used
  to delineate the parts.
• Syntax for call method
• object.method(parameters)

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Your First Cup of Java (for Win32)

• /The HelloWorldApp class implements an
  application that displays "Hello World!" to the
  standard output.
• /
• public class HelloWorldApp
• public static void main(String args)
• // Display "Hello World!" System.out.println("H
  ello World!")
• System.out.print("Hello World!")
• System.out.println( I am learning Java!")

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Step by Step (1)

• Create a source file programmer understand
• You can use any text editor to create and edit
  source files.
• Be Careful When You Type
• Type all code, commands, and file names exactly
  as shown. The Java compiler and interpreter are
  case-sensitive, so you must capitalize
  consistently.
• Save as HelloWorldApp.java

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Step by Step (2)

• Compile the source file into a bytecode file -
  Java Virtual Machine (Java VM) understand.
• Open MS-DOS Prompt
• Go to the directory that you save your source
  file
• Enter
• Javac HelloWorldApp.java 
• Get a class file HelloWorldApp.class

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Step by Step (3)

• Run the program contained in the bytecode file -
  computer understand.
• Enter
• Java HelloWorldApp
• Have you got the greeting words?

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Algorithm to Program

• Program a Java class with a main method that
  is the translation of the algorithm.
• Givens, Results and intermediates all get
  translated to LOCAL VARIABLES
• They all must be declared and given a type.
• Program template

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Exercises

• Define turnEngineOff() and decelerate() methods
  for Car class
• Translate the Example 2 of algorithm into Java
  code

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Fundamental Programming Structures in Java - Data
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Data Type

• Eight primitive types
• Six number types (4 integer and two
  floating-point type)
• int
• short
• long
• byte
• bloat
• double
• One character type char
• One Boolean type boolean

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Integers (1)

• Numbers without fractional parts
• Negative values are allowed
• Long integer numbers have a suffix L. Hexadecimal
  numbers have a prefix 0x

Type Storage requirement Range (inclusive)
int 4 bytes -2,147,483,648 to 2,147,483,647 (just over 2 billion)
short 2 bytes -32,768 to 32,767
long 8 bytes -9,223,372,036,854,775,808L to 9,223,372,036,854,775,807L
byte 1 bytes -128 to 127
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Integers (2)

• Under Java, the ranges of the integer types do
  not depend on the machine on which you will run
  your Java code.
• Platform-independent integer type brings a small
  performance penalty (not the worst one).

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Floating-Point Types

• Denote numbers with fractional parts.
• double means the numbers have twice the precision
  of the float type
• Without a suffix F, a number is always considered
  to be of type double.

Type Storage requirement Range
float 4 bytes approximately 3.40282347E38F (6 7 significant decimal digits)
double 8 bytes approximately 1.79769313486231570E 308F (15 significant decimal digits)
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The Character Type

• Denotes characters in the Unicode encoding
  scheme.
• Designed to handle all characters in all written
  languages -- 35,000 are in use
• \u prefix with four hexadecimal digits indicates
  a Unicode value
• www.unicode.org
• Single quotes are used to denote char constants.
• H is a character
• H is a string

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Special characters
Escape Sequence Name Unicode Value
\b backspace \u0008
\t tab \u0009
\n linefeed \u000a
\r carriage return \u000d
\ double quote \u0022
\ single quote \u0027
\\ backslash \u005c
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Boolean Type

• Used for logic testing
• Only two values
• TRUE or FALSE
• Not numbers

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Variables (1)

• Java requests declaring the type of a variable
• Declare a variable by placing the type first,
  followed by the name of the variable
• e.g.
• byte b //for space-sensitive //consideration
  s
• int anIntegerVariable
• long aLongVariable
• char ch

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Variables (2)

• Multiple declaration
• int i, j //both are integers
• recommend to declare one variable per line
• You can put declaration anywhere in your code,
  but you can only declare a variable once in any
  block in a method.

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Expressions

• An expression is a series of variables,
  operators, and method calls (constructed
  according to the syntax of the language) that
  evaluates to a single value
• The job of an expression
• To perform the computation indicated by the
  elements of the expression
• To return a value that is the result of the
  computation
• e.g.
• System.out.println("The largest byte value is "
  largestByte)

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Assignments and Initializations

• After you declare a variable, you should
  initialize it by means of an assignment statement
• Never have un-initialized variables
• Assignment
• Previously declared variable on the left
• an equal sign ()
• some Java expression with an appropriate value on
  the right

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Example of Assignments and Initializations

• Correct assignments
• int foo //this is a declaration
• foo 37 //this is an assignment
• char yesChar
• yesChar Y
• int i 10 //this is an initialization

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Conversions Between Numeric Types (1)

• A binary operations on numeric values of
  different types will be treated in the following
  fashion
• If any of the operands is of type double, the
  other one will be converted to a double
• Otherwise, if any of the operands is of type
  float, the other one will be converted to a
  float
• Otherwise, if any of the operands is of type
  long, the other one will be converted to a long.

79
Conversions Between Numeric Types (2)

• Numeric conversion loss of information
• casts give the target type in parentheses,
  followed by the variable name.
• e.g. double x 9.997
• int nx (int) x
• Value of nx is 9.
• Round use the Math.round method.
• e.g. double x 9.997
• int nx (int)Math.round(x)
• Value of nx is 10.

Return type is long
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Conversions Between Numeric Types (3)

• Certain assignment conversion by assigning the
  value of a variable of one type to another
  without an explicit cast
• byte short int long
• double float

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Constants

• Use keyword final to denote a constant
• final indicates that you can only assign to the
  variable once
• e.g.
• public class UsesConstants
• public static void main(String args)
• final double CM_PER_INCH 2.54
• double paperWidth 8.5
• double paperHeight 11
• System.out.println(Paper size in centimeter
  paperWidth CM_PER_INCH by
• paperHeight CM_PER_INCH )

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Class Constants

• Use static final for class constants
• Appear outside method
• e.g.
• public class UserConstants2
• public static final double G 9.81
• // gravitation in meters/second squared
• public static void main(String args)
• System.out.println(G
• meters per second squared)

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Arithmetic Operators (1)

• Arithmetic operators - / are used in Java for
  addition, subtraction, multiplication and
  division
• The / operator denotes integer division if both
  arguments are integers, and floating-point
  division otherwise.
• denoted integer remainder.
• e.g.
• 15 / 6 is 2
• 15 6 is 3
• 11.0 / 4 is 2.75

84
Arithmetic Operators (2)

• You can use arithmetic operators in your variable
  initialization
• e.g.
• int n 5
• int a 2 n // a is 10
• Shortcut for using binary arithmetic operators
• e.g.
• x 4 equal to x x 4
• y 3 equal to y y 3

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Exponentiation

• Java has no operator for raising a quantity to a
  power
• Use Math.pow method.
• e.g.
• double y Math.pow(x, n)
• y is set to x raised to the power n, which is xn
• The return type of Math.pow is double.

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Increment and Decrement Operators (1)

• Increment operator
• adds 1 to the current value of the variable
• e.g.
• int n 12
• n
• Value of n is 13
• Decrement operator --
• subtracts 1 to the current value of the variable
• e.g.
• int m 7
• m--
• Value of m is 6.

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Increment and Decrement Operators (2)

• Not be applied to number.
• e.g 4 is error
• Two forms make difference in expressions
  recommend against using and -- inside other
  expressions
• postfix evaluate to the old value of the
  variable
• prefix does addition or subtraction first
• e.g
• int m 7
• int n 7
• int a 2 m // a is 16, m is 8
• int b 2 n //b is 14, n is 8

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Rational and Boolean Operators

• Results of rational and Boolean operators are of
  type boolean
• Test for equality
• Test for inequality !
• Less than lt
• Greater than gt
• Less than or equal lt
• Greater than or equal gt
• Logic and
• Logic or
• e.g. (3 ! 7) is true

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Bitwise Operators (1)

• Work with any of the integer types
• Work directly with the bits that make up the
  integers use masking techniques to get a
  individual bits in a number
• and
• or
• xor
• not
• e.g. if foo is an integer,
• int fouthBitFromRight (foo 8) / 8
• gives the fourth bit from the right in the
  binary representation of foo is one, and a zero
  if not

90
Bitwise Operators (2)

• gtgt and ltlt operators shift a bit pattern to the
  right or left
• gtgt operator fills the top bits with the sign bit
• gtgtgt operator fills the top bits with zero
• There is no ltltlt operator

91
Parentheses and Operator Hierarchy

• Use parentheses to indicate the order in which
  you want operations to be carried out
• If no parentheses are used, operations are
  performed in the hierarchical order indicated
• Operators on the same level are processed from
  left to right, except for those that are right
  associative

92
Operator Precedence
Operators Associativity
. ()(method call) left to right
! -- (unary) (unary) ()(cast) new right to left
/ left to right
- left to right
ltlt gtgt gtgtgt left to right
lt lt gt gt instanceof left to right
! left to right
left to right
left to right
left to right
left to right
left to right
? left to right
- / ltlt gtgt gtgtgt right to left
93
Strings

• Denotes a sequences of Unicode characters
• Java has not a built-in string type
• Use methods in String class, each quoted string
  is an instance of the String class
• e.g.
• String e //and empty string
• String greeting Hello

94
Concatenation of Strings

• Java allows to use the sign to
  join(concatenate) two strings together
• e.g.
• String expletive Expletive
• String PG13 deleted
• String message expletive PG13
• The sign join two strings together in the order
  received, exactly as they are given
• When you concatenate a string with a value that
  is not a string, the value is converted to a
  string
• e.g.
• String fating PG 13 is PG13

95
Substrings

• Exact a substring from a larger string with the
  substring method of the String class.
• e.g
• String greeting Hello
• String s greeting.substring(0, 4)
• s is Hell
• The first character in a string has position 0.
• The length of string s.substring(a, b) is always
  b - a

96
String Editing

• To find out the length of a string, use length()
• e.g.
• String greeting Hello
• int n greeting.length() //is 5
• To get at individual characters of a string
  s.charAt(n), n is between 0 and s.length() - 1
• e.g.
• String greeting Hello
• char ch greeting.charAt(1) // is e
• No direct way to change a character in strings
  use substring() and concatenation to do that

97
Testing Strings for Equality (1)

• To test if or not two strings are equal
  s.equals(t). -- Return true if the strings and t
  are equal, false otherwise
• e.g.
• Hello.equals(command)
• To test if two strings are identical except for
  the upper/lower case letter distinction
  s.equalsIgnoreCase(t)
• e.g.
• Hello.equalsIgnoreCase(hello) is true

98
Testing Strings for Equality (2)

• Do not use to test if strings are equal, it
  only determines whether or not the strings are
  stored in the same location
• e.g.
• String greeting Hello
• if (greeting Hello) . . . //probably
  true
• if (greeting.substring(0,4) Hell) .
  //probably false
• Never use to compare strings
• Go to the Java document to find API of String

99
Input from a Command Line Parameter

• public class Test
• public static void main(String args)
• double x args0 
• double y args1
• System.out.println(I got x
• and y
• from command line)
• C\gtjava Test 3.02

100
Formatting Output (1)

• System.out.print(x) print x with the maximum
  number of non-zero digits for that type
• e.g.
• x 10000.0 / 3.0
• System.out.print(x)
• prints 3333.3333333333335
• To arrange your output neatly NumberFormat class
  in java.txt package offers formatters for
  numbers, currency values and percentage values
• e.g. for United States locale should print
• 3,333.333
• 3,333.33
• 333,333

101
Formatting Output (2)

• To obtain a formatter for the default locale
• NumberFormat.getNumberInstance()
• NumberFormat.getCurrencyInstance()
• NumberFormat.getPercentInstance()
• Customize output
• Set minimum or maximum number of integer digits
• setMinimumIntegerDigits()
• setMaximumIntegerDigits()
• Set minimum or maximum number of fractional
  digits
• setMinimumFractionDigits() default 0
• setMaximumFractionDigits()

102
Example of formatting Output

• 1.double x 1000.0 / 3.0
• NumberFormat nf NumberFormat.getNumberInstance
  ()
• String fx nf.format(x)
• //the string is 3,333.33
• 2. double x 1000.0 / 3.0
• NumberFormat nf NumberFormat.getNumberInstance
  ()
• nf.setMaximumFractionDigits(4)
• nf.setMinimumIntegerDigits(6)
• String fx nf.format(x)
• //the string is 003,333.3333

103
Formatter for different locales

• Object Locale.GERMAN is the local for German
  number formatting rules
• e.g.
• double x 10000.0 / 3.0
• NumberFormat nf NumberFormat.getNumberInstance
  ( Locale.GERMAN)
• Sysetm.out.println(nf.format(x))
• NumberForamt cf NumberFormat.getCurrencyInstan
  ce( Locale.GERMAN)
• Sysetm.out.println(cf.format(x))
• Prints
• 3.333,333
• 3.333,33 DM

104
Create your own format

• e.g To define a format that shows number with six
  digits after the decimal point, but no thousands
  separator
• DecimalFormat df new decimalFormat(0.)
  
• System.out.println(df.format(x))

105
Formatting characters for DecimalFormat class
Symbol Meaning
0 A digit
A digit dont show if it is a leading or trailing zero
. Location of decimal separator
, Location of grouping separator
Separates formats for positive and negative numbers
- Negative prefix
Divide by 100 and show as percentage
Any other symbols Include symbol in output string
106
Excises

1. Write a program that tests whether a floating
   point number is zero. (Hint You shouldn't
   generally use the equality operator with
   floating point numbers, since floating point
   numbers by nature are hard to match exactly.
   Instead, test whether the number is close to
   zero.)
2. Change the code for Algorithm 1 and 2 to get
   parameter from command line.
3. Write a program that calculates the number of US
   dollars equivalent to a given number of French
   francs. Assume an exchange rate of 6.85062 francs
   per dollar. If you want to control the format of
   the numbers your program displays, you can use
   the DecimalFormat class, which is discussed in
   Customizing Formats

107

• 4. Write a program that uses the bits in a single
  integer to represent the true/false data shown in
  the following figure
• Include in the program a variable named status,
  and have the program print the meaning of status.
  For example, if status is 1 (only bit 0 is set)
  the program should print something like this
  Ready to receive requests
• Show your code.
• What is the output when status is 8?
• What is the output when status is 7?

108
Fundamental Programming Structures in Java -
Control
109
Statements (1)

• Statements are roughly equivalent to sentences in
  natural languages. It forms a complete unit of
  execution.
• The following types of expressions can be made
  into a statement by terminating the expression
  with a semicolon ()
• Assignment expressions
• Any use of or --
• Method calls
• Object creation expressions
• e.g
• aValue 8933.234
• aValue
• System.out.println(aValue)
• Integer integerObject new Integer(4)

110
Statements (2)

• Two other kinds of statements.
• declaration statement declares a variable.
• e.g.
• double aValue 8933.234
• control flow statement regulates the order in
  which statements get executed.

111
Know more about blocks (1)

• Block is any number of simple Java statements
  that are surrounded by a pair of braces
• Blocks define the scope of variables
• Blocks can be nested inside another
• e.g.
• public static void main(String args)
• int n
•  
• int k // k local to block and //defines
  only until the //end of the block

112
Know more about blocks (2)

• It is not possible to declare identically named
  variables in two nested blocks
• e.g.
• public static void main(String args)
• int n
•  
• int k
• int n // error cant redefine n // in
  inner block

113
Conditional Statements (1) if Statement

• Syntax
• if (condition) statement
• if (condition) block
• e.g. if (yourSales gt target)
• performance Satisfactory
• bonus 100

NO
youSales gt target
YES
performance Satisfactory
bonus 100
114
Conditional Statements (2) if-else Statement

• Syntax
• if (condition) statement1 else statement2
• if (condition) block1 else block2
• e.g.
• if (yourSales gt target)
• performance Satisfactory
• bonus 100
• else
• performance Unsatisfactory
• bonus 0

115

• Use else groups with the closest if
• e.g.if (yourSales gt 2 target)
• performance Excellent
• bonus 1000
• else if (yourSales gt 1.5 target)
• performance Fine
• bonus 500
• else if (yourSales gt target)
• performance Satisfactory
• bonus 100
• else
• System.out.println(You are fired!)

116
? Operator

• Java supports the ternary ? operator
• The expression
• condition ? e1 e2 evaluates to e1 if the
  condition is true, to e2 otherwise.
• e.g.
• (x lt y) ? xy give the smaller of x and y
• int x 1
• int y 3
• int z (x lt y) ? xy // z is 1

117
Multiple Selections - switch Statement

• Conditionally perform statements based on an
  integer or char expression
• Select only against a char or against all the
  integer types but long.
• Can not use a range of values
• Each break statement terminates the enclosing
  switch statement, and the flow of control
  continues with the first statement following the
  switch block.
• Use the default statement at the end of the
  switch to handle all values that aren't
  explicitly handled by one of the case statements

118

• public class SwitchDemo
• public static void main(String args)
• int day 3
• switch (month)
• case 1 System.out.println(Monday") break
• case 2System.out.println(Tuesday")
• break
• case 3
• System.out.println(Wednesday") break
  
• case 4
• System.out.println(Thursday")
  break
• case 7 System.out.println("June") break

119

• public class SwitchDemo2
• public static void main(String args)
• int month 2
• int year 2000
• int numDays 0
• switch (month)
• case 1
• case 3
• case 5
• case 7
• case 8
• case 10
• case 12 numDays 31 break
• case 4
• case 6
• case 9
• case 11 numDays 30
• break
• case 2 if ( ((year 4 0) !(year 100
  0)) (year 400 0) ) numDays
  29

120

• int month 8
• . . .
• switch (month)
• case 1 System.out.println("January") break
  
• case 2 System.out.println("February") break
  
• case 3 System.out.println("March") break
• case 11 System.out.println("November")
  break
• case 12 System.out.println("December") break
  
• default
• System.out.println("Hey, that's not a valid
  month!")
• break

121
Breaks

• break statement is used to resume program
  execution at the statement immediately following
  the current statement.
• e.g.
• while (years lt 100)
• balance ( balance payment )
• ( 1 interest )
• if ( balance gt goal ) break
• years

122
Labeled Breaks

• If followed by a label, the program resumes
  execution at the labeled statement.
• e.g.
• next_client
• while (years lt 100)
• balance ( balance payment )
• ( 1 interest ) if ( balance gt goal )
  break next_client
• years

123
Indeterminate Loops (1)- while loop

• Syntax
• while (condition) block
• The while loop only executes the body of the loop
  while a condition is true

NO
condition
YES
Statemets
124

• public class WhileDemo
• public static void main(String args)
• String copyFromMe
• "Copy this string until you "
• "encounter the letter 'g'."
• StringBuffer copyToMe
• new StringBuffer()
• int i 0
• char c copyFromMe.charAt(i)
• while (c ! 'g')
• copyToMe.append(c)
• c copyFromMe.charAt(i)
• System.out.println(copyToMe)

125
Indeterminate Loops (2)- do-while loop

• Syntax
• do block while (condition)
• Instead of evaluating the expression at the top
  of the loop, do-while evaluates the expression at
  the bottom. The statements are executed at least
  once

Statemets
condition
YES
NO
126

• public class DoWhileDemo
• public static void main(String args)
• String copyFromMe
• "Copy this string until you "
• "encounter the letter 'g'."
• StringBuffer copyToMe
• new StringBuffer()
• int i 0
• char c copyFromMe.charAt(i)
• do
• copyToMe.append(c)
• c copyFromMe.charAt(i) while (c !
  'g')
• System.out.println(copyToMe)

127
Determinate Loops for loop

• Syntax
• for (initialization termination increment )
• block
• Equal to
• initialization
• while (termination)
• block
• increment

128

• /Adds the numbers from 1 to 10 and displays the
  result
• /
• public class ForDemo
• public static void main(String args)
• int sum 0
• for (int current 1 current lt 10 current)
  
• sum current
• System.out.println("Sum " sum)

129
continue Statement

• Skip the current iteration of a for, while , or
  do-while loop
• The unlabeled form skips to the end of the
  innermost loop's body and evaluates the boolean
  expression that controls the loop, basically
  skipping the remainder of this iteration of the
  loop.
• The labeled form skips the current iteration of
  an outer loop marked with the given label.

130

• public class ContinueDemo
• public static void main(String args)
• StringBuffer searchMe
• new StringBuffer( "peter piper picked
• a peck of pickled peppers")
• int max searchMe.length()
• int numPs 0
• for (int i 0 i lt max i)
• //interested only in p's
• if (searchMe.charAt(i) ! 'p') continue
  //process p's numPs
• searchMe.setCharAt(i, 'P')
• System.out.println("Found " numPs
• " p's in the string.") System.out.println(s
  earchMe)

131

• public class ContinueWithLabelDemo
• public static void main(String args)
• String searchMe
• "Look for a substring in me"
• String substring "sub"
• boolean foundIt false
• int max searchMe.length()
• - substring.length()
• test for (int i 0 i lt max i)
• int n substring.length()
• int j i
• int k 0
• while (n-- ! 0)
• if (searchMe.charAt(j) !
• substring.charAt(k))
• continue test
• foundIt true
• break test

132
return Statement

• Exit from the current method.
• The flow of control returns to the statement that
  follows the original method call.
• Two forms
• Returns a value put the value (or an expression
  that calculates the value) after the return
  keyword. The data type of the value returned by
  return must match the type of the method's
  declared return value.
• e.g. return count
• Doesn't return a value. When a method is declared
  void,
• e.g. return

133
Excises

• 1. What's wrong with the following code snippet
• if (i 1)
• /do something /
• 2. Consider the following code snippet.
• if (aNumber gt 0)
• if (aNumber 0) System.out.println("first
  string")
• else System.out.println("second string")
• System.out.println("third string")

134

• What output do you think the code will produce if
  aNumber is 3?
• Write a test program containing the code snippet
  make aNumber 3. What is the output of the
  program? Is it what you predicted? Explain why
  the output is what it is. In other words, what is
  the control flow for the code snippet?
• Using only spaces and line breaks, reformat the
  code snippet to make the control flow easier to
  understand.
• Use braces and to further clarify the code
  and reduce the possibility of errors by future
  maintainers of the code.