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Workshop on Java Programming

September 29, October 13, October 20

YANPING CHEN

ychen@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 statementJohn wants to know the total and average cost of the three books he just bought.

Givens: descriptions and names of the values that are knowA, B, C numbers representing the cost of each of John’s 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 statementWrite 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 knowA, 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 algorithm’s HEADER specifies the name of the algorithm, an order for the givens and an order for the results

• An algorithm’s 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 = A+B+C

– 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

• HEADERAlg2(A, B, C, D)

Return: (Avg) • BODY

– Sum = A + B + C + D //an Intermediate Variable

– Avg = Sum /4/25*100

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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 1Avg Alg1 (18, 25, 20)

• Example 2Avg 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 A=10, B=7, 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?

TextMessage

Message

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)

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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 variablee.g. In Autoexec.bat

SET PATH=d:\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

Public class FirstSample

{

public static void main(String[] args)

{

//Do nothing

System.out.println("Hello World!");

}

}

Access modifier Everything in Javalives in a class Name of

classEntrance fromcommand line

Comments Command-lineargument

A blockof code

Call the method println ofSystem.out object

Semicolon atthe end of every statement

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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 ‘@’ 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("Hello 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.

TypeStorage

requirementRange

float 4 bytes

approximately ±3.40282347E+38F

(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 variablee.g.

byte b; //for space-sensitive

//considerations

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.

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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 typeis 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 methode.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 215 % 6 is 311.0 / 4 is 2.75

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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 operatorse.g.

x += 4; equal to x = x + 4y %= 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

variablee.g.

int n = 12;n++;

Value of n is 13

• Decrement operator: --– subtracts 1 to the current value of the

variablee.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

firste.g

int m = 7;int n = 7;int a = 2 * ++m; // a is 16, m is 8int 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: <– Greater than: >– Less than or equal: <=– Greater than or equal: >=– 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) / 8gives the fourth bit from the right in the binary representation of foo is one, and a zero if not

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Bitwise Operators (2)

• >> and << operators shift a bit pattern to the right or left

• >> operator fills the top bits with the sign bit

• >>> operator fills the top bits with zero

• There is no <<< operator

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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

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Operator Precedence

Operators Associativity

[] . ()(method call) left to right

! ~ ++ -- + (unary) – (unary) ()(cast) new right to left

* / % left to right

+ - left to right

<< >> >>> left to right

< <= > >= 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

= += -= *= /= %= &= |= ^= <<= >>= >>>= right to left

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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 classe.g.

String e = “”; //and empty string

String greeting = “Hello”;

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Concatenation of Strings

• Java allows to use the + sign to join(concatenate) two strings togethere.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 stringe.g.

String fating = “PG” + 13 is “PG13”

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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

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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() - 1e.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

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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 otherwisee.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

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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 locatione.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

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Input from a Command Line Parameter

public class Test{

public static void main(String[] args){

double x = args[0]; double y = args[1];System.out.println(“I got ” + x

+“ and ” + y + “ from command line);

}}

C:\>java Test 3.02

100

Formatting Output (1)• System.out.print(x) print x with the

maximum number of non-zero digits for that typee.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 valuese.g. for United States locale should print

3,333.333$3,333.33333,333%

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 0setMaximumFractionDigits()

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 rulese.g.

double x = 10000.0 / 3.0;NumberFormat nf = NumberFormat.getNumberInstance(

Locale.GERMAN);Sysetm.out.println(nf.format(x));NumberForamt cf = NumberFormat.getCurrencyInstance(

Locale.GERMAN);Sysetm.out.println(cf.format(x));

Prints:3.333,3333.333,33 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; don’t 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

a. Show your code.

b. What is the output when status is 8?

c. 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 blockse.g.

public static void main(String[] args){

int n;  {

int k; int n; // error – can’t

redefine n // in inner block

}}

113

Conditional Statements (1) – if Statement

Syntax:if (condition) statement;if (condition) { block }

e.g. if (yourSales >= target){

performance = “Satisfactory”;bonus = 100;

}

youSales >= target

performance = “Satisfactory”

bonus = 100

YES

NO

114

Conditional Statements (2) – if-else Statement

Syntax:if (condition) statement1 else statement2;

if (condition) {block1} else {block2}e.g.

if (yourSales >= target){

performance = “Satisfactory”;bonus = 100;

}else

{performance = “Unsatisfactory”;bonus = 0;

}

115

• Use else groups with the closest ife.g.if (yourSales >= 2 * target)

{performance = “Excellent”;bonus = 1000;

}else if (yourSales >= 1.5 * target){

performance = “Fine”;bonus = 500;

}else if (yourSales >= 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 < y) ? x:y give the smaller of x and y

int x = 1;

int y = 3;

int z = (x < y) ? x:y; // 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 2:System.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;

else numDays = 28; break; } System.out.println("Number of Days = " +

numDays); } }

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 <= 100)

{

balance = ( balance + payment )

* ( 1 + interest );

if ( balance > 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 <= 100)

{

balance = ( balance + payment )

* ( 1 + interest ); if ( balance > 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

condition

Statemets

YES

NO

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

condition

Statemets

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 <= 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 < 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(searchMe); }

}

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 <= 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;

} System.out.println(foundIt ? "Found it" :

"Didn't find it"); }

}

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

Excises1. What's wrong with the following

code snippet:

if (i = 1)

{

/*do something */

}

2. Consider the following code snippet. if (aNumber >= 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.