Introduction to Computers and Java Recitation - 01/11/2008

CS 180

Department of Computer Science,

Purdue University

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Recitation

• Review of important material or relevant material not yet covered

• Introduction to projects• A chance to ask questions

Come prepared and participate• A quiz every week

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Announcements

Class website:

http://www.cs.purdue.edu/~cs180/ Syllabus Lecture and recitation slides are available Assignments Course policies (note the integrity policy)

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Announcements

Class newsgroup: purdue.class.cs180 Project information and answers to questions

will be posted to the newsgroup; posts by grad TAs are official

You are responsible for knowing all information posted, including changes to assignments

A newsgroup client such as Mozilla Thunderbird or Microsoft Outlook can access the class newsgroup.

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Compilers

• A compiler translates a program from a high-level language to a low-level language the computer can run.

• You compile a program by running the compiler on the high-level-language version of the program called the source program.

• Compilers produce machine- or assembly-language programs called object programs.

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Java Byte-Code

• The Java compiler does not translate a Java program into assembly language or machine language for a particular computer.

• Instead, it translates a Java program (from a .java file) into byte-code (a .class file).

– Byte-code is the machine language for a hypothetical computer (or interpreter) called the Java Virtual Machine.

• A byte-code program is easy to translate into machine language for any particular computer.

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Compiling, Interpreting and Running

• Compile command• The compiler translates the Java source program (.java)

into the Java byte-code (.class).• javac Program1.java creates Program1.class

• Run command• The byte-code interpreter for your computer translates

each byte-code instruction into machine language and runs the resulting machine-language instructions.

• java Program1 runs the program

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Portability

• After compiling a Java program into byte-code, that byte-code (a .class file) can be used on any computer with a byte-code interpreter and without a need to recompile.

• Byte-code can be sent over the Internet and used anywhere in the world.

• This makes Java suitable for Internet applications.

Compiling and running a Java Program

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Object-Oriented Programming

• Our world consists of objects (people, trees, cars, etc.).

• Represents a discrete concept

• Objects can perform actions which affect themselves and other objects in the world.

• Object-oriented programming (OOP) treats a program as a collection of objects that interact by means of actions.

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OOP Design Principles

• OOP adheres to three primary design principles:• Encapsulation• Polymorphism• Inheritance.

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Encapsulation

• Encapsulation refers to the clubbing of data with the methods that operate on that data.

• Encapsulation provides a means of using the class, but it omits the details of how the class works

• Encapsulation often is called information hiding.

• An object of type ‘Car’ can have several properties (carburettor, engine, fuel etc) and methods (start the engine, brake etc) clubbed together.

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Encapsulation Example

• An automobile consists of several parts and pieces and is capable of doing many useful things.• Awareness of the accelerator pedal, the brake

pedal, and the steering wheel is important to the driver.

• Awareness of the fuel injectors, the automatic braking control system, and the power steering pump are not important to the driver.

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Inheritance

• Inheritance is a way to form new classes by inheriting properties from existing classes.

• The new class is also referred to as a derived class or sub-class of the superclass.

• Advantage – reusability of code. Inherited characteristics do not need to be repeated in the sub-class.

• At each level, classifications become more specialized by adding other characteristics.

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Inheritance, contd.

• Example of inheritance hierarchy –

More about inheritance in chapter 7

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Polymorphism

• from the Greek meaning “many forms”• The same program instruction adapts to

mean different things in different contexts.– A method name, used as an instruction, produces

results that depend on the class of the object that used the method.

– Example – A bank account will compute monthly interest depending on which form (or which type of account - saving, checking etc) it is.

• more about polymorphism in Chapter 7

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Reusable Components

• Most programs are created by combining components that exist already.

• Reusing components saves time and increases reliability.

• New components should be designed to be reusable by other applications.

Making Components Reusable

• To make components reusable-• Specify exactly how objects of the class

interact with other objects.• Design a class so that objects are general,

rather than unique to a particular application.• The more general the class is, the more reusable it

is.

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Errors

• An error in a program is called a bug.• Eliminating errors is called debugging.• three kinds or errors

– syntax errors– runtime errors– logic errors

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Syntax Errors

• Grammatical mistakes in a program– the grammatical rules for writing a program are very

strict

• The compiler catches syntax errors and prints an error message.

• example: using a period where a program expects a semi-colon

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Runtime Errors

• Errors that are detected when your program is running, but not during compilation

• Run-time errors occur when an unnatural issue is experienced at run-time. They may not occur every time a program is run.

• When the computer detects an error, it terminates the program and prints an error message.

• example: attempting to divide by 0

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Logic Errors

• Errors that are not detected during compilation or while running, but which cause the program to produce incorrect results.

• because of incorrect logic used in the program.• difficult to catch as there is no error message

• Example: an attempt to calculate a Fahrenheit temperature from a Celsius temperature by multiplying by 9/5 and adding 23 instead of 32.

• A logic error can be detected by comparing the expected result with the actual result.

Quiz

What are the three types of errors and when are they reported?