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C1 D6

C1 D6. Obj: cont 1.3 and 1.4, to become familiar with identifiers and to understand how programming languages work HW: p.51 #1.8 – 1.9 (Short Answers) Chapter 1 Test in two class days Do Now: How is the program on p.33 different from the program on p.27? . Identifiers.

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C1 D6

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  1. C1 D6 Obj: cont 1.3 and 1.4, to become familiar with identifiers and to understand how programming languages work HW: p.51 #1.8 – 1.9 (Short Answers) Chapter 1 Test in two class days • Do Now: How is the program on p.33 different from the program on p.27?

  2. Identifiers • Identifiers are the words a programmer uses in a program • Some identifiers are already defined • Ex: class, public, static, void, main, println, String,… • Some are made up by the programmer: • Ex: Lincoln

  3. An identifier can be made up of letters, digits, the underscore character ( _ ), and the dollar sign • Identifiers cannot begin with a digit • Java is case sensitive - Total, total, and TOTAL are different identifiers • By convention, Java programmers use different case styles for different types of identifiers, such as • title case for class names - Lincoln • upper case for constants - MAXIMUM

  4. Identifiers • Sometimes we choose identifiers ourselves when writing a program (such as Lincoln) • Sometimes we are using another programmer's code, so we use the identifiers that they chose (such as println)

  5. Often we use special identifiers called reserved words that already have a predefined meaning in the language • A reserved word cannot be used in any other way

  6. Reserved Words • The Java reserved words are listed in your text on p.31 • You do not need to memorize this list, but you should know whether or not a word is on this list. • For example, you should know that you cannot use the word “while” other than for it’s pre-defined use.

  7. White Space • Spaces, blank lines, and tabs are called white space • White space is used to separate words and symbols in a program • Extra white space is ignored • A valid Java program can be formatted in many ways

  8. Programs should be formatted to enhance readability, using consistent indentation • SeeLincoln2.java (page 33) • SeeLincoln3.java (page 34) • These are both the same exact program as on p.27, but they are more difficult for a human to read.

  9. Language Levels • There are four programming language levels: • machine language • assembly language • high-level language (ex: Java, understandable by programmers) • fourth-generation language (doesn’t require knowledge of a programming language) • See p.36

  10. Each type of CPU has its own specific machine language • The other levels were created to make it easier for a human being to read and write programs

  11. Programming Languages • A program must be translated into machine language before it can be executed on a particular type of CPU • This can be accomplished in several ways • A compiler is a software tool which translates source code into a specific target language

  12. JCreator, our editor, is actually an IDE (Integrated Development Environment) • It combines an editor, a compiler, and other Java support tools. • Other examples of IDEs are NetBeans and Eclipse.

  13. Java Translation Java source code Java bytecode Java Compiler (ex: JCreator) Java interpreter Bytecode compiler Machine code

  14. Syntax and Semantics • The syntax rules of a language define how we can put together symbols, reserved words, and identifiers to make a valid program • The semantics of a program statement define what that statement means (its purpose or role in a program)

  15. A program that is syntactically correct is not necessarily logically (semantically) correct • A program will always do what we tell it to do, not what we meant to tell it to do

  16. Errors • A program can have three types of errors • The compiler will find syntax errors and other basic problems (compile-time errors) • If compile-time errors exist, an executable version of the program is not created

  17. A problem can occur during program execution, such as trying to divide by zero, which causes a program to terminate abnormally (run-time errors) • A program may run, but produce incorrect results, perhaps using an incorrect formula (logical errors)

  18. errors errors Basic Program Development Edit and save program Compile program Execute program and evaluate results

  19. If time: • Do p.52 #1.1

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