Identifiers. Identifiers are the words a programmer uses in a program Some identifiers are already defined. Some are made up by the programmer:

Transcription

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 # (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? (on Document Camera) 1

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 2

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 3

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) 4

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 5

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. 6

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8 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

9 Programs should be formatted to enhance readability, using consistent indentation See Lincoln2.java (page 33) See Lincoln3.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

10 Language Levels There are four programming language levels: machine language assembly language high-level language (ex: Java, understandable by programmers) 10

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12 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 12

13 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 13

14 Eclipse, 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 JCreator. 14

15 Java Translation Java source code Java bytecode Java Compiler (ex: Eclipse) Java interpreter Bytecode compiler Machine code 15

16 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) 16

17 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 17

18 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 18

19 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) 19

20 Basic Program Development Edit and save program errors errors Compile program Execute program and evaluate results 20

21 If time: Finish programming exercises ! 21