© 2006 Pearson Education 1 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? C1 D6

© 2006 Pearson Education 2 Identifiers  Identifiers are the words a programmer uses in a program  Some identifiers are already defined Ex: class, public, static, void, main, println, Ex: class, public, static, void, main, println, String,… String,…  Some are made up by the programmer: Ex: Lincoln Ex: Lincoln

© 2006 Pearson Education 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 title case for class names - Lincoln upper case for constants - MAXIMUM upper case for constants - MAXIMUM

© 2006 Pearson Education 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 )

© 2006 Pearson Education 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

© 2006 Pearson Education 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.

© 2006 Pearson Education 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

© 2006 Pearson Education 8  Programs should be formatted to enhance readability, using consistent indentation  See Lincoln2.java (page 33) Lincoln2.java  See Lincoln3.java (page 34) Lincoln3.java  These are both the same exact program as on p.27, but they are more difficult for a human to read.

© 2006 Pearson Education 9 Language Levels  There are four programming language levels: machine language machine language assembly language assembly language high-level language (ex: Java, understandable by programmers) high-level language (ex: Java, understandable by programmers) fourth-generation language (doesn’t require knowledge of a programming language) fourth-generation language (doesn’t require knowledge of a programming language)  See p.36

© 2006 Pearson Education 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

© 2006 Pearson Education 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

© 2006 Pearson Education 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.

© 2006 Pearson Education 13 Java Translation Java source code Machine code Java bytecode Java interpreter Bytecode compiler Java Compiler (ex: JCreator)

© 2006 Pearson Education 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)

© 2006 Pearson Education 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

© 2006 Pearson Education 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 If compile-time errors exist, an executable version of the program is not created

© 2006 Pearson Education 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)

© 2006 Pearson Education 18 Basic Program Development errors Edit and save program Compile program Execute program and evaluate results

© 2006 Pearson Education 19 If time:  Do p.52 #1.1