Java Syntax and Style JavaMethods An Introduction to Object-Oriented Programming Maria Litvin Gary Litvin Copyright © 2003 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved. TM /** * Chapter 5 */

5-2 Objectives: l Learn to distinguish the required syntax from the conventional style l Learn when to use comments and how to mark them l Review reserved words and standard names l Learn the proper style for naming classes, methods, and variables l Learn to space and indent blocks of code

5-3 Comments l Comments are notes in plain English inserted in the source code. l Comments are used to: –document the program’s purpose, author, revision history, copyright notices, etc. –describe fields, constructors, and methods –explain obscure or unusual places in the code –temporarily “comment out” fragments of code

5-4 Formats for Comments l A “block” comment is placed between /* and */ marks: /* Exercise 5-2 for Java Methods Author: Miss Brace Date: 3/5/2010 Rev. 1.0 */ l A single-line comment goes from // to the end of the line: wt * = ; // Convert to kilograms

5-5 Documentation Comments l Used by the special utility program javadoc to automatically generate documentation in HTML format from the source code l Should precede a class, a method, or a field l Can use special javadoc – describes a parameter of a - describes the method’s return value

5-6 javadoc Comments (cont’d) /** * Returns total sales from all vendors; * sets totalSales * to 0. * total amount of sales from all vendors */ /** indicates a javadoc comment Can use HTML tags Common style

5-7 Reserved Words l In Java a number of words are reserved for a special purpose. l Reserved words use only lowercase letters. l Reserved words include: –primitive data types: int, double, char, boolean, etc. –storage modifiers: public, private, static, final, etc. –control statements: if, else, switch, while, for, etc. –built-in constants: true, false, null l There are about 50 reserved words total.

5-8 Programmer-Defined Names l In addition to reserved words, Java uses standard names for library packages and classes: String, Graphics, javax.swing, JApplet, JButton, ActionListener, java.awt l The programmer gives names to his or her classes, methods, fields, and variables.

5-9 Names (cont’d) l Syntax: A name can include: –upper- and lowercase letters –digits –underscore characters l Syntax: A name cannot begin with a digit. l Style: Names should be descriptive to improve readability.

5-10 Names (cont’d) l Programmers follow strict style conventions. l Style: Names of classes begin with an uppercase letter, subsequent words are capitalized: public class FallingCube l Style: Names of methods, fields, and variables begin with a lowercase letter, subsequent words are capitalized. private final int delay = 30; public void dropCube()

5-11 Names (cont’d) l Method names often sound like verbs: setBackground, getText, dropCube, start l Field names often sound like nouns: cube, delay, button, whiteboard l Constants sometimes use all caps: PI, CUBESIZE l It is OK to use standard short names for temporary “throwaway” variables: i, k, x, y, str

5-12 Syntax vs. Style l Syntax is part of the language. The compiler checks it. l Style is a convention widely adopted by software professionals. l The main purpose of style is to improve the readability of programs.

5-13 Syntax l The compiler catches syntax errors and generates error messages. l Text in comments and literal strings within double quotes are excluded from syntax checking. l Before compiling, carefully read your code a couple of times to check for syntax and logic errors.

5-14 Syntax (cont’d) l Pay attention to and check for: –matching braces { }, parentheses ( ), and brackets [ ] –missing and extraneous semicolons –correct symbols for operators +, -, =, <, <=, ==, ++, &&, etc. –correct spelling of reserved words, library names and programmer-defined names, including case

5-15 Syntax (cont’d) l Common syntax errors: Missing closing brace Public static int abs (int x) { If (x < 0); { x = - x } return x; public static int sign (int x)... Extraneous semicolon Spelling (p  P, if  If) Missing semicolon

5-16 Style l Arrange code on separate lines; insert blank lines between fragments of code. l Use comments. l Indent blocks within braces.

5-17 Style (cont’d) public boolean moveDown(){if (cubeY<6*cubeX) {cubeY+=yStep; return true;}else return false;} public boolean moveDown() { if (cubeY < 6 * cubeX) { cubeY += yStep; return true; } else { return false; } Before:After: Compiles fine!

5-18 Style (cont’d) public void fill (char ch) { int rows = grid.length, cols = grid[0].length; int r, c; for (r = 0; r < rows; r++) { for (c = 0; c < cols; c++) { grid[r][c] = ch; } Add blank lines for readability Add spaces around operators and after semicolons

5-19 Blocks, Indentation l Java code consists mainly of declarations and control statements. l Declarations describe objects and methods. l Control statement describe actions. l Declarations and control statements end with a semicolon. l No semicolon is used after a closing brace (except certain array declarations).

5-20 l Braces divide code into nested blocks. l A block in braces indicates a number of statements that form one compound statement. l Statements inside a block are indented, usually by two spaces or one tab. Blocks, Indentation (cont’d)

5-21 Blocks, Indentation (cont’d) public void fill (char ch) { int rows = grid.length, cols = grid[0].length; int r, c; for (r = 0; r < rows; r++) { for (c = 0; c < cols; c++) { grid[r][c] = ch; }

5-22 Review: l Name as many uses of comments as you can. l What does javadoc do? l Explain the difference between syntax and style. l Why is style important? l Roughly how many reserved words does Java have?

5-23 Review (cont’d): l Explain the convention for naming classes, methods and variables. Which of the following are syntactically valid names for variables: C, _denom_, my. num, AvgScore? Which of them are in good style? l What can happen if you put an extra semicolon in your program? l What are braces used for in Java? l Is indentation required by Java syntax or style?