Chapter 3 Syntax, Errors, and Debugging
Objectives Construct and use numeric and string literals. Name and use variables and constants. Create arithmetic expressions. Understand the precedence of different arithmetic operators. Concatenate two strings or a number and a string. 2
Objectives (cont.) Know how and when to use comments in a program. Tell the difference between syntax errors, run-time errors, and logic errors. Insert output statements to debug a program. 3
Objectives (cont.) Understand the difference between Cartesian coordinates and screen coordinates. Work with color and text properties. 4
Vocabulary Arithmetic expression Comments Coordinate system Exception Graphics context Literal 5
Vocabulary (cont.) Logic error Origin Package Pseudocode Reserved words Run-time error 6
Vocabulary (cont.) Screen coordinate system Semantics Syntax Virus 7
Language Elements Every language, including Java has: Vocabulary: Set of all of the words and symbols in the language Syntax: Rules for combining words into sentences (statements) Semantics: Rules for interpreting the meaning of statements 8
Language Elements (cont.) Table 3-1: Some Java vocabulary 9
Language Elements (cont.) Programming vs. natural languages Programming languages have small vocabularies and simple syntax and semantics. Programming language syntax must be absolutely correct. Programming language statements are interpreted literally. Every detail must be present. 10
Basic Java Syntax and Semantics 11 Two categories of data types: 1. Primitive data types: Numbers, characters, and Booleans 2. Objects ex; Strings Syntax for manipulating primitive data types differs than for objects Primitive data types are combined in expressions with operators. Objects are sent messages.
Basic Java Syntax and Semantics (cont.) 12 Objects must be instantiated before use. Unlike primitives String objects are a little different; are objects & are sent messages, but don t need to be instantiated before use. Six numeric data types int and double are most commonly used Also short, long, byte, and float Each uses a different number of bytes for storage. Each represents a different range of values. Learn more later.
Basic Java Syntax and Semantics (cont.) Table 3-2: Some Java numeric data types 13
Basic Java Syntax and Semantics (cont.) Literals: Items whose values do not change. Examples the number 5.0 or the string Java Restricted to primitive data & strings. Variable is a named location in memory. Changing a variable s value is equivalent to replacing the value at the memory location. A variable s data type cannot change. 14
Basic Java Syntax and Semantics (cont.) Figure 3-1: Changing the value of a variable 15
Basic Java Syntax and Semantics (cont.) 16 Variable declaration statement: Declares the identifier and data type for a variable int age; (declares one int variable) int a, b, c; (declares three int variables) int x, y, z = 7; double p, q = 1.41, pi = 3.14, t; String name = Bill Jones ; Scanner reader = new Scanner(System.in); double d = 2.45; (declares and initializes a variable) You can declare several variables in a single declaration & simultaneously assign them initial values Declares the object variable reader, instantiates or creates a Scanner object that is attached to the keyboard input stream, System.in, and finally assigns the object to the object variable, reader.
Basic Java Syntax and Semantics (cont.) Constants are variables whose value cannot change. final double PI = 3.14; Customary to write the names of constants in uppercase. 17
Basic Java Syntax and Semantics (cont.) 18 Assignment statements: <variable> = <expression>; Value of expression assigned to variable Arithmetic expressions: EXAMPLE: double celsius, fahrenheit; String name; Scanner reader; fahrenheit = reader.nextdouble(); celsius = (fahrenheit 32.0) * 5.0/9.0; Multiplication and division have higher precedence than addition name = Bill and Smith ; subtraction. reader = new Scanner(System.in); Operators of same precedence evaluated from left to right. Parentheses are used to change evaluation order.
Basic Java Syntax and Semantics (cont.) 19 Table 3-5: Common operators and their precedence
Basic Java Syntax and Semantics (cont.) The semantics of division (/) differ for integers and floating-point operators. int / int yields an int. double / double yields EXAMPLES: a double. The modulus operator (%) yields 5.0/2.0 yields a remainder. 2.5 11 % 3 yields 2. Division: 5/2 yields 2 Modulus: 9 % 5 yields 4 9.3 % 5.1 yields 4.2 20
Basic Java Syntax and Semantics (cont.) 21 Table 3-6: Examples of expressions and their values
Basic Java Syntax and Semantics (cont.) Arithmetic overflow: Assigning a value to a variable that is outside of the ranges of values that the data type can represent Mixed-mode arithmetic: Expressions involving integer and floating-point values Lower-precision data types (int) temporarily converted to high-precision data types (double) 22
Basic Java Syntax and Semantics (cont.) Type casting: Temporarily converting one data type to another Can type cast a single variable or an entire expression int i; double d; Place the desired i = (int)3.14; data type i equals within 3, truncation parentheses toward 0 d = (double)5/4; d equals 1.25 before the variable or expression that will be cast to another data type. int x = (int)(d + 1.6); 23
Basic Java Syntax and Semantics (cont.) 24 String concatenation: Append a String or value to another String Use the + operator String s = string1 + string2 ; String s2 = String1 + intvariable1; Escape character (\): Used in codes to represent characters that cannot be directly typed into a program \t is a tab character
Basic Java Syntax and Semantics (cont.) The String class s length method gives the number of characters in a String. Classes implement methods, and objects are instances of classes. Objects can respond to a message only if their class implements the method. Must implement a method with a matching signature 25
Basic Java Syntax and Semantics (cont.) Method signature: Method name Number and data types of method parameters Method and variable names are user defined symbols. Cannot use Java keywords (reserved words) Packages: Used to organize related classes into a single unit for distribution 26
Basic Java Syntax and Semantics (cont.) 27 Table 3-7: Java s reserved words
Terminal I/O for Different Data Types Table 3-8: Methods in class Scanner 28
Terminal I/O for Different Data Types (cont.) 29 Example 3-1: Tests three types of input data
Comments Explanatory sentences inserted in a program Compiler ignores them Purpose is to make program more readable Two varieties: End of line comments: All text following a double slash (//) on a single line Multiline comments: All text occurring between a /* and a */ 30
Comments (cont.) Typical uses of comments: Begin a program with a statement of its purpose Explain the purpose of a variable declaration Explain the purpose of a major segment of code Explain the workings of complex or tricky sections of code 31
Programming Errors Three types of programming errors: Syntax errors: When a syntax rule is violated Detected during compilation Compiler helps identify error Run-time errors: Occurs during execution Dividing by 0 Detected when program runs JVM indicates type of error and location 32
Programming Errors (cont.) Three types of programming errors (cont.): Logic errors (design errors or bugs): Incorrect logic implemented in the program Code may be correct in every other way, but does not do what it is supposed to do. Must thoroughly test and debug the program when an error is found. Desk checking: Examine code immediately after it is written 33
Debugging One debugging method is to add extra lines of code to print values of selected variables at strategic points in the program. 34
Graphics and GUIs: Drawing Shapes and Text Defining a specialized graphics panel: Define a new class that extends the JPanel class Inherits all of the properties and methods of a JPanel, but can add additional instance variables and methods 35
Graphics and GUIs: Drawing Shapes and Text (cont.) Example 3.5: Empty color panel 36
Graphics and GUIs: Drawing Shapes and Text (cont.) Every graphics application uses a coordinate system. Positions of items on a window specified in terms of two-dimensional points Java uses the screen coordinate system: The origin (point with coordinates (0,0)) located at upper-left corner of a panel or frame Every window, frame, or other type of window has own coordinate system 37
Graphics and GUIs: Drawing Shapes and Text (cont.) Figure 3-7: Orientation of Java s coordinate system 38
Graphics and GUIs: Drawing Shapes and Text (cont.) Graphics class: Used to draw on a panel Every panel maintains an instance of this class. The graphics context Shapes drawn on a panel by the Graphics class have a foreground color. Change color via the setcolor() method. 39
Graphics and GUIs: Drawing Shapes and Text (cont.) 40 Table 3-9: Common method in the Graphics class
Graphics and GUIs: Drawing Shapes and Text (cont.) 41 Table 3-9: Common method in the Graphics class (cont.)
Graphics and GUIs: Drawing Shapes and Text (cont.) 42 Table 3-9: Common method in the Graphics class (cont.)
Graphics and GUIs: Drawing Shapes and Text (cont.) Every panel instance has a paintcomponent() method Called by the JVM when the panel needs to be drawn on the screen Contains instructions for how to draw the panel For custom panels, can write own paintcomponent() method, but must also call the superclass s paintcomponent() method 43
Graphics and GUIs: Drawing Shapes and Text (cont.) 44 Example 3.6: Colored panel containing a red text message in a blue rectangle
Graphics and GUIs: Drawing Shapes and Text (cont.) Figure 3-8: Displaying a shape and text in a panel 45
Graphics and GUIs: Drawing Shapes and Text (cont.) The width and height of a panel can be found using the getwidth() and getheight() methods, respectively. Font class: Information about a specific font Font name, size, and style Font font = new Font( Arial, Font.BOLD, 10); 46
Summary Use the int data type for whole numbers and double for floating-point numbers. Variable and method names consist of a letter followed by additional letters or digits. Keywords cannot be used as names. Final variables behave as constants; their values cannot change after they are declared. 47
Summary (cont.) Arithmetic expressions are evaluated according to precedence. Some expressions yield different results for integer and floating-point operands. Strings may be concatenated. The compiler catches syntax errors. The JVM catches run-time errors. 48
Summary (cont.) Logic errors, if caught, are detected by the programmer or user at run-time. Can find and remove logic errors by inserting debugging output statements to view the values of variables. The programmer can modify the color with which images are drawn and the properties of text fonts for a given graphics object. 49
Summary (cont.) Java uses a screen coordinate system to locate the positions of pixels in a window or panel. Origin is the upper-left corner of the drawing area. x and y axes increase to the right and downward. 50