1. Chapter 3 Syntax, Errors, and Debugging
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2. Ch 1 Quiz
Score Ranges: 44-50: : : : : : 1.5
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3. 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.
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4. 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.
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5. Objectives (cont.)
Understand the difference between Cartesian coordinates and screen coordinates.
Work with color and text properties.
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6. Vocabulary Arithmetic expression Comments Coordinate system Exception
Graphics context
Literal
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7. Vocabulary (cont.) Logic error Origin Package Pseudocode
Reserved words
Run-time error
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8. Vocabulary (cont.) Screen coordinate system Semantics Syntax Virus
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9. 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
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10. Language Elements (cont.)
Table 3-1: Some Java vocabulary
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11. 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.
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12. Basic Java Syntax and Semantics
Two categories of data types:
1. Primitive data types: Numbers, characters, and Booleans
2. Objects
Syntax for manipulating primitive data types differs than for objects
Primitive data types are combined in expressions with operators.
Objects are sent messages.
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13. Basic Java Syntax and Semantics (cont.)
Objects must be instantiated before use.
Unlike primitives
String objects are a little different.
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.
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14. Basic Java Syntax and Semantics (cont.)
Table 3-2: Some Java numeric data types
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15. Basic Java Syntax and Semantics (cont.)
Literals: Items whose values do not change.
The number 5.0 or the string “Java”
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.
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16. Basic Java Syntax and Semantics (cont.)
Figure 3-1: Changing the value of a variable
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17. Basic Java Syntax and Semantics (cont.)
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)
double d = 2.45; (declares and initializes a variable)
Constants are variables whose value cannot change.
final double PI = 3.14;
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18. Basic Java Syntax and Semantics (cont.)
Assignment statements:
<variable> = <expression>;
Value of expression assigned to variable
Arithmetic expressions:
Multiplication and division have higher precedence than addition and subtraction.
Operators of same precedence evaluated from left to right.
Parentheses are used to change evaluation order.
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19. Basic Java Syntax and Semantics (cont.)
Table 3-5: Common operators and their precedence
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20. 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 a double.
The modulus operator (%) yields a remainder.
11 % 3 yields 2.
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21. Basic Java Syntax and Semantics (cont.)
Table 3-6: Examples of expressions and their values
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22. 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)
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23. 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
Place the desired data type within parentheses before the variable or expression that will be cast to another data type.
int x = (int)(d + 1.6);
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24. Basic Java Syntax and Semantics (cont.)
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
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25. 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
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26. 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
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27. Basic Java Syntax and Semantics (cont.)
Table 3-7: Java’s reserved words
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28. Terminal I/O for Different Data Types
Table 3-8: Methods in class Scanner
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29. Terminal I/O for Different Data Types (cont.)
Example 3-1: Tests three types of input data
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30. 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 */
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31. 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
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32. 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
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33. 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
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34. Debugging
One debugging method is to add extra lines of code to print values of selected variables at strategic points in the program.
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35. 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
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36. Graphics and GUIs: Drawing Shapes and Text (cont.)
Example 3.5: Empty color panel
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37. 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
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38. Graphics and GUIs: Drawing Shapes and Text (cont.)
Figure 3-7: Orientation of Java’s coordinate system
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39. 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.
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40. Graphics and GUIs: Drawing Shapes and Text (cont.)
Table 3-9: Common method in the Graphics class
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41. Graphics and GUIs: Drawing Shapes and Text (cont.)
Table 3-9: Common method in the Graphics class (cont.)
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42. Graphics and GUIs: Drawing Shapes and Text (cont.)
Table 3-9: Common method in the Graphics class (cont.)
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43. 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
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44. Graphics and GUIs: Drawing Shapes and Text (cont.)
Example 3.6: Colored panel containing a red text message in a blue rectangle
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45. Graphics and GUIs: Drawing Shapes and Text (cont.)
Figure 3-8: Displaying a shape and text in a panel
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46. 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);
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47. 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.
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48. 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.
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49. 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.
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50. 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.
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