Chapter 3 Syntax, Errors, and Debugging

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Chapter 3Syntax, Errors, and Debugging

• Fundamentals of Java

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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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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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Objectives (cont.)

• Understand the difference between Cartesian
  coordinates and screen coordinates.
• Work with color and text properties.

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Vocabulary

• Arithmetic expression
• Comments
• Coordinate system
• Exception
• Graphics context
• Literal

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Vocabulary (cont.)

• Logic error
• Origin
• Package
• Pseudocode
• Reserved words
• Run-time error

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Vocabulary (cont.)

• Screen coordinate system
• Semantics
• Syntax
• Virus

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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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Language Elements (cont.)
Table 3-1 Some Java vocabulary
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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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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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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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Basic Java Syntax and Semantics (cont.)
Table 3-2 Some Java numeric data types
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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 variables value is equivalent to
  replacing the value at the memory location.
• A variables data type cannot change.

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Basic Java Syntax and Semantics (cont.)
Figure 3-1 Changing the value of a variable
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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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Basic Java Syntax and Semantics (cont.)

• Assignment statements
• ltvariablegt ltexpressiongt
• 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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Basic Java Syntax and Semantics (cont.)
Table 3-5 Common operators and their precedence
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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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Basic Java Syntax and Semantics (cont.)
Table 3-6 Examples of expressions and their
values
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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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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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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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Basic Java Syntax and Semantics (cont.)

• The String classs 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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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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Basic Java Syntax and Semantics (cont.)
Table 3-7 Javas reserved words
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Terminal I/O for Different Data Types
Table 3-8 Methods in class Scanner
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Terminal I/O for Different Data Types (cont.)
Example 3-1 Tests three types of input data
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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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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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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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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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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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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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Graphics and GUIs Drawing Shapes and Text (cont.)
Example 3.5 Empty color panel
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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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Graphics and GUIs Drawing Shapes and Text (cont.)
Figure 3-7 Orientation of Javas coordinate
system
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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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Graphics and GUIs Drawing Shapes and Text (cont.)
Table 3-9 Common method in the Graphics class
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Graphics and GUIs Drawing Shapes and Text (cont.)
Table 3-9 Common method in the Graphics class
(cont.)
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Graphics and GUIs Drawing Shapes and Text (cont.)
Table 3-9 Common method in the Graphics class
(cont.)
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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 superclasss
  paintComponent() method

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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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Graphics and GUIs Drawing Shapes and Text (cont.)
Figure 3-8 Displaying a shape and text in a panel
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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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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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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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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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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.