This document covers fundamentals of Java syntax, errors, and debugging. It discusses Java language elements like variables, data types, expressions, and methods. It also describes different types of errors like syntax errors and logic errors. Finally, it discusses graphics and GUIs, including drawing shapes and text using a coordinate system.

1. Chapter 3
Syntax, 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 variable’s value is equivalent to
replacing the value at the memory location.
– A variable’s 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:
– <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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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 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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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: Java’s 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 Java’s 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 superclass’s 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.