Transcript Chapter 3 Syntax, Errors, and Debugging

Chapter 3
Syntax, Errors, and Debugging
Fundamentals of Java
Objectives





2
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.
Fundamentals of Java
Objectives (cont.)



3
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.
Fundamentals of Java
Objectives (cont.)


4
Understand the difference between Cartesian
coordinates and screen coordinates.
Work with color and text properties.
Fundamentals of Java
Vocabulary






5
Arithmetic expression
Comments
Coordinate system
Exception
Graphics context
Literal
Fundamentals of Java
Vocabulary (cont.)






6
Logic error
Origin
Package
Pseudocode
Reserved words
Run-time error
Fundamentals of Java
Vocabulary (cont.)




7
Screen coordinate system
Semantics
Syntax
Virus
Fundamentals of Java
Language Elements

Every language, including Java has:
–
–
–
8
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
Fundamentals of Java
Language Elements (cont.)
Table 3-1: Some Java vocabulary
9
Fundamentals of Java
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
10
detail must be present.
Fundamentals of Java
Basic Java Syntax and Semantics

Two categories of data types:
–
–

Syntax for manipulating primitive data types
differs than for objects
–
–
11
1. Primitive data types: Numbers, characters,
and Booleans
2. Objects
Primitive data types are combined in expressions
with operators.
Objects are sent messages.
Fundamentals of Java
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
12
represents a different range of values.
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)
Table 3-2: Some Java numeric data types
13
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)

Literals: Items whose values do not change.
–

Variable is a named location in memory.
–
–
14
The number 5.0 or the string “Java”
Changing a variable’s value is equivalent to
replacing the value at the memory location.
A variable’s data type cannot change.
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)
Figure 3-1: Changing the value of a variable
15
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)

Variable declaration statement: Declares
the identifier and data type for a variable
–
–
–

Constants are variables whose value cannot
change.
–
16
int age; (declares one int variable)
int a, b, c; (declares three int variables)
double d = 2.45; (declares and initializes
a variable)
final double PI = 3.14;
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)


Assignment statements:
–
<variable> = <expression>;
–
Value of expression assigned to variable
Arithmetic expressions:
–
–
–
17
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.
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)
18
Table 3-5: Common operators and their precedence
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)

The semantics of division (/) differ for
integers and floating-point operators.
–
–

The modulus operator (%) yields a remainder.
–
19
int / int yields an int.
double / double yields a double.
11 % 3 yields 2.
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)
20
Table 3-6: Examples of expressions and their values
Fundamentals of Java
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
–
21
Lower-precision data types (int) temporarily
converted to high-precision data types
(double)
Fundamentals of Java
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
22
x = (int)(d + 1.6);
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)

String concatenation: Append a String
or value to another String
–
–
–

Escape character (\): Used in codes to
represent characters that cannot be directly
typed into a program
–
23
Use the + operator
String s = “string1” + “string2”;
String s2 = “String1” + intVariable1;
“\t” is a tab character
Fundamentals of Java
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
24
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)

Method signature:
–
–

Method and variable names are user
defined symbols.
–

25
Method name
Number and data types of method parameters
Cannot use Java keywords (reserved words)
Packages: Used to organize related classes
into a single unit for distribution
Fundamentals of Java
Basic Java Syntax and
Semantics (cont.)
26
Table 3-7: Java’s reserved words
Fundamentals of Java
Terminal I/O for Different
Data Types
Table 3-8: Methods in class Scanner
27
Fundamentals of Java
Terminal I/O for Different
Data Types (cont.)
Example 3-1: Tests three types of input data
28
Fundamentals of Java
Comments




29
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 */
Fundamentals of Java
Comments (cont.)

Typical uses of comments:
–
–
–
–
30
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
Fundamentals of Java
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
31
Fundamentals of Java
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.
–
32
Desk checking: Examine code immediately after
it is written
Fundamentals of Java
Debugging

33
One debugging method is to add extra lines
of code to print values of selected variables
at strategic points in the program.
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text

Defining a specialized graphics panel: Define
a new class that extends the JPanel class
–
34
Inherits all of the properties and methods of a
JPanel, but can add additional instance
variables and methods
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)
Example 3.5: Empty color panel
35
Fundamentals of Java
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
36
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)
Figure 3-7: Orientation of Java’s coordinate system
37
Fundamentals of Java
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
38
color via the setColor() method.
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)
Table 3-9: Common method in the Graphics class
39
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)
Table 3-9: Common method in the Graphics class (cont.)
40
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)
Table 3-9: Common method in the Graphics class (cont.)
41
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)

Every panel instance has a
paintComponent() method
–
–
–
42
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
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)
Example 3.6: Colored panel containing a
red text message in a blue rectangle
43
Fundamentals of Java
Graphics and GUIs: Drawing
Shapes and Text (cont.)
Figure 3-8: Displaying a shape and text in a panel
44
Fundamentals of Java
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
–
–
45
Font name, size, and style
Font font = new Font(“Arial”,
Font.BOLD, 10);
Fundamentals of Java
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.


46
Fundamentals of Java
Summary (cont.)





47
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.
Fundamentals of Java
Summary (cont.)



48
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.
Fundamentals of Java
Summary (cont.)

Java uses a screen coordinate system to
locate the positions of pixels in a window or
panel.
–
–
49
Origin is the upper-left corner of the drawing
area.
x and y axes increase to the right and
downward.
Fundamentals of Java