liang, introduction to java programming,revised by dai-kaiyu 1 chapter 2 primitive data types and...

Liang, Introduction to Java Programming,revised by Dai-kaiyu1

Chapter 2 Primitive Data Types and Operations

Chapter 1 Introduction to Computers, Programs, and Java

Chapter 2 Primitive Data Types and Operations

Chapter 3 Control Statements

Chapter 5 Arrays

Chapter 4 Methods

Basic computer skills such as using Windows, Internet Explorer, and Microsoft Word

Prerequisites for Part I

事实不可扭曲，意见大可自由

—— C.P.Scott


Objectives To write Java programs to perform simple calculations (§2.2). To use identifiers to name variables, constants, methods, and classes (§2.3). To use variables to store data (§2.4-2.5). To program with assignment statements and assignment expressions (§2.5). To use constants to store permanent data (§2.6). To declare Java primitive data types: byte, short, int, long, float, double, char, and

boolean (§2.7 – 2.10). To use Java operators to write expressions (§2.7 – 2.10). To know the rules governing operand evaluation order, operator precedence, and

operator associativity (§2.11 – 2.12). To represent a string using the String type. (§2.13) To obtain input using the JOptionPane input dialog boxes (§2.14). To obtain input from console (§2.16 Optional). To format output using JDK 1.5 printf (§2.17). To become familiar with Java documentation, programming style, and naming

conventions (§2.18). To distinguish syntax errors, runtime errors, and logic errors (§2.19). To debug logic errors (§2.20).


Introducing Programming with an Example

Example 2.1 Computing the Area of a Circle

ComputeAreaComputeArea

RunRun

Program = Algorithm + Data structure


Trace a Program Executionpublic class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); }}

no valueradius

allocate memory for radius



no valueradius

memory

no valuearea

allocate memory for area



20radius

no valuearea

assign 20 to radius



20radius

memory

1256.636area

compute area and assign it to variable area



20radius

memory

1256.636area

print a message to the console

A string constant should not cross lines


Identifiers An identifier is a sequence of characters that consist of

letters, digits, underscores (_), and dollar signs ($).

An identifier must start with a letter, an underscore (_), or a dollar sign ($). It cannot start with a digit. An identifier cannot be a reserved word.

An identifier can be of any length.

Are used for naming variables, constants, methods, classes, and packages


Memory ConceptsVariables

Every variable has a name, a type, a size and a value

Name corresponds to location in memory

When new value is placed into a variable, replaces (and destroys) previous value

Reading variables from memory does not change them


2.6 Memory Concepts

Visual Representation Sum = 0; number1 = 1; number2 = 2;

Sum = number1 + number2; after execution of statement

sum 0

sum 3

number1

number2

1

2

number1

number2

1

2


Variables

Used to store data in a program

// Compute the first arearadius = 1.0;area = radius * radius * 3.14159;System.out.println("The area is " + area + " for radius "+radius);

// Compute the second arearadius = 2.0;area = radius * radius * 3.14159;System.out.println("The area is “ + area + " for radius "+radius);


Declaring Variables

Giving the name and the data type of variables

int x; // Declare x to be an // integer variable;

double radius; // Declare radius to // be a double variable;

char a; // Declare a to be a // character variable;


Java data types

Data type is the classification of forms of information

Data type is declared using keywords

Java is strongly typed


Assignment Statements

Give a value to the declared variable

x = 1; // Assign 1 to x;

radius = 1.0; // Assign 1.0 to radius;

a = 'A'; // Assign 'A' to a;


Assignment Expressions

Expression: a computation involving values, variables, and operators that evaluates to a value.

X = 5 * ( 3 / 2 ) + 3 * 2;

Assignment expressions:

i = j = k = 1;


Declaring and Initializingin One Step

int x = 1;

double d = 1.4;

float f = 1.4;

Is this statement correct?

A variable in method must be assigned a value before it can be used


Constants

final double PI = 3.14159;

final int SIZE = 3;

final datatype CONSTANTNAME = VALUE;


Numerical Data Types

byte 8 bits

short 16 bits

int 32 bits

long 64 bits

float 32 bits

double 64 bits

type size range

byte 1byte -128 ~ 127

short 2bytes -215 ~ 215-1

int 4bytes -231 ~ 231-1

long 8bytes -263 ~ 263-1


Operators

+, -, *, /, and %

5 / 2 yields an integer 2.

5.0 / 2 yields a double value 2.5

5 % 2 yields 1 (the remainder of the division)


Remainder Operatordetermine whether a number is even or odd using %.

Suppose you know January 1, 2005 is Saturday, you can find that the day for February 1, 2005 is Tuesday using the following expression:

Saturday is the 6th day in a week A week has 7 days

January has 31 days The 2nd day in a week is Tuesday

(6 + 31) % 7 is 2


NOTE

Calculations involving floating-point numbers are approximated because these numbers are not stored with complete accuracy.

Integers are stored precisely. Therefore, calculations with integers yield a precise integer result.

Show case FloatIsNotPrecise.java


Number Literals

A literal is a constant value that appears directly in the program. For example, 34, 1,000,000, and 5.0 are literals in the following statements:

int i = 34;

long x = 1000000;

double d = 5.0;

byte b = 1000 Wrong

A compilation error would occur if the literal were too large for the variable to hold.


Number LiteralsAn integer literal is assumed to be of the int type, whose value is between -231 (-2147483648) to 231–1 (2147483647). To denote an integer literal of the long type, append it with the letter L or l.

long j = 10; Is this statement correct?

By default, a floating-point literal is treated as a double type value. You can make a number a float by appending the letter f or F, and make a number a double by appending the letter d or D.

Floating-point literals can also be specified in scientific notation, for example, 1.23456e+2

correct


Arithmetic Expressions

)94

(9))(5(10

5

43

y

x

xx

cbayx

is translated to

(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)


Shortcut Assignment Operators

Operator Example Equivalent

+= i+=8 i = i+8

-= f-=8.0 f = f-8.0

*= i*=8 i = i*8

/= i/=8 i = i/8

%= i%=8 i = i%8


Increment andDecrement Operators

Operator Name Description++var preincrement The expression (++var)

increments var by 1 and evaluates to the new value in var after the

increment.var++ postincrement The expression (var++) evaluates

to the original value in var and increments var by 1.

--var predecrement The expression (--var) decrements var by 1 and evaluates

to the new value in var after the decrement.

var-- postdecrement The expression (var--) evaluates to the original value

in var and decrements var by 1.


Increment andDecrement Operators, cont.

int i = 10; int newNum = 10 * i++;

int newNum = 10 * i; i = i + 1;

Same effect as

int i = 10; int newNum = 10 * (++i);

i = i + 1; int newNum = 10 * i;

Same effect as


Assignment Expressions and Assignment Statements

Prior to Java 2, all the expressions can be used as statements. Since Java 2, only the following types of expressions can be statements:

variable op= expression; // Where op is +, -, *, /, or %

++variable;

variable++;

--variable;

variable--;

补充了解


Numeric Type Conversion

Consider the following statements:

long k = i * 3 + 4;

double d = i * 3.1 + k / 2;


Conversion Rules

When performing a binary operation involving two operands of different types, Java automatically converts the operand based on the following rules: 1. If one of the operands is double, the other is converted into double.2. Otherwise, if one of the operands is float, the other is converted into float.3. Otherwise, if one of the operands is long, the other is converted into long.4. Otherwise, both operands are converted into int.


Conversion Rules

Datatype of oprand1

Datatype of oprand2

After convertion

byte 、 short 、 char

　　　 int 　　　　 int

byte 、 short 、 char 、 int

　　　 long 　　　　 long

byte 、 short 、 char 、 int 、 long

　　　 float 　　　　 float

byte 、 short 、 char 、 int 、 long 、 float

　　　 double 　　　　 double


Type Casting

Implicit casting double d = 3; (type widening)

Explicit casting int i = (int)3.0; (type narrowing) int i = (int)3.9; (Fraction part is truncated) What is wrong? int x = 5 / 2.0;


Type Casting

For a variable of type int , explicit casting must be used

int i = 1;

byte b = i ;

For a literal of type integer, if in the persission range of short or byte. Explicit casting is not needed

byte i = 1;

wrong


Character Data Type

char letter = 'A'; (ASCII)

char numChar = '4'; (ASCII)

char letter = '\u0041'; (Unicode)

char numChar = '\u0034'; (Unicode)

Four hexadecimal digits.

NOTE: The increment and decrement operators can also be used on char variables to get the next or preceding Unicode character. the following statements display character b.

char ch = 'a';

System.out.println(++ch);


Unicode Format

Encoding: convert a character to its binary representation

Java characters use Unicode, a 16-bit encoding scheme

Unicode can represent 65,536 characters

Unicode takes two bytes, preceded by \u, expressed in four hexadecimal numbers that run from '\u0000' to '\uFFFF'. So, Unicode can represent 65535 + 1 characters.

Unicode \u03b1 \u03b2 \u03b3 for three Greek letters


Escape Sequences for Special Characters

Description Escape Sequence Unicode

Backspace \b \u0008

Tab \t \u0009

Linefeed \n \u000A

Carriage return \r \u000D

Backslash \\ \u005C

Single Quote \' \u0027

Double Quote \" \u0022Show case Welcome4.java


Appendix B: ASCII Character Set

ASCII is a 7-bits encoding scheme

ASCII Character Set is a subset of the Unicode from \u0000 to \u007f


ASCII Character Set, cont.

ASCII Character Set is a subset of the Unicode from \u0000 to \u007f


Casting between char and Numeric Types

int i = 'a'; // Same as int i = (int)'a';

char c = 97; // Same as char c = (char)97;

char letter = ‘A’;

System.out.println( letter+10);

System.out.println((char)(letter+10));

Integerchar :lower sixteen bits are used

Floating-pointchar :first cast into an int

Char numeric type : Unicode is used


The boolean Type and Operators

six comparison operators (relational operators)

The result of the comparison is a Boolean value: true or false(can’t using 1 or 0). boolean b = (1 > 2);

Operator Name

< less than

<= less than or equal to

> greater than

>= greater than or equal to

== equal to

!= not equal to


Boolean Operators

Operator Name

! not

&& and

|| or

^ exclusive or

Also called logic operators


Truth Table for Operator !

p !p

true false

false true

Example

!(1 > 2) is true, because (1 > 2) is false.

!(1 > 0) is false, because (1 > 0) is true.


Truth Table for Operator &&

p1 p2 p1 && p2

false false false

false true false

true false false

true true true

Example

(3 > 2) && (5 >= 5) is true, because (3 > 2) and (5 >= 5) are both true.

(3 > 2) && (5 > 5) is false, because (5 > 5) is false.


Truth Table for Operator ||

p1 p2 p1 || p2

false false false

false true true

true false true

true true true

Example

(2 > 3) || (5 > 5) is false, because (2 > 3) and (5 > 5) are both false.

(3 > 2) || (5 > 5) is true, because (3 > 2) is true.


Truth Table for Operator ^

p1 p2 p1 ^ p2

false false false

false true true

true false true

true true false

Example

(2 > 3) ^ (5 > 1) is true, because (2 > 3) is false and (5 > 1) is true.

(3 > 2) ^ (5 > 1) is false, because both (3 > 2) and (5 > 1) are true.

One of p1 and p2 is true, but not both


Examples

System.out.println("Is " + num + " divisible by 2 and 3? " +

((num % 2 == 0) && (num % 3 == 0)));

System.out.println("Is " + num + " divisible by 2 or 3? " +

((num % 2 == 0) || (num % 3 == 0)));

System.out.println("Is " + num +

" divisible by 2 or 3, but not both? " +

((num % 2 == 0) ^ (num % 3 == 0)));


Leap Year?

A year is a leap year if it is divisible by 4 but not by 100 or if it is divisible by 400.

boolean isLeapYear =

((year % 4 == 0) && (year % 100 != 0)) ||

(year % 400 == 0);


The & and | Operators

&&: conditional AND operator (shortcut)&: unconditional AND operator||: conditional OR operator (shortcut)|: unconditional OR operator

exp1 && exp2(1 < x) && (x < 100)

(1 < x) & (x < 100)


The & and | Operators

If x is 1, what is x after this expression?(x > 1) & (x++ < 10)

If x is 1, what is x after this expression?(1 > x) && ( 1 > x++)

How about (1 == x) | (10 > x++)?(1 == x) || (10 > x++)?


Operator Precedence

How to evaluate 3 + 4 * 4 > 5 * (4 + 3) – 1?

The expression in the parentheses is evaluated first All binary operators except assignment operators are

left-associative. If operators with the same precedence are next to each

other, their associativity determines the order of evaluation. a – b + c – d is equivalent to ((a – b) + c) – d Assignment operators are right-associative. Therefore,

the expression a = b += c = 5 is equivalent to a = (b += (c = 5))


Operator Precedence var++, var-- +, - (Unary plus and minus), ++var,--var (type) Casting ! (Not) *, /, % (Multiplication, division, and remainder) +, - (Binary addition and subtraction) <, <=, >, >= (Comparison) ==, !=; (Equality) & (Unconditional AND) ^ (Exclusive OR) | (Unconditional OR) && (Conditional AND) Short-circuit AND || (Conditional OR) Short-circuit OR =, +=, -=, *=, /=, %= (Assignment operator)

. Preceeds casting


ExampleApplying the operator precedence and associativity rule, the expression 3 + 4 * 4 > 5 * (4 + 3) - 1 is evaluated as follows:

3 + 4 * 4 > 5 * (4 + 3) - 1 3 + 4 * 4 > 5 * 7 – 1 3 + 16 > 5 * 7 – 1 3 + 16 > 35 – 1 19 > 35 – 1 19 > 34 false

(1) inside parentheses first

(2) multiplication

(3) multiplication

(4) addition

(5) subtraction

(6) greater than


Operand Evaluation Order

Operands are evaluated from left to right in Java.

The left-hand operand of a binary operator is evaluated before any part of the right-hand operand is evaluated.

int a = 0;int x = ++a + a;

int a = 0;

int x = a + (++a); x becomes 1

x becomes 2


Rule of Evaluating an Expression

· Rule 1: Evaluate whatever subexpressions you can possibly evaluate from left to right. · Rule 2: The operators are applied according to their precedence, as shown in Table 2.11.· Rule 3: The associativity rule applies for two operators next to each other with the same precedence.


Rule of Evaluating an Expression

· Applying the rule, the expression 3 + 4 * 4 > 5 * (4 + 3) - 1 is evaluated as follows:

3 + 4 * 4 > 5 * (4 + 3) - 1 3 + 16 > 5 * (4 + 3) - 1 19 > 5 * (4 + 3) - 1 19 > 5 * 7 - 1 19 > 35 – 1 19 > 34 false

(1) 4 * 4 is the first subexpression that can be evaluated from left.

(2) 3 + 16 is evaluated now.

(3) 4 + 3 is now the leftmost subexpression

that should be evaluated.

(4) 5 * 7 is evaluated now.

(5) 35 – 1 is evaluated now.

(6) 19 > 34 is evaluated now.


The String Type represent a string of characters, use the data type called String. String message = "Welcome to Java"; String is actually a predefined class in the Java library The String type is not a primitive type. It is known as

a reference type. Any Java class can be used as a reference type for a variable.


String Concatenation

// Three strings are concatenatedString message = "Welcome " + "to " + "Java"; // String Chapter is concatenated with number 2String s = "Chapter" + 2; // s becomes Chapter2 // String Supplement is concatenated with character BString s1 = "Supplement" + 'B'; // s becomes SupplementB


String Concatenation

+ operator Addition String concatenation int y=5;

System.out.println("y+2="+y+2);

System.out.println(y+2+"y+2=");

System.out.println("y+2="+(y+2));

Show case Concatination.java


Obtaining Input

Two common ways of obtaining input.

1. Using JOptionPane input dialogs (§2.14)

2. Using the JDK 1.5 Scanner class (Supplement T)


Getting Input from Input Dialog Boxes

String string = JOptionPane.showInputDialog(

null, “Prompting Message”, “Dialog Title”,

JOptionPane.QUESTION_MESSAGE));


Message dialog type Icon Description

JOptionPane.ERROR_MESSAGE

Displays a dialog that indicates an error to the user.

JOptionPane.INFORMATION_MESSAGE

Displays a dialog with an informational message to the user. The user can simply dismiss the dialog.

JOptionPane.WARNING_MESSAGE

Displays a dialog that warns the user of a potential problem.

JOptionPane.QUESTION_MESSAGE

Displays a dialog that poses a question to the user. This dialog normally requires a response, such as clicking on a Yes or a No button.

JOptionPane.PLAIN_MESSAGE no icon Displays a dialog that simply contains a

message, with no icon.

JOptionPane constants for message dialogs.


Two Ways to Invoke the Method There are several ways to use the showInputDialog method. For example:

String string = JOptionPane.showInputDialog(null, x,

y, JOptionPane.QUESTION_MESSAGE));

where x is a string for the prompting message, and y is a string for the title of the input dialog box.

JOptionPane. showInputDialog(x);

where x is a string for the prompting message.


Converting Strings to Integers

To convert a string into an int value, you can use the static parseInt method in the Integer class as follows:

int intValue = Integer.parseInt(intString);

where intString is a numeric string such as “123”.


Converting Strings to Doubles

To convert a string into a double value, you can use the static parseDouble method in the Double class as follows:

double doubleValue =Double.parseDouble(doubleString);

where doubleString is a numeric string such as “123.45”.


Detailed case study: Adding Integers

Upcoming program Use input dialogs to input two values from user Use message dialog to display sum of the two values

Liang, Introduction to Java Programming,revised by Dai-kaiyu

68

1 // Fig. 2.9: Addition.java2 // An addition program.3 4 // Java extension packages5 import javax.swing.JOptionPane; // import class JOptionPane6 7 public class Addition {8 9 // main method begins execution of Java application10 public static void main( String args[] )11 {12 String firstNumber; // first string entered by user13 String secondNumber; // second string entered by user14 int number1; // first number to add15 int number2; // second number to add16 int sum; // sum of number1 and number217 18 // read in first number from user as a String19 firstNumber =20 JOptionPane.showInputDialog( "Enter first integer" );21 22 // read in second number from user as a String23 secondNumber =24 JOptionPane.showInputDialog( "Enter second integer" );25 26 // convert numbers from type String to type int27 number1 = Integer.parseInt( firstNumber ); 28 number2 = Integer.parseInt( secondNumber );29 30 // add the numbers31 sum = number1 + number2;32

Declare variables: name and data type.

Input first integer as a String, assign to firstNumber.

Add, place result in sum.

Convert strings to integers.

Liang, Introduction to Java Programming,revised by Dai-kaiyu

69

33 // display the results34 JOptionPane.showMessageDialog(35 null, "The sum is " + sum, "Results",36 JOptionPane.PLAIN_MESSAGE );37 38 System.exit( 0 ); // terminate application39 40 } // end method main41 42 } // end class Addition



Location of JOptionPane for use in the program

Begins public class Addition Recall that file name must be Addition.java

Lines 10-11: main

Declaration firstNumber and secondNumber are variables

5 import javax.swing.JOptionPane;

7 public class Addition {

12 String firstNumber; // first string entered by user13 String secondNumber; // second string entered by user



Variables Location in memory that stores a value

• Declare with name and data type before use firstNumber and secondNumber are of data type String

(package java.lang)• Hold strings

Variable name: any valid identifier Declarations end with semicolons ;

• Can declare multiple variables of the same type at a time• Use comma separated list

Can add comments to describe purpose of variables

String firstNumber, secondNumber;

12 String firstNumber; // first string entered by user13 String secondNumber; // second string entered by user


Good Practice

•meaningful varible name, self-documenting•String firstNumber;

•String secondNumber ；

•begin with lowercase letter



Declares variables number1, number2, and sum of type int

int holds integer values : i.e., 0, -4, 97 Data types float and double can hold decimal numbers Data type char can hold a single character: i.e., x, $, \n, 7

• Single letter, single digit, single special character and escape sequences.

Primitive data types(Built in data types) • Boolean, char, byte, short, int, long, float, double

14 int number1; // first number to add15 int number2; // second number to add16 int sum; // sum of number1 and number2



Reads String from the user, representing the first number to be added

Method JOptionPane.showInputDialog displays the following:

Message called a prompt - directs user to perform an action Argument appears as prompt text If wrong type of data entered (non-integer) or click Cancel,

error occurs (fault tolerant)

19 firstNumber =20 JOptionPane.showInputDialog( "Enter first integer" );



Result of call to showInputDialog given to firstNumber using assignment operator =

Assignment statement = binary operator - takes two operands

• Expression on right evaluated and assigned to variable on left Read as: firstNumber gets value of

JOptionPane.showInputDialog( "Enter first integer" )

19 firstNumber =20 JOptionPane.showInputDialog( "Enter first integer" );



Similar to previous statement Assigns variable secondNumber to second integer input

Method Integer.parseInt Converts String argument into an integer (type int)

• Class Integer in java.lang Integer returned by Integer.parseInt is assigned to variable

number1 (line 27)• Remember that number1 was declared as type int

Line 28 similar

23 secondNumber =24 JOptionPane.showInputDialog( "Enter second integer" );

27 number1 = Integer.parseInt( firstNumber ); 28 number2 = Integer.parseInt( secondNumber );



Assignment statement Calculates sum of number1 and number2 (right hand

side) Uses assignment operator = to assign result to variable

sum Read as: sum gets the value of number1 + number2 number1 and number2 are operands

31 sum = number1 + number2;

Good Practice :

Space on either side of binary operator(make it stand out)



Use showMessageDialog to display results "The sum is " + sum

Uses the operator + to "add" the string literal "The sum is" and sum

Concatenation of a String and another data type• Results in a new string• Automatic conversion of integer to string.

If sum contains 117, then "The sum is " + sum results in the new string "The sum is 117"

Note the space in "The sum is "

34 JOptionPane.showMessageDialog(35 null, "The sum is " + sum, "Results",36 JOptionPane.PLAIN_MESSAGE );



Different version of showMessageDialog Requires four arguments (instead of two as before) First argument: null for now Second: string to display Third: string in title bar Fourth: type of message dialog with icon

• Line 36 no icon: JOptionPane.PLAIN_MESSAGE

34 JOptionPane.showMessageDialog(35 null, "The sum is " + sum, "Results",36 JOptionPane.PLAIN_MESSAGE );


Example 2.2 Entering Input from Dialog Boxes

InputDialogDemoInputDialogDemo RunRun

This program first prompts the user to enter a year as an int value and checks if it is a leap year, it then prompts you to enter a double value and checks if it is positive.

A year is a leap year if it is divisible by 4 but not by 100, or it is divisible by 400.

((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0)


Example 2.3 Computing Loan Payments

ComputeLoanComputeLoan RunRun

This program lets the user enter the interest rate, number of years, and loan amount and computes monthly payment and total payment.

12)1(11

numOfYearserestRatemonthlyInt

erestRatemonthlyIntloanAmount


Example 2.4 Monetary Units

This program lets the user enter the amount in decimal representing dollars and cents and output a report listing the monetary equivalent in single dollars, quarters, dimes, nickels, and pennies. Your program should report maximum number of dollars, then the maximum number of quarters, and so on, in this order.

ComputeChangeComputeChange RunRun


Trace ComputeChange

int remainingAmount = (int)(amount * 100); // Find the number of one dollars int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100; // Find the number of quarters in the remaining amount int numberOfQuarters = remainingAmount / 25; remainingAmount = remainingAmount % 25; // Find the number of dimes in the remaining amount int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10; // Find the number of nickels in the remaining amount int numberOfNickels = remainingAmount / 5; remainingAmount = remainingAmount % 5; // Find the number of pennies in the remaining amount int numberOfPennies = remainingAmount;

1156remainingAmount

remainingAmount initialized

Suppose amount is 11.56


Trace ComputeChange


1156remainingAmount


11numberOfOneDollars

numberOfOneDollars assigned


Trace ComputeChange


56remainingAmount



remainingAmount updated


Trace ComputeChange


56remainingAmount



2numberOfOneQuarters

numberOfOneQuarters assigned


Trace ComputeChange


6remainingAmount



2numberOfQuarters

remainingAmount updated


Example 2.5 Displaying Current Time

Write a program that displays current time in GMT in the format hour:minute:second such as 1:45:19.

The currentTimeMillis method in the System class returns the current time in milliseconds since the midnight, January 1, 1970 GMT. (1970 was the year when the Unix operating system was formally introduced.) You can use this method to obtain the current time, and then compute the current second, minute, and hour as follows.

ShowCurrentTimeShowCurrentTime RunRun


Getting Input Using Scanner

1. Create a Scanner object

Scanner scanner = new Scanner(System.in);

2. Use the methods next(), nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), or nextBoolean() to obtain to a string, byte, short, int, long, float, double, or boolean value. For example,

System.out.print("Enter a double value: ");Scanner scanner = new Scanner(System.in);double d = scanner.nextDouble();TestScannerTestScanner RunRun

Optional Supplement T


Formatting Output

Use the new JDK 1.5 printf statement.

System.out.printf(format, item);

Where format is a string that may consist of substrings and format specifiers. A format specifier specifies how an item should be displayed. An item may be a numeric value, character, boolean value, or a string. Each specifier begins with a percent sign.

JDK 1.5Feature


Frequently-Used Specifiers JDK 1.5Feature

Specifier Output Example

%b a boolean value true or false

%c a character 'a'

%d a decimal integer 200

%f a floating-point number 45.460000

%e a number in standard scientific notation 4.556000e+01

%s a string "Java is cool" int count = 5;

double amount = 45.56;

System.out.printf("count is %d and amount is %f", count, amount);

display count is 5 and amount is 45.560000

items

Table 2.13 for detailed usage examples


Programming Style and Documentation

Appropriate CommentsNaming ConventionsProper Indentation and Spacing LinesBlock Styles

Coding conventions


Appropriate Comments

Include a summary at the beginning of the program to explain what the program does, its key features, its supporting data structures, and any unique techniques it uses.

Include your name, class section, instructor, date, and a brief description at the beginning of the program.

Javadoc brief intro


Naming Conventions

Choose meaningful and descriptive names.Variables and method names:

Use lowercase. If the name consists of several words, concatenate all in one, use lowercase for the first word, and capitalize the first letter of each subsequent word in the name. For example, the variables radius and area, and the method computeArea.


Naming Conventions, cont.

Class names: Capitalize the first letter of each word in the

name. For example, the class name ComputeArea.

Constants: Capitalize all letters in constants, and use

underscores to connect words. For example, the constant PI and MAX_VALUE


Proper Indentation and Spacing

Indentation Indent two spaces.

Spacing Use blank line to separate segments of the code.


Block Styles

Use end-of-line style for braces.

public class Test { public static void main(String[] args) { System.out.println("Block Styles"); } }

public class Test { public static void main(String[] args) {

System.out.println("Block Styles"); } }

End-of-line style

Next-line style


Programming Errors

Syntax Errors Detected by the compiler

Runtime Errors Causes the program to abort

Logic Errors Produces incorrect result


Syntax Errorspublic class ShowSyntaxErrors { public static void main(String[] args) { i = 30; System.out.println(i + 4); }}

Not declared


Runtime Errors

public class ShowRuntimeErrors { public static void main(String[] args) { int i = 1 / 0; }}

Devided by zero


Logic Errors

public class ShowLogicErrors {

// Determine if a number is between 1 and 100 inclusively

public static void main(String[] args) {

// Prompt the user to enter a number

String input = JOptionPane.showInputDialog(null,

"Please enter an integer:",

"ShowLogicErrors", JOptionPane.QUESTION_MESSAGE);

int number = Integer.parseInt(input);

// Display the result

System.out.println("The number is between 1 and 100, " +

"inclusively? " + ((1 > number) && (number < 100)));

System.exit(0);

}

}

Wrong expression


Debugging

Logic errors are called bugs. The process of finding and correcting errors is called debugging.

Debugger is a program that facilitates debugging. You can use a debugger to

Execute a single statement at a time.Trace into or stepping over a method.Set breakpoints.Display variables.Display call stack.Modify variables.


Debugging in JBuilder

The debugger utility is integrated in JBuilder. You can pinpoint bugs in your program with the help of the JBuilder debugger without leaving the IDE. The JBuilder debugger enables you to set breakpoints and execute programs line by line. As your program executes, you can watch the values stored in variables, observe which methods are being called, and know what events have occurred in the program.

JBuilder Optional


Setting BreakpointsA breakpoint is a stop sign placed on a line of source code that tells the debugger to pause when this line is encountered. Using the breakpoint, you can quickly move over the sections you know work correctly and concentrate on the sections causing problems.

JBuilder Optional


Setting Breakpoints, cont.Cutter area

Breakpoint

As you debug your program, you can set as many breakpoints as you want, and can remove breakpoints at any time during debugging. The project retains the breakpoints you have set when you exit the project. The breakpoints are restored when you reopen it.

JBuilder Optional


Starting the debugger1. Set breakpoints.

2. Choose the program (e.g., ShowCurrentTime.java in the project pane, and right-click the mouse button to display the context menu. Click Debug Using Defaults in the context menu to start debugging.

JBuilder Optional


Console View

Console view

Console view displays output and errors. You can also enter input from the console view.

JBuilder Optional


Stack View

Call stacks

Stack view displays the methods and local variables in the call stack.

JBuilder Optional


Watch View

Data watches

You can add variables to the watch view. Watch view displays the contents of the variables in the watch view.

JBuilder Optional


Adding Variables to Watch View

There are several ways to add variables to the watch view. A simple way is to highlight the variable and then right-click the mouse to display the context menu. Choose Add Watch in the context menu to add the variable to the watch view.

JBuilder Optional


Controlling Program Execution

The program pauses at the first breakpoint line encountered. This line, called the current execution point, is highlighted and has a green arrow to the left. The execution point marks the next line of source code to be executed by the debugger.

When the program pauses at the execution point, you can issue debugging commands to control the execution of the program. You also can inspect or modify the values of variables in the program.

When JBuilder is in the debugging mode, the Run menu contains the debugging commands. Most of the commands also appear in the toolbar under the message pane. The toolbar contains additional commands that are not in the Run menu. Here are the commands for controlling program execution:

JBuilder Optional


Debugger Commands in the Toolbar

Reset program

Resume program

Pause program

Step over

Step into

Step out

JBuilder Optional


Debugger Commands in the MenuJBuilder Optional


Debugger Commands

Step Over executes a single statement. If the statement contains a call to a method, the entire method is executed without stepping through it.

Step Into executes a single statement or steps into a method.

Step Out executes all the statements in the current method and returns to its caller.

Run to Cursor runs the program, starting from the current execution point, and pauses and places the execution point on the line of code containing the cursor, or at a breakpoint.

Run to End of Method runs the program until it reaches the end of the current method or a breakpoint.

Resume Program continues the current debugging session or restarts one that has finished or been reset.

Reset Program ends the current program and releases it from memory. Use Reset to restart an application from the beginning, as when you make a change to the code and want to run again from the beginning, or if variables or data structures become corrupted with unwanted values. This command terminates debugging and returns to the normal editing session.

Show Execution Point positions the cursor at the execution point in the content pane.

JBuilder Optional


Tracing Execution

You may trace the program line by line and see the contents of the variables in the stack view. This is most convenient.

JBuilder Optional

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