Lập trình Java - Phần 1

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Chapter 1: Introduction to Java
Chapter 2: Primitive Data Types and Operations
Chapter 3: Control Statements
Chapter 4: Methods
Chapter 5: Arrays
Part I: Fundamentals of Programming
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Chapter 1:
Introduction to Java
What Is Java?
Getting Started With Java Programming
Create, Compile and Running a Java Application
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What Is Java?
History
Applications of Java
Compare to C/C++/C#
Characteristics of Java
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History
James Gosling and Sun Microsystems (1983-1984)
Oak
In a project of electric device developing
Java, May 20, 1995, Sun World
HotJava
The first Java-enabled Web browser
JDK Evolutions
J2SE, J2ME, and J2EE
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JDK Versions
JDK 1.0 (1995)
JDK 1.1 (1996)
Java 2 SDK v 1.2 (JDK 1.2, 1998)
Java 2 SDK v 1.3 (JDK 1.3, 2000)
Java 2 SDK v 1.4 (JDK 1.4, 2002)
Java 5 SDK v 1.5 (JDK 1.5, 2005)
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JDK Editions
Java Standard Edition (J2SE)
J2SE can be used to develop client-side standalone applications or applets.
Java Enterprise Edition (J2EE)
J2EE can be used to develop server-side applications such as Java servlets and Java ServerPages.
Java Micro Edition (J2ME).
J2ME can be used to develop applications for mobile devices such as cell phones.
JDK Editions (2)
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Standard desktop &
workstation applications
Heavy duty server
systems
Small & memory
constrained devices
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Java IDE Tools
Eclipse
Borland Jbuilder
NetBean
Microsoft Visual J++, J#
WebGain Café
IBM Visual Age for Java
Forte by Sun Microsystems
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Applications of Java
JAVA
Mobile
Console
GUI
Network
Web
Database
RMI
Applet
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Console
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GUI (with JFC)
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Applet & Web
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Database & Web
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Mobile
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Compare to C/C++/C#
Similar to C/C++/C#
Difference:
Compiler
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Traditional Compiler (C/C++)
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Java Compiler
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Characteristics of Java
Java is simple
Java is object-oriented
Java is distributed
Java is robust
Java is secure
Java is architecture-neutral
Java is multithreaded
Java is dynamic
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Getting Started with Java Programming
A Simple Java Application
Compiling Programs
Executing Applications
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A Simple Application
Example 1.1
//This application program prints Welcome
//to Java!
package chapter1;

public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
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Creating and Compiling Programs
On command line
javac file.java
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Executing Applications
On command line
java classname
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Example
javac Welcome.java

java Welcome

output:...
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Compiling and Running a Program
 Where are the files stored in the directory?
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Anatomy of a Java Program
Comments
Package
Reserved words
Modifiers
Statements
Blocks
Classes
Methods
The main method
Input processing
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Comments
Similar to C/C++
In Java, comments are preceded by two slashes (//) in a line, or enclosed between /* and */ in one or multiple lines.
When the compiler sees //, it ignores all text after // in the same line. When it sees /*, it scans for the next */ and ignores any text between /* and */.
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Package
The second line in the program (package chapter1;) specifies a package name, chapter1, for the class Welcome.
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Reserved Words
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Statements
Similar to C/C++, every statement in Java ends with a semicolon (;).
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Blocks
A pair of braces in a program forms a block that groups components of a program
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Classes
The class is the essential Java construct.
A class is a template or blueprint for objects. To program in Java, you must understand classes and be able to write and use them.
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Methods
What is System.out.println "Welcome to Java!"? It is a method: a collection of statements that performs a sequence of operations to display a message on the console.
It can be used even without fully understanding the details of how it works.
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main Method
The main method provides the control of program flow. The Java interpreter executes the application by invoking the main method.
 
The main method looks like this:
public static void main(String[] args) {
// Statements;
}
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Displaying Text in a Message Dialog Box
You can use the showMessageDialog method in the JOptionPane class.
JOptionPane is one of the many predefined classes in the Java system.
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The showMessageDialog Method
JOptionPane.showMessageDialog(null, "Welcome to Java!", "Example 1.2", JOptionPane.INFORMATION_MESSAGE);
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The exit Method
Use Exit to terminate the program and stop all threads.

NOTE: When your program starts, a thread is spawned to run the program. When the showMessageDialog is invoked, a separate thread is spawned to run this method. The thread is not terminated even you close the dialog box. To terminate the thread, you have to invoke the exit method.
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Input processing
Examples:
public static void main(String[] args) {
InputStreamReader io = new InputStreamReader(System.in);
BufferedReader console = new BufferedReader(io);
try{
System.out.print("Nhap chuoi str = ");
String str = console.readLine();
System.out.print(str);
}
catch(Exception ex){}
}
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Chapter 2: Primitive Data Types and Operations
Introduce Programming with an Example
Identifiers, Variables, and Constants
Primitive Data Types
byte, short, int, long, float, double, char, boolean
Expressions
Operators, Precedence, Associativity, Operand Evaluation Order: ++, --, *, /, %, +=, -=, *=, /=, %=, ^, &, |, +, -,
Getting Input from Input Dialog Boxes
Case Studies (Computing Mortgage, and Computing Changes)
Style and Documentation Guidelines
Syntax Errors, Runtime Errors, and Logic Errors
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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 cannot be true, false, or
null.
An identifier can be of any length.
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Variables
// Compute the first area
radius = 1.0;
area = radius*radius*3.14159;
System.out.println("The area is “ + area + " for radius "+radius);

// Compute the second area
radius = 2.0;
area = radius*radius*3.14159;
System.out.println("The area is “ + area + " for radius "+radius);
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Declaring 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;
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Assignment Statements
x = 1; // Assign 1 to x;
radius = 1.0; // Assign 1.0 to radius;
a = `A`; // Assign `A` to a;

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Declaring and Initializing
in One Step
int x = 1;
double d = 1.4;
float f = 1.4;
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Constants
final datatype CONSTANTNAME = VALUE;

final double PI = 3.14159;
final int SIZE = 3;
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Numerical Data Types
byte 8 bits
short 16 bits
int 32 bits
long 64 bits
float 32 bits
double 64 bits
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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)
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NOTE
Calculations involving floating-point numbers are approximated because these numbers are not stored with complete accuracy.
For example,
System.out.println(1 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);
displays 0.5000000000000001, not 0.5, and
System.out.println(1.0 - 0.9);
displays 0.09999999999999998, not 0.1.
Integers are stored precisely. Therefore, calculations with integers yield a precise integer result.
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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 l = 1000000;
double d = 5.0;
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Integer Literals
An integer literal can be assigned to an integer variable as long as it can fit into the variable. A compilation error would occur if the literal were too large for the variable to hold.
For example, the statement byte b = 1000 would cause a compilation error, because 1000 cannot be stored in a variable of the byte type.
An 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.
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Floating-Point Literals
Floating-point literals are written with a decimal point. By default, a floating-point literal is treated as a double type value.
For example, 5.0 is considered a double value, not a float 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.
For example, you can use 100.2f or 100.2F for a float number, and 100.2d or 100.2D for a double number.
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Scientific Notation
Floating-point literals can also be specified in scientific notation.
For example, 1.23456e+2, same as 1.23456e2, is equivalent to 123.456, and 1.23456e-2 is equivalent to 0.0123456.
E (or e) represents an exponent and it can be either in lowercase or uppercase.
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Arithmetic Expressions
is translated to
(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)
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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
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Increment and
Decrement Operators
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Increment and
Decrement Operators, cont.
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Increment and
Decrement Operators, cont.
Using increment and decrement operators makes expressions short, but it also makes them complex and difficult to read.
Avoid using these operators in expressions that modify multiple variables, or the same variable for multiple times such as this: int k = ++i + i.
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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--;
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Numeric Type Conversion
Consider the following statements:
byte i = 100;
long k = i*3+4;
double d = i*3.1+k/2;

int x = k; //(Wrong)
long k = x; //(fine,implicit casting)
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Type Casting
double
float
long
int
short
byte
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Type Casting, cont.
Implicit casting
double d = 3; (type widening)

Explicit casting
int i = (int)3.0; (type narrowing)
What is wrong? int x = 5/2.0;
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Character Data Type
char letter = `A`; (ASCII)
char numChar = `4`; (ASCII)
char letter = `u0041`; Unicode)
char numChar = `u0034`;(Unicode)

Special characters
char tab = ‘ ’;
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Unicode Format
Description Escape Sequence Unicode
Backspace  u0008
Tab u0009
Linefeed u000a
Carriage return u000d
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Casting between char and Numeric Types
int i = `a`; // Same as int //i = (int)`a`;
char c = 97; // Same as char //c = (char)97;
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The boolean Type and Operators
boolean lightsOn = true;
boolean lightsOn = false;
boolean b = (1 > 2);

&& (and) (1 < x) && (x < 100)
|| (or) (lightsOn) || (isDayTime)
! (not) !(isStopped)
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Comparison Operators
Operator Name
< less than
<= less than or equal to
> greater than
>= greater than or equal to
== equal to
!= not equal to
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Boolean Operators
Operator Name
! not
&& and
|| or
^ exclusive or
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Truth Table for Operator !
Truth Table for Operator !

Operand !Operand
true false
false true
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Truth Table for Operator &&
Operand1 Operand2 Operand1 && Operand2
false false false
false true false
true false false
true true true
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Truth Table for Operator ||
Operand1 Operand2 Operand1 || Operand2
false false false
false true true
true false true
true true true
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Truth Table for Operator ^
Operand1 Operand2 Operand1 ^ Operand2
false false false
false true true
true false true
true true false
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The & and | Operators
&&: conditional AND operator
&: unconditional AND operator
||: conditional OR operator
|: unconditional OR operator

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

(1 < x) & (x < 100)
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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++)?
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Operator Precedence
How to evaluate

3 + 4 * 4 > 5 * (4 + 3) - ++i
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Operator Precedence
var++, var--
+, - (Unary plus and minus), ++var,--var
(type) Casting
! (Not)
*, /, % (Multiplication, division, and modulus)
+, - (Binary addition and subtraction)
<, <=, >, >= (Comparison)
==, !=; (Equality)
& (Unconditional AND)
^ (Exclusive OR)
| (Unconditional OR)
&& (Conditional AND) Short-circuit AND
|| (Conditional OR) Short-circuit OR
=, +=, -=, *=, /=, %= (Assignment operator)
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Operator Associativity
When two operators with the same precedence are evaluated, the associativity of the operators determines the order of evaluation.
All binary operators except assignment operators are left-associative.
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))
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Operand Evaluation Order
The precedence and associativity rules specify the order of the operators, but do not specify the order in which the operands of a binary operator are evaluated.
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.
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Operand Evaluation Order, cont.
If no operands have side effects that change the value of a variable, the order of operand evaluation is irrelevant. Interesting cases arise when operands do have a side effect.
For example, x becomes 1 in the following code, because a is evaluated to 0 before ++a is evaluated to 1. 
int a = 0;
int x = a + (++a);
But x becomes 2 in the following code, because ++a is evaluated to 1, then a is evaluated to 1.
int a = 0;
int x = ++a + a;
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Getting Input from Input Dialog Boxes
String string = JOptionPane.showInputDialog(null, “Prompt Message”, “Dialog Title”,
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.
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Convertting Strings to Integers
The input returned from the input dialog box is a string. If you enter a numeric value such as 123, it returns “123”. To obtain the input as a number, you have to convert a string into a number.
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”.
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Convertting 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”.
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Programming Style and Documentation
Appropriate Comments
Naming Conventions
Proper Indentation and Spacing Lines
Block Styles
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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, instruction, date, and a brief description at the beginning of the program.
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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.
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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. For example, the constant PI.
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Proper Indentation and Spacing
Indentation
Indent two spaces.

Spacing
Use blank line to separate segments of the code.
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Block Styles
Use end-of-line style for braces.
 
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Programming Errors
Syntax Errors
Detected by the compiler
Runtime Errors
Causes the program to abort
Logic Errors
Produces incorrect result
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Compilation Errors
public class ShowSyntaxErrors {
public static void main(String[] args) {
i = 30;
System.out.println(i+4);
}
}
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Runtime Errors
public class ShowRuntimeErrors {
public static void main(String[] args) {
int i = 1 / 0;
}
}
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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);
}
}
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Chapter 3
Control Statements
Selection Statements
Using if and if...else
Nested if Statements
Using switch Statements
Conditional Operator
Repetition Statements
Looping: while, do-while, and for
Nested loops
Using break and continue
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Selection Statements
if Statements
switch Statements
Conditional Operators
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if Statements
if (booleanExpression) {
statement(s);
}
Example:
if ((i > 0) && (i < 10)) {
System.out.println("i is an " +
"integer between 0 and 10");
}
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Caution
Adding a semicolon at the end of an if clause is a common mistake.
if (radius >= 0);
{
area = radius*radius*PI;
System.out.println(
"The area for the circle of radius " +
radius + " is " + area);
}
This mistake is hard to find, because it is not a compilation error or a runtime error, it is a logic error.
This error often occurs when you use the next-line block style.
Wrong
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The if...else Statement
if (booleanExpression) {
statement(s)-for-the-true-case;
}
else {
statement(s)-for-the-false-case;
}
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if...else Example
if (radius >= 0) {
area = radius*radius*PI;
System.out.println("The area for the “
+ “circle of radius " + radius +
" is " + area);
}
else {
System.out.println("Negative input");
}
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Multiple Alternative if Statements
if (score >= 90)
grade = ‘A’;
else
if (score >= 80)
grade = ‘B’;
else
if (score >= 70)
grade = ‘C’;
else
if (score >= 60)
grade = ‘D’;
else
grade = ‘F’;
if (score >= 90)
grade = ‘A’;
else if (score >= 80)
grade = ‘B’;
else if (score >= 70)
grade = ‘C’;
else if (score >= 60)
grade = ‘D’;
else
grade = ‘F’;
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Note
The else clause matches the most recent if clause in the same block. For example, the following statement
int i = 1; int j = 2; int k = 3;
if (i > j)
if (i > k)
System.out.println("A");
else
System.out.println("B");
is equivalent to
int i = 1; int j = 2; int k = 3;
if (i > j)
if (i > k)
System.out.println("A");
else
System.out.println("B");
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Note, cont.
Nothing is printed from the preceding statement. To force the else clause to match the first if clause, you must add a pair of braces:
int i = 1;
int j = 2;
int k = 3;
if (i > j) {
if (i > k)
System.out.println("A");
}
else
System.out.println("B");
This statement prints B.
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Nested if Statements
Example 3.1 Using Nested if Statements
This program reads in number of years and loan amount and computes the monthly payment and total payment.
The interest rate is determined by number of years.
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switch Statements
switch (year) {
case 7: annualInterestRate = 7.25;
break;
case 15: annualInterestRate = 8.50;
break;
case 30: annualInterestRate = 9.0;
break;
default: System.out.println(
"Wrong number of years, enter 7, 15, or 30");
}
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switch Statement Flow Chart
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switch Statement Rules
The switch-expression must yield a value of char, byte, short, or int type and must always be enclosed in parentheses.
The value1, ..., and valueN must have the same data type as the value of the switch-expression.
The resulting statements in the case statement are executed when the value in the case statement matches the value of the switch-expression. (The case statements are executed in sequential order.)
The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.
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switch Statement Rules, cont.
The default case, which is optional, can be used to perform actions when none of the specified cases is true.
The order of the cases (including the default case) does not matter. However, it is a good programming style to follow the logical sequence of the cases and place the default case at the end.
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Caution
Do not forget to use a break statement when one is needed.
For example: The following code always displays Wrong number of years regardless of what numOfYears is. Suppose the numOfYears is 15. The statement annualInterestRate = 8.50 is executed, then the statement annualInterestRate = 9.0, and finally the statement System.out.println("Wrong number of years").

switch (numOfYears) {
case 7: annualInterestRate = 7.25;
case 15: annualInterestRate = 8.50;
case 30: annualInterestRate = 9.0;
default: System.out.println("Wrong number of years");
}
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Conditional Operator
if (x > 0) y = 1
else y = -1;

is equivalent to

y = (x > 0) ? 1 : -1;

Ternary operator
Binary operator
Unary operator
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Conditional Operator
if (num % 2 == 0)
System.out.println(num + “is even”);
else
System.out.println(num + “is odd”);


System.out.println(
(num % 2 == 0)? num + “is even” :
num + “is odd”);
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Conditional Operator, cont.
(booleanExp) ? exp1 : exp2
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Repetitions
while Loops
do-while Loops
for Loops
break and continue
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while Loop Flow Chart
while (continuation-condition) {
// loop-body;
}
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while Loop Flow Chart, cont.
int i = 0;
while (i < 100) {
System.out.println(
"Welcome to Java!");
i++;
}
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Caution
Don’t use floating-point values for equality checking in a loop control. Since floating-point values are approximations, using them could result in imprecise counter values and inaccurate results. This example uses int value for data. If a floating-point type value is used for data, (data != 0) may be true even though data is 0.

// data should be zero
double data = Math.pow(Math.sqrt(2), 2) - 2;
 
if (data == 0)
System.out.println("data is zero");
else
System.out.println("data is not zero");
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do-while Loop
do {
// Loop body;
} while (continue-condition);
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for Loops
for (initial-action; loop-continuation-condition; action-after-each-iteration) {
//loop body;
}

int i = 0;
while (i < 100) {
System.out.println("Welcome to Java! ” + i);
i++;
}
Example:
int i;
for (i = 0; i < 100; i++) {
System.out.println("Welcome to Java! ” + i);
}
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for Loop Flow Chart
for (initial-action;
loop-continuation-condition;
action-after-each-iteration) {
//loop body;
}
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for Loop Example
int i;
for (i = 0; i<100; i++) {
System.out.println(
"Welcome to Java");
}
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Which Loop to Use?
The three forms of loop statements, while, do, and for, are expressively equivalent; that is, you can write a loop in any of these three forms.
I recommend that you use the one that is most intuitive and comfortable for you. In general, a for loop may be used if the number of repetitions is known, as, for example, when you need to print a message 100 times. A while loop may be used if the number of repetitions is not known, as in the case of reading the numbers until the input is 0. A do-while loop can be used to replace a while loop if the loop body has to be executed before testing the continuation condition.
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Caution
Adding a semicolon at the end of the for clause before the loop body is a common mistake, as shown below:

for (int i=0; i<10; i++);
{
System.out.println("i is " + i);
}
Wrong
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Caution, cont.
Similarly, the following loop is also wrong:
int i=0;
while (i<10);
{
System.out.println("i is " + i);
i++;
}
In the case of the do loop, the following semicolon is needed to end the loop.
int i=0;
do {
System.out.println("i is " + i);
i++;
} while (i<10);
Wrong
Correct
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The break Keyword
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The continue Keyword
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Chapter 4: Methods
Introducing Methods
Benefits of methods, Declaring Methods, and Calling Methods
Passing Parameters
Pass by Value
Overloading Methods
Ambiguous Invocation
Scope of Local Variables
Method Abstraction
The Math Class
Case Studies
Recursion (Optional)
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Introducing Methods
Method Structure
A method is a collection of statements that are grouped together to perform an operation.
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Introducing Methods, cont.
parameter profile refers to the type, order, and number of the parameters of a method.
method signature is the combination of the method name and the parameter profiles.
The parameters defined in the method header are known as formal parameters.
When a method is invoked, its formal parameters are replaced by variables or data, which are referred to as actual parameters.
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Declaring Methods
public static int max(int num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
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Calling Methods, cont.
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Calling Methods, cont.
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CAUTION
A return statement is required for a nonvoid method. The following method is logically correct, but it has a compilation error, because the Java compiler thinks it possible that this method does not return any value.
public static int xMethod(int n) {
if (n > 0) return 1;
else if (n == 0) return 0;
else if (n < 0) return –1;
}
To fix this problem, delete if (n<0) in the code.
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Passing Parameters
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
}
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Pass by Value, cont.
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Overloading Methods
Example 4.3 Overloading the max Method

public static double max(double num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
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Ambiguous Invocation
Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match.
This is referred to as ambiguous invocation. Ambiguous invocation is a compilation error.
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Ambiguous Invocation, cont
public class AmbiguousOverloading {
public static void main(String[] args) {
System.out.println(max(1, 2));
}
 
public static double max(int num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}

public static double max(double num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
}
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Scope of Local Variables
A local variable: a variable defined inside a method.
Scope: the part of the program where the variable can be referenced.
The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared before it can be used.
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Scope of Local Variables, cont.
You can declare a local variable with the same name multiple times in different non-nesting blocks in a method, but you cannot declare a local variable twice in nested blocks.
Thus, the following code is correct.
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Scope of Local Variables, cont.
// Fine with no errors
public static void correctMethod() {
int x = 1;
int y = 1;
// i is declared
for (int i = 1; i < 10; i++) {
x += i;
}
// i is declared again
for (int i = 1; i < 10; i++) {
y += i;
}
}
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Scope of Local Variables, cont.
// With no errors
public static void incorrectMethod() {
int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
int x = 0;
x += i;
}
}
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Method Abstraction
You can think of the method body as a black box that contains the detailed implementation for the method.
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Benefits of Methods
Write once and reuse it any times.
Information hiding. Hide the implementation from the user.
Reduce complexity.
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The Math Class
Class constants:
PI
E
Class methods:
Trigonometric Methods
Exponent Methods
Rounding Methods
min, max, abs, and random Methods
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Trigonometric Methods
sin(double a)
cos(double a)
tan(double a)
acos(double a)
asin(double a)
atan(double a)
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Exponent Methods
exp(double a)
Returns e raised to the power of a.
log(double a)
Returns the natural logarithm of a.
pow(double a, double b)
Returns a raised to the power of b.
sqrt(double a)
Returns the square root of a.
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Rounding Methods
double ceil(double x)
x rounded up to its nearest integer. This integer is returned as a double value.
double floor(double x)
x is rounded down to its nearest integer. This integer is returned as a double value.
double rint(double x)
x is rounded to its nearest integer. If x is equally close to two integers, the even one is returned as a double.
int round(float x)
Return (int)Math.floor(x+0.5).
long round(double x)
Return (long)Math.floor(x+0.5).
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min, max, abs, and random
max(a, b)and min(a, b)
Returns the maximum or minimum of two parameters.
abs(a)
Returns the absolute value of the parameter.
random()
Returns a random double value
in the range [0.0, 1.0).
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Chapter 5: Arrays
Introducing Arrays
Declaring Array Variables, Creating Arrays, and Initializing Arrays
Passing Arrays to Methods
Copying Arrays
Multidimensional Arrays
Search and Sorting Methods
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Introducing Arrays
Array is a data structure that represents a collection of the same types of data.
An Array of 10 Elements
of type double
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Declaring Array Variables
datatype[] arrayname;
Example:
double[] myList;

datatype arrayname[];
Example:
double myList[];
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Creating Arrays
arrayName = new datatype[arraySize];
Example:
myList = new double[10];
myList[0] references the first element in the array.
myList[9] references the last element in the array.
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Declaring and Creating
in One Step
datatype[] arrayname = new
datatype[arraySize];
double[] myList = new double[10];
datatype arrayname[] = new
datatype[arraySize];
double myList[] = new double[10];
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The Length of Arrays
Once an array is created, its size is fixed. It cannot be changed. You can find its size using:
arrayVariable.length

For example:
myList.length
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Initializing Arrays
Using a loop:
for (int i = 0; i < myList.length; i++)
myList[i] = i;
Declaring, creating, initializing in one step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand syntax must be in one statement.
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Declaring, creating, initializing Using the Shorthand Notation
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent to the following statements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;
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CAUTION
Using the shorthand notation, you have to declare, create, and initialize the array all in one statement.
Splitting it would cause a syntax error. For example, the following is wrong:
double[] myList;
myList = {1.9, 2.9, 3.4, 3.5};
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Example 5.1
Testing Arrays
Objective: The program receives 6 numbers from the keyboard, finds the largest number and counts the occurrence of the largest number entered from the keyboard.
Suppose you entered 3, 5, 2, 5, 5, and 5, the largest number is 5 and its occurrence count is 4.
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Example 5.2
Assigning Grades
Objective: read student scores (int) from the keyboard, get the best score, and then assign grades based on the following scheme:
Grade is A if score is >= best–10;
Grade is B if score is >= best–20;
Grade is C if score is >= best–30;
Grade is D if score is >= best–40;
Grade is F otherwise.
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Passing Arrays to Methods
Java uses pass by value to pass parameters to a method. There are important differences between passing a value of variables of primitive data types and passing arrays.

For a parameter of a primitive type value, the actual value is passed. Changing the value of the local parameter inside the method does not affect the value of the variable outside the method.

For a parameter of an array type, the value of the parameter contains a reference to an array; this reference is passed to the method. Any changes to the array that occur inside the method body will affect the original array that was passed as the argument.
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Copying Arrays
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Copying Arrays
Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new int[sourceArray.length];

for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];
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The arraycopy Utility
arraycopy(sourceArray, src_pos, targetArray, tar_pos, length);

Example:
System.arraycopy(sourceArray, 0, targetArray, 0, sourceArray.length);
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Multidimensional Arrays
Declaring Variables of Multidimensional Arrays and Creating Multidimensional Arrays
int[][] matrix = new int[10][10];
or
int matrix[][] = new int[10][10];
matrix[0][0] = 3;
for (int i=0; i for (int j=0; j {
matrix[i][j] = (int)(Math.random()*1000);
}

double[][] x;
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Multidimensional Array Illustration
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Declaring, Creating, and Initializing Using Shorthand Notations
You can also use a shorthand notation to declare, create and initialize a two-dimensional array. For example,
int[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} };
This is equivalent to the following statements: 
int[][] array = new int[4][3];
array[0][0] = 1; array[0][1] = 2; array[0][2] = 3;
array[1][0] = 4; array[1][1] = 5; array[1][2] = 6;
array[2][0] = 7; array[2][1] = 8; array[2][2] = 9;
array[3][0] = 10; array[3][1] = 11; array[3][2] = 12;
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Lengths of Multidimensional Arrays
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};

array.length
array[0].length
array[1].length
array[2].length
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Ragged Arrays
Each row in a two-dimensional array is itself an array. So, the rows can have different lengths. Such an array is known as a ragged array.
For example,
int[][] matrix = {
{1, 2, 3, 4, 5},
{2, 3, 4, 5},
{3, 4, 5},
{4, 5},
{5}
};