02 - java fundamentals

Object Oriented Programming Mihai Dasclu

2CB, 2013 - 2014

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, teaching

//assistant, date, and a brief description at the

//beginning of the program

// strategic (before, overall) vs. tactical (intended for single lines of code can make code unreadable)

// include copyright information

// add descriptive information before function

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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.

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

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Proper Indentation and Spacing Indentation

Indent two spaces (or \t in Eclipse / NetBeans)

Spacing Use blank line to separate segments of the code

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Block Styles Next-line & End-of-line

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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");

}

}

Programming Errors Syntax Errors

Detected by the compiler

Runtime Errors Causes the program to abort

Logic Errors Produces incorrect result

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Syntax Errors

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public class ShowSyntaxErrors {


i = 30;

System.out.println(i + 4);

}

}

Runtime Errors

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public class ShowRuntimeErrors {


int i = 1 / 0;

}

}

Logic Errors

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public class ShowLogicErrors {

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


// 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);

}

}

Debugging Bugs = logic errors are called

Debugging = the process of finding and correcting errors

Common approach -> use a combination of methods to narrow down to the part of the program where the bug is located Hand-trace the program (i.e., catch errors by reading

the program) Insert print statements in order to show the values of

the variables or the execution flow of the program This approach might work for a short, simple program.

But for a large, complex program, the most effective approach for debugging is to use a debugger utility. Ob

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Debugger A program that facilitates debugging & used to:

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

http://www.vogella.com/articles/EclipseDebugging/article.html

https://netbeans.org/features/java/debugger.html

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Elementary programming

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Trace a Program Execution public class ComputeArea {

/** Main method */


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);

}

}

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no value radius

allocate memory for radius

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/** Main method */


double radius;

double area;

// Assign a radius

radius = 20;

// Compute area


// Display results



}

}

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no value radius

memory

no value area

allocate memory for area

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/** Main method */


double radius;

double area;

// Assign a radius

radius = 20;

// Compute area


// Display results



}

}

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20 radius

no value area

assign 20 to radius

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/** Main method */


double radius;

double area;

// Assign a radius

radius = 20;

// Compute area


// Display results



}

}

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20 radius

memory

1256.636 area

compute area and assign it to variable area

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/** Main method */


double radius;

double area;

// Assign a radius

radius = 20;

// Compute area


// Display results



}

}

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20 radius

memory

1256.636 area

print a message to the console

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Reading Input from the Console Create a Scanner object Scanner input = new Scanner(System.in);!

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 input = new Scanner(System.in);!double d = input.nextDouble(); Obj

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Getting Input from Input Dialog Boxes String input =

JOptionPane.showInputDialog( "Enter an input");

String string = JOptionPane.showInputDialog(null, Prompting Message, Dialog Title, JOptionPane.QUESTION_MESSAGE);

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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. Obje

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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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Constants final datatype CONSTANTNAME = VALUE;

final double PI = 3.14159;

final int SIZE = 3;

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Primitive Data Types & Operators byte: 8-bit signed [-128; 127]

short: 16-bit signed [-32,768; 32,767]

int: 32-bit signed [-2,147,483,648; 2,147,483,647] generally the default choice

long: 64-bit signed [-9,223,372,036,854,775,808; 9,223,372,036,854,775,807] exceed int

float: single-precision 32-bit IEEE 754 floating point; should never be used for precise values, such as currency (java.math.BigDecimal)

double: double-precision 64-bit IEEE 754 floating point; usually the default choice for decimal values

boolean: true and false; its "size" is not precisely defined

char: 16-bit Unicode character ['\u0000' (or 0); '\uffff' (or 65,535]

+, -, *, /, %

Shortcut assignment with =

++, -- (pre and post)

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Default Values

Data Type Default Value (for fields)

byte 0 short 0 int 0 long 0L float 0.0f double 0.0d char '\u0000' String (or any object) null boolean false

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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;

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.

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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;

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range increases

byte, short, int, long, float, double

Character Data Type char letter = 'A'; (ASCII)

char numChar = '4'; (ASCII)

char letter = '\u0041'; (Unicode)

char numChar = '\u0034'; (Unicode)

NOTE: The increment and decrement operators can also be used on char variables to get the next or preceding Unicode character. For example, the following statements display character b. char ch = 'a'; System.out.println(++ch);

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Four hexadecimal digits

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 \" \u0022

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The String Type Char type represents only one character. To represent a

string of characters:

String message = "Welcome to Java";

A predefined class in the Java library

Not a primitive type, but a reference type

// Three strings are concatenated String message = "Welcome " + "to " + "Java";

// String Chapter is concatenated with number 2 String s = "Chapter" + 2; // s becomes Chapter2

// String Supplement is concatenated with character B String s1 = "Supplement" + 'B'; // s1 becomes SupplementB

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Converting Strings to Integers or Doubles 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.

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);

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The boolean Type and Operators Often in a program you need to compare two values,

such as whether i is greater than j => Boolean value: true or false. boolean b = (1 > 2);

Operator Name

< less than

greater than

>= greater than or equal to

== equal to

!= not equal to Obj

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if Statements if (boolean-expression) {

statement(s);

}

//else {

// statement(s)-for-the-false-case;

//}

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Multiple Alternative if Statements The else clause matches the most recent if clause

in the same block.

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if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F';

Equivalent

if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F';

int i = 1; int j = 2; int k = 3; if (i > j) if (i > k) System.out.println("A"); else System.out.println("B");

int i = 1; int j = 2; int k = 3; if (i > j) if (i > k) System.out.println("A"); else System.out.println("B");

Equivalent

Common Errors 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. O

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Wrong

Efficiency

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Equivalent

Equivalent if (number % 2 == 0) even = true; else even = false;

boolean even = number % 2 == 0;

if (even == true) System.out.println( "It is even.");

if (even) System.out.println( "It is even.");

Logical Operator Operator Name

! not

&& and

|| or

^ exclusive or

& bitwise and

| bitwise or

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switch Statement Rules (1)

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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. Note that value1, ..., and valueN are constant expressions, meaning that they cannot contain variables in the expression, such as 1 + x.

switch (switch-expression) { case value1: statement(s)1; break; case value2: statement(s)2; break; case valueN: statement(s)N; break; default: statement(s)-for-default; }

switch Statement Rules (2)

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!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.

switch (switch-expression) { case value1: statement(s)1; break; case value2: statement(s)2; break; case valueN: statement(s)N; break; default: statement(s)-for-default; }

!The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression. The case statements are executed in sequential

order, but the order of the cases (including the default case) does not matter. However, it is good programming style to follow the logical sequence of the cases and place the default case at the end.

Trace switch statement (1)

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switch (ch) { case 'a': System.out.println(ch); case 'b': System.out.println(ch); case 'c': System.out.println(ch); }

Suppose ch is 'a':


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This is 'a':


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Execute this line


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Execute this line


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Execute this line


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switch (ch) { case 'a': System.out.println(ch);

break; case 'b': System.out.println(ch);

break; case 'c': System.out.println(ch);

break; }

Suppose ch is 'a':


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break; }

This is 'a':


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break; }

Execute this line

Conditional Operator (boolean-expression) ? exp1 : exp2

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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Formatting Output Use the printf statement.

System.out.printf(format, items);

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.

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" Obj

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Operator Precedence var++, var--

+, - (Unary plus and minus), ++var,--var

(type) Casting

! (Not)

*, /, % (Multiplication, division, and remainder)

+, - (Binary addition and subtraction)

= (Comparison)

==, !=; (Equality)

^ (Exclusive OR)

&& (Conditional AND) Short-circuit AND

|| (Conditional OR) Short-circuit OR

=, +=, -=, *=, /=, %= (Assignment operator) Obj

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Operator Precedence and Associativity The expression in the parentheses is evaluated first.

(Parentheses can be nested, in which case the expression in the inner parentheses is executed first.) When evaluating an expression without parentheses, the operators are applied according to the precedence rule and the associativity rule.

If operators with the same precedence are next to each other, their associativity determines the order of evaluation. All binary operators except assignment operators are left-associative.

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)) O

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Loops

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Loops - Motivations Suppose you need to print a string (e.g., "Welcome to Java!") a hundred times. It would be tedious to have to write the following statement a hundred times:

System.out.println("Welcome to Java!");

How do you solve this problem?

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System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!");

System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!");

100 times

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Introducing while Loops while (loop-continuation-condition) {

// loop-body;

Statement(s);

}

Often the number of times a loop is executed is not predetermined => use an input value to signify the end of the loop (a sentinel value)

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int count = 0; while (count < 100) { System.out.println("Welcome to Java"); count++; }

Trace it!

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Caution Dont use floating-point values for equality checking in a loop

control

Consider the following code for computing 1 + 0.9 + 0.8 + ... + 0.1:

double item = 1; double sum = 0;

while (item != 0) { // No guarantee item will be 0

sum += item;

item -= 0.1;

}

System.out.println(sum);

This loop seems OK on the surface, but can become an infinite loop (because the floating-point arithmetic is approximated) Ob

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do-while & for Loops do {

// Loop body;

Statement(s);

} while (loop-continuation-condition);

for (initial-action; loop-continuation-condition; action-after-each-iteration) {

// loop body;

Statement(s);

}

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int i; for (i = 0; i < 100; i++) { System.out.println("Welcome to Java!"); }

Trace it!

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Note The initial-action in a for loop a list of zero

or more comma-separated expressions

The action-after-each-iteration in a for loop a list of zero or more comma-separated statements for (int i = 1; i < 100; System.out.println(i++));

for (int i = 0, j = 0; (i + j < 10); i++, j++) {

// Do something

}

Therefore, the two for loops are correct

Rarely used in practice

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Note If the loop-continuation-condition in a for loop is

omitted, it is implicitly true.

In case of infinite loops, it is better to use the equivalent while loop to avoid confusion

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Equivalent for ( ; ; ) { // Do something }

while (true) { // Do something }

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Caution Adding a semicolon at the end of the for clause before the loop body is a common mistake:

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Logic Errors

for (int i=0; i

Caution, cont. 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

Which Loop to Use? The three forms of loop statements, while, do-

while, and for, are expressively equivalent:

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while (loop-continuation-condition) { // Loop body }

for ( ; loop-continuation-condition; ) { // Loop body } Equivalent

for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; }

initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; }

Equivalent

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Recommendations Use the one that is most intuitive and comfortable

for you!

In general: a for loop a fixed number of repetitions (e.g., print a

message 100 times) a while loop unknown number of repetitions (e.g.

reading the numbers until the input is 0) a do-while loop replace a while loop if the loop body

has to be executed before testing the continuation condition

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Minimizing Numerical Errors Numeric errors involving floating-point numbers

are inevitable.

Sums a series that starts with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows: 0.01 + 0.02 + 0.03 and so on.

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public class TestSum { public static void main(String[] args) { // Initialize sum float sum = 0; // Add 0.01, 0.02, ..., 0.99, 1 to sum for (float i = 0.01f; i

Problem: Monte Carlo Simulation The Monte Carlo simulation refers to a technique

that uses random numbers and probability to solve problems.

Wide range of applications in computational mathematics, physics, chemistry, and finance

Use Monto Carlo simulation for estimating .

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circleArea / squareArea = / 4

4 * number hits / 1.000.000

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x

y

1 -1

1

-1

Using break and continue w1: while (number < 20) {

number++;

sum += number;

if (sum >= 100) break w1;

}

while (number < 20) {

number++;

if (number == 10 || number == 11) continue;

sum += number;

}

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Methods

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Problem

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int sum = 0; for (int i = 1; i

Solution

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public static int sum(int i1, int i2) { int sum = 0; for (int i = i1; i

Defining Methods (1) A collection of statements grouped together to

perform specific operations

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public static int max(int num1, int num2) {

int result; if (num1 > num2) result = num1; else result = num2; return result;

}

modifier return value

type method name

formal parameters

return value

method body

method header

parameter list

Define a method Invoke a method

int z = max(x, y);

actual parameters (arguments)

method signature

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Defining Methods (2) Signature = combination of method name +

parameter list

Formal Parameters = variables defined in the method header

Actual parameter or argument = when invoking a method is, a value is passed to the parameter

A method may return a value (otherwise void). The returnValueType = data type of the value the method returns

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Calling Methods & Call stack

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public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

pass the value of i & j Trace call

(a) The main method is invoked.

Space required for the main method k: j: 2 i: 5

(b) The max method is invoked.

Space required for the max method num2: 2 num1: 5

(d) The max method is finished and the return value is sent to k.

(e) The main method is finished.

Stack is empty


Space required for the main method k: 5 j: 2 i: 5

(c) The max method is being executed.

Space required for the max method result: 5 num2: 2 num1: 5


CAUTION A return statement is required for a value-returning

method

The method shown below in (a) is logically correct, but it has a compilation error because the Java compiler thinks it possible that this method does not return any value

To fix this problem: delete if (n < 0) in (a), so that the compiler will see a return statement to be reached regardless of how the if statement is evaluated

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public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return 1; }

(a)

Should be

(b)

public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else return 1; }

Passing Parameters Suppose you invoke the method using

nPrintln(Welcome to Java, 5);

What is the output?

Suppose you invoke the method using

nPrintln(Computer Science, 15);

What is the output?

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public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); }

Overloading Methods Overloading the max Method

public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; }

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02 - java fundamentals

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