Lesson Overview

Program শুধু data store করে না। Stored data-এর ওপর calculation, comparison এবং decision-making operation চালায়।

উদাহরণ:

• দুটি number যোগ করা
• Product price এবং quantity থেকে total বের করা
• একজন learner-এর age 18 বা তার বেশি কি না check করা
• Email এবং password দুটিই valid কি না নির্ধারণ করা
• একটি counter-এর value বাড়ানো
• Variable-এর existing value update করা

Java-তে এসব operation করার জন্য operator ব্যবহার করা হয়।

যেমন:

int total = 10 + 20;

এখানে + একটি arithmetic operator।

আরেকটি example:

boolean isAdult = age >= 18;

এখানে >= একটি comparison operator।

এই lesson-এ আমরা শিখব:

• Operator কী
• Operand কী
• Expression কী
• Arithmetic operators
• Assignment operators
• Comparison operators
• Logical operators
• Unary operators
• Increment এবং decrement
• Operator precedence
• Parentheses
• Short-circuit evaluation
• String concatenation
• Integer এবং decimal expressions
• Common operator-related error

---

Learning Objectives

এই lesson শেষ করার পর আপনি পারবেন:

• Operator, operand এবং expression ব্যাখ্যা করতে
• Arithmetic operator ব্যবহার করে calculation করতে
• Assignment এবং compound assignment operator ব্যবহার করতে
• Comparison operator দিয়ে values compare করতে
• Logical operator ব্যবহার করে multiple condition combine করতে
• Unary operator ব্যবহার করতে
• Increment এবং decrement operator বুঝতে
• Prefix এবং postfix operation-এর পার্থক্য ব্যাখ্যা করতে
• Operator precedence অনুযায়ী expression evaluate করতে
• Parentheses ব্যবহার করে calculation order control করতে
• Short-circuit evaluation বুঝতে
• String concatenation এবং numeric addition আলাদা করতে
• Common operator-related bug শনাক্ত করতে

---

What Is an Operator?

Operator হলো একটি symbol, যা এক বা একাধিক value-এর ওপর একটি operation চালায়।

Example:

int total = 10 + 20;

এখানে:

10

প্রথম value।

20

দ্বিতীয় value।

+

Operator।

10 + 20

একটি expression।

Result:

30

---

What Is an Operand?

Operator যে value বা variable-এর ওপর কাজ করে, তাকে operand বলা হয়।

Example:

10 + 20

এখানে:

• 10 প্রথম operand
• 20 দ্বিতীয় operand
• + operator

Variable-ও operand হতে পারে:

firstNumber + secondNumber

এখানে:

• firstNumber operand
• secondNumber operand

---

What Is an Expression?

Expression হলো value, variable এবং operator-এর একটি combination, যা evaluate হয়ে একটি result তৈরি করে।

Example:

10 + 20

Result:

30

আরেকটি expression:

price * quantity

আরেকটি:

age >= 18

Result হতে পারে:

true

আরেকটি:

isEmailValid && isPasswordValid

Result হতে পারে:

true

অথবা:

false

---

Statement and Expression

Expression একটি value তৈরি করে।

10 + 20

Statement একটি complete instruction।

int total = 10 + 20;

এখানে:

10 + 20

Expression।

সম্পূর্ণ line:

int total = 10 + 20;

Statement।

---

Types of Java Operators

এই lesson-এ আমরা প্রধানত শিখব:

1. Arithmetic operators
2. Assignment operators
3. Comparison operators
4. Logical operators
5. Unary operators
6. Increment and decrement operators
7. Conditional operator-এর basic introduction

---

Arithmetic Operators

Arithmetic operator numeric calculation-এর জন্য ব্যবহার করা হয়।

Java-এর basic arithmetic operators:

Operator	কাজ
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Remainder বা modulus

---

Addition Operator

Addition operator:

+

Example:

int firstNumber = 10;
int secondNumber = 20;

int total = firstNumber + secondNumber;

System.out.println(total);

Output:

30

---

Adding More Than Two Values

int mathematicsMark = 80;
int englishMark = 90;
int scienceMark = 85;

int totalMarks =
        mathematicsMark
        + englishMark
        + scienceMark;

System.out.println(totalMarks);

Output:

255

---

Subtraction Operator

Subtraction operator:

-

Example:

int totalAmount = 1000;
int spentAmount = 350;

int remainingAmount =
        totalAmount - spentAmount;

System.out.println(remainingAmount);

Output:

650

---

Multiplication Operator

Multiplication operator:

*

Example:

int productPrice = 500;
int quantity = 3;

int totalPrice =
        productPrice * quantity;

System.out.println(totalPrice);

Output:

1500

---

Division Operator

Division operator:

/

Example:

int total = 20;
int count = 4;

int average = total / count;

System.out.println(average);

Output:

5

---

Integer Division

দুটি integer divide করলে result-ও integer হয়।

int result = 5 / 2;

System.out.println(result);

Output:

2

Mathematical result:

2.5

কিন্তু Java integer division fractional অংশ বাদ দেয়।

---

Getting a Decimal Result

কমপক্ষে একটি operand floating-point type হতে হবে।

double result = 5.0 / 2;

System.out.println(result);

Output:

2.5

এটিও valid:

double result = 5 / 2.0;

এটিও:

double result = 5.0 / 2.0;

---

Common Average Bug

Wrong:

int totalMarks = 250;
int subjectCount = 3;

double average =
        totalMarks / subjectCount;

System.out.println(average);

Output:

83.0

Expected mathematical result প্রায়:

83.3333...

কারণ division আগে integer arithmetic হিসেবে হয়েছে।

Correct:

double average =
        totalMarks / (double) subjectCount;

অথবা:

double average =
        totalMarks / 3.0;

---

Division by Zero

Integer division by zero runtime error তৈরি করে।

int result = 10 / 0;

Possible exception:

ArithmeticException

---

Floating-Point Division by Zero

double result = 10.0 / 0.0;

System.out.println(result);

Output:

Infinity

আরেকটি:

double result = 0.0 / 0.0;

Output:

NaN

NaN মানে:

Not a Number

---

Remainder Operator

Remainder operator:

%

এটি division-এর পরে remainder return করে।

Example:

int remainder = 10 % 3;

System.out.println(remainder);

Output:

1

কারণ:

10 ÷ 3 = 3
Remainder = 1

---

Checking Even and Odd Numbers

একটি number even হলে 2 দিয়ে divide করার remainder 0।

int number = 10;

boolean isEven =
        number % 2 == 0;

System.out.println(isEven);

Output:

true

Odd check:

boolean isOdd =
        number % 2 != 0;

---

Other Uses of the Remainder Operator

% ব্যবহার করা যায়:

• Even বা odd check
• Cycle repeat করা
• Time conversion
• Pagination calculation
• Alternating behaviour
• Digit extraction

Example:

int seconds = 125;

int remainingSeconds =
        seconds % 60;

System.out.println(remainingSeconds);

Output:

5

---

Arithmetic with Negative Values

int firstValue = -10;
int secondValue = 4;

System.out.println(firstValue + secondValue);
System.out.println(firstValue - secondValue);
System.out.println(firstValue * secondValue);
System.out.println(firstValue / secondValue);
System.out.println(firstValue % secondValue);

Output:

-6
-14
-40
-2
-2

Java integer division result zero-এর দিকে truncate করে।

---

Arithmetic Type Promotion

Different numeric type-এর মধ্যে calculation হলে result wider type-এ promote হতে পারে।

int quantity = 3;
double price = 99.50;

double total = quantity * price;

System.out.println(total);

Output:

298.5

double wider type হওয়ায় result double।

---

Integer Arithmetic May Overflow Before Assignment

Wrong:

long result =
        2_000_000_000
        + 2_000_000_000;

System.out.println(result);

Operands দুটি int হওয়ায় addition আগে int হিসেবে হতে পারে এবং overflow হতে পারে।

Correct:

long result =
        2_000_000_000L
        + 2_000_000_000L;

System.out.println(result);

Output:

4000000000

---

Assignment Operator

Basic assignment operator:

=

Example:

int age = 30;

ডান পাশের value বাম পাশের variable-এ assign হয়।

30 → age

---

Reassignment

int score = 80;

score = 90;

System.out.println(score);

Output:

90

---

Assignment Is Right-to-Left

int firstValue;
int secondValue;

firstValue = secondValue = 10;

Evaluation:

secondValue = 10
firstValue = secondValue

Final values:

firstValue = 10
secondValue = 10

যদিও এটি valid, readability-এর জন্য separate statements ভালো।

int firstValue = 10;
int secondValue = 10;

---

Compound Assignment Operators

Compound assignment operator existing value-এর সঙ্গে operation চালিয়ে result আবার একই variable-এ assign করে।

Operator	Equivalent
+=	value = value + ...
-=	value = value - ...
*=	value = value * ...
/=	value = value / ...
%=	value = value % ...

---

Addition Assignment

Long form:

int score = 10;

score = score + 5;

Short form:

int score = 10;

score += 5;

Final value:

15

---

Subtraction Assignment

int balance = 1000;

balance -= 250;

System.out.println(balance);

Output:

750

Equivalent:

balance = balance - 250;

---

Multiplication Assignment

int value = 10;

value *= 3;

System.out.println(value);

Output:

30

---

Division Assignment

int value = 20;

value /= 4;

System.out.println(value);

Output:

5

---

Remainder Assignment

int value = 17;

value %= 5;

System.out.println(value);

Output:

2

---

Compound Assignment and Type Conversion

Compound assignment কিছু implicit conversion করতে পারে।

Example:

byte value = 10;

value += 5;

এটি compile করতে পারে।

কিন্তু:

byte value = 10;

value = value + 5;

এটি compile নাও করতে পারে, কারণ byte + int result int।

Equivalent behaviour conceptually:

value = (byte) (value + 5);

এই কারণে compound assignment convenient হলেও silent narrowing conversion-এর বিষয়ে সতর্ক থাকা উচিত।

---

Comparison Operators

Comparison operator দুটি value compare করে এবং একটি boolean result দেয়।

Java comparison operators:

Operator	Meaning
==	Equal to
!=	Not equal to
>	Greater than
<	Less than
>=	Greater than or equal to
<=	Less than or equal to

---

Equality Operator

Equality operator:

==

Example:

int age = 18;

boolean isExactlyEighteen =
        age == 18;

System.out.println(isExactlyEighteen);

Output:

true

---

Assignment vs Equality

Assignment:

age = 18;

Comparison:

age == 18

একটি = value assign করে।

দুটি == equality compare করে।

---

Not Equal Operator

!=

Example:

int score = 80;

boolean isNotPerfect =
        score != 100;

System.out.println(isNotPerfect);

Output:

true

---

Greater Than Operator

>

Example:

int score = 80;

boolean passedWithHighScore =
        score > 75;

Result:

true

---

Less Than Operator

<

Example:

int stock = 5;

boolean lowStock =
        stock < 10;

Result:

true

---

Greater Than or Equal To

>=

Example:

int age = 18;

boolean isAdult =
        age >= 18;

Result:

true

---

Less Than or Equal To

<=

Example:

int score = 40;

boolean needsImprovement =
        score <= 40;

Result:

true

---

Range Checking

ধরা যাক valid mark range:

0 to 100

int mark = 85;

boolean validMark =
        mark >= 0
        && mark <= 100;

Result:

true

এখানে logical && operator ব্যবহার করা হয়েছে, যা আমরা পরে শিখব।

---

Comparing Characters

char values Unicode numeric value অনুযায়ী compare করা যায়।

char firstLetter = 'A';
char secondLetter = 'B';

System.out.println(firstLetter < secondLetter);

Output:

true

কারণ 'A'-এর numeric value 'B'-এর চেয়ে ছোট।

---

Comparing Floating-Point Values

Direct equality risky হতে পারে।

double result = 0.1 + 0.2;

System.out.println(result == 0.3);

Output হতে পারে:

false

Approximate comparison:

double expected = 0.3;
double tolerance = 0.000001;

boolean approximatelyEqual =
        Math.abs(result - expected)
        < tolerance;

---

Comparing Strings

String content compare করতে == ব্যবহার করা উচিত নয়।

Wrong:

String firstValue =
        new String("Java");

String secondValue =
        new String("Java");

System.out.println(
        firstValue == secondValue
);

== reference identity compare করে।

Correct:

System.out.println(
        firstValue.equals(secondValue)
);

---

Logical Operators

Logical operator boolean expression combine বা reverse করতে ব্যবহৃত হয়।

Java-এর common logical operators:

Operator	Meaning
&&	Logical AND
`
!	Logical NOT

---

Logical AND

AND operator:

&&

Result true হবে যখন দুই পাশের condition-ই true।

Truth table:

Left	Right	Result
true	true	true
true	false	false
false	true	false
false	false	false

---

AND Example

boolean emailValid = true;
boolean passwordValid = true;

boolean canLogin =
        emailValid && passwordValid;

System.out.println(canLogin);

Output:

true

যদি password invalid হয়:

boolean emailValid = true;
boolean passwordValid = false;

Result:

false

---

Age Range with AND

int age = 25;

boolean withinRange =
        age >= 18
        && age <= 60;

এখানে দুই condition-ই true হতে হবে।

---

Logical OR

OR operator:

||

কমপক্ষে একটি condition true হলে result true।

Truth table:

Left	Right	Result
true	true	true
true	false	true
false	true	true
false	false	false

---

OR Example

boolean isAdmin = false;
boolean isInstructor = true;

boolean canAccessHarbor =
        isAdmin || isInstructor;

System.out.println(canAccessHarbor);

Output:

true

---

Multiple Allowed Roles

boolean isInstructor = false;
boolean isTeamMember = true;
boolean isAdmin = false;

boolean hasAccess =
        isInstructor
        || isTeamMember
        || isAdmin;

Result:

true

---

Logical NOT

NOT operator:

!

Boolean value reverse করে।

boolean active = true;

boolean inactive = !active;

System.out.println(inactive);

Output:

false

---

NOT Example

boolean enrollmentClosed = false;

if (!enrollmentClosed) {
    System.out.println("Enrollment is open");
}

!enrollmentClosed result true।

---

Combining Logical Operators

int age = 20;
boolean emailVerified = true;
boolean accountBlocked = false;

boolean canEnroll =
        age >= 18
        && emailVerified
        && !accountBlocked;

canEnroll true হবে যদি:

• Age অন্তত 18
• Email verified
• Account blocked নয়

---

Use Parentheses for Clarity

boolean allowed =
        isAdmin
        || isInstructor && isActive;

Operator precedence অনুযায়ী && আগে evaluate হবে।

Equivalent:

boolean allowed =
        isAdmin
        || (isInstructor && isActive);

কিন্তু যদি requirement হয় admin বা instructor—দুজনকেই active হতে হবে:

boolean allowed =
        (isAdmin || isInstructor)
        && isActive;

Parentheses logic পরিষ্কার করে।

---

Short-Circuit Evaluation

&& এবং || short-circuit operator।

এর অর্থ, result আগেই জানা গেলে Java দ্বিতীয় operand evaluate নাও করতে পারে।

---

Short-Circuit AND

leftCondition && rightCondition

যদি leftCondition false হয়, পুরো expression false।

তাই Java right side evaluate করে না।

Example:

String value = null;

boolean valid =
        value != null
        && !value.isBlank();

যদি value == null, প্রথম condition false।

তাই:

value.isBlank()

call হবে না।

এতে NullPointerException avoid হয়।

---

Wrong Condition Order

Risky:

boolean valid =
        !value.isBlank()
        && value != null;

এখানে value null হলে প্রথমেই method call হবে এবং error হবে।

Correct:

boolean valid =
        value != null
        && !value.isBlank();

---

Short-Circuit OR

leftCondition || rightCondition

যদি left side true হয়, পুরো expression true।

Right side evaluate হয় না।

Example:

boolean hasAccess =
        isAdmin
        || checkInstructorPermission();

যদি isAdmin true হয়, method call নাও হতে পারে।

---

& and | with Booleans

Java-তে single:

&
|

boolean operand-এর সঙ্গেও ব্যবহার করা যায়।

Difference:

• && short-circuit AND
• & দুই side-ই evaluate করে
• || short-circuit OR
• | দুই side-ই evaluate করে

Example:

boolean result =
        firstCondition & secondCondition;

Business condition-এর জন্য সাধারণত && এবং || ব্যবহার করা উচিত।

Single & এবং | bitwise operation-এও ব্যবহৃত হয়, যা advanced topic।

---

Unary Operators

Unary operator একটি operand-এর ওপর কাজ করে।

Common unary operators:

Operator	Meaning
+	Positive value
-	Negative value
!	Boolean negation
++	Increment
--	Decrement

---

Unary Plus

int value = +10;

Result:

10

Unary plus খুব কম প্রয়োজন হয়।

---

Unary Minus

int value = 10;

int negativeValue = -value;

System.out.println(negativeValue);

Output:

-10

---

Negating a Negative Value

int value = -10;

int positiveValue = -value;

System.out.println(positiveValue);

Output:

10

---

Logical Negation

boolean active = true;

System.out.println(!active);

Output:

false

Double negation:

System.out.println(!!active);

Output:

true

Double negation সাধারণ Java code-এ rarely needed।

---

Increment Operator

Increment operator:

++

Variable-এর value 1 বাড়ায়।

int count = 10;

count++;

System.out.println(count);

Output:

11

Equivalent:

count = count + 1;

অথবা:

count += 1;

---

Decrement Operator

Decrement operator:

--

Variable-এর value 1 কমায়।

int count = 10;

count--;

System.out.println(count);

Output:

9

---

Prefix Increment

++count

প্রথমে value increment হয়, তারপর expression-এ নতুন value ব্যবহৃত হয়।

int count = 10;

int result = ++count;

System.out.println(count);
System.out.println(result);

Output:

11
11

---

Postfix Increment

count++

Expression-এ প্রথমে পুরোনো value ব্যবহৃত হয়, তারপর variable increment হয়।

int count = 10;

int result = count++;

System.out.println(count);
System.out.println(result);

Output:

11
10

---

Prefix vs Postfix

Prefix

int value = 5;
int result = ++value;

Step:

1. value becomes 6
2. result gets 6

---

Postfix

int value = 5;
int result = value++;

Step:

1. result gets 5
2. value becomes 6

---

Prefix Decrement

int value = 5;
int result = --value;

Final:

value = 4
result = 4

---

Postfix Decrement

int value = 5;
int result = value--;

Final:

value = 4
result = 5

---

Avoid Complex Increment Expressions

Technically valid:

int result =
        count++ + ++count;

কিন্তু code খুব confusing।

Avoid করুন।

Better:

count++;

count++;

int result = previousValue + count;

অথবা logic আরও পরিষ্কারভাবে rewrite করুন।

Increment এবং decrement simple standalone statement হিসেবে ব্যবহার করা ভালো।

count++;

---

Increment Works Only on Variables

Invalid:

10++;

কারণ literal update করা যায় না।

Correct:

int value = 10;

value++;

---

Operator Precedence

একটি expression-এ multiple operator থাকলে কোনটি আগে evaluate হবে, তা operator precedence নির্ধারণ করে।

Example:

int result = 10 + 5 * 2;

Multiplication-এর precedence addition-এর চেয়ে বেশি।

তাই:

5 × 2 = 10
10 + 10 = 20

Output:

20

---

Using Parentheses

int result = (10 + 5) * 2;

Parentheses-এর expression আগে evaluate হয়।

10 + 5 = 15
15 × 2 = 30

Output:

30

---

Common Precedence Order

Simplified high-to-low order:

1. Parentheses
2. Unary operators
3. Multiplication, division, remainder
4. Addition, subtraction
5. Relational operators
6. Equality operators
7. Logical AND
8. Logical OR
9. Assignment

Example symbols:

()
++ -- ! + -
* / %
+ -
< <= > >=
== !=
&&
||
= += -= *= /= %=

---

Multiplication and Division Have Equal Precedence

Equal-precedence operators সাধারণত left-to-right evaluate হয়।

int result = 20 / 5 * 2;

Step:

20 / 5 = 4
4 * 2 = 8

Output:

8

---

Addition and Subtraction Left-to-Right

int result = 10 - 5 + 2;

Step:

10 - 5 = 5
5 + 2 = 7

Output:

7

---

Division Order Matters

int firstResult = 20 / 5 / 2;

Step:

20 / 5 = 4
4 / 2 = 2

Output:

2

কিন্তু:

int secondResult = 20 / (5 / 2);

5 / 2 integer division:

2

তারপর:

20 / 2 = 10

Output:

10

---

Comparison and Logical Precedence

boolean result =
        age >= 18
        && score >= 40;

Comparison আগে evaluate হয়:

age >= 18
score >= 40

তারপর:

result1 && result2

---

Parentheses Improve Readability

Precedence জানা থাকলেও complex condition-এ parentheses ব্যবহার করুন।

Less clear:

boolean allowed =
        isAdmin
        || isInstructor
        && isActive;

Clearer:

boolean allowed =
        isAdmin
        || (isInstructor && isActive);

---

String Concatenation Operator

+ numeric addition এবং String concatenation—দুই কাজেই ব্যবহার হয়।

Numeric:

int result = 10 + 20;

Output:

30

String:

String result = "Java" + " Foundation";

Output:

Java Foundation

---

Mixed String and Number Expressions

System.out.println("Total: " + 10 + 20);

Output:

Total: 1020

কারণ expression left-to-right evaluate হয়।

---

Correct Numeric Calculation

System.out.println(
        "Total: " + (10 + 20)
);

Output:

Total: 30

---

More Concatenation Examples

System.out.println(10 + 20 + " Total");

Output:

30 Total

কারণ first operation numeric addition।

System.out.println("Total " + 10 + 20);

Output:

Total 1020

কারণ first operation String concatenation।

---

Compound String Assignment

String message = "Hello";

message += " Java";

System.out.println(message);

Output:

Hello Java

Equivalent:

message = message + " Java";

String immutable হলেও variable নতুন String reference করতে পারে।

---

Conditional Operator

Conditional বা ternary operator:

condition ? valueIfTrue : valueIfFalse

Example:

int age = 20;

String status =
        age >= 18
                ? "Adult"
                : "Minor";

System.out.println(status);

Output:

Adult

---

Ternary Operator Structure

condition
        ? resultWhenTrue
        : resultWhenFalse;

Example:

int score = 75;

String result =
        score >= 40
                ? "Passed"
                : "Failed";

---

When to Use Ternary

Ternary useful যখন:

• Simple condition
• একটি value choose করতে হবে
• Expression concise থাকে

Good:

String status =
        active ? "Active" : "Inactive";

Avoid deeply nested ternary:

String grade =
        score >= 80 ? "A"
                : score >= 70 ? "B"
                : score >= 60 ? "C"
                : "F";

এটি technically valid, কিন্তু readability কমতে পারে।

Complex decision-এর জন্য if-else ভালো।

---

Expressions with Different Types

int quantity = 3;
double price = 99.50;

double total =
        quantity * price;

Result double।

---

Character Arithmetic

char letter = 'A';

int nextCode = letter + 1;

System.out.println(nextCode);

Output:

66

Character arithmetic-এর result সাধারণত int।

Character পেতে casting প্রয়োজন:

char nextLetter =
        (char) (letter + 1);

System.out.println(nextLetter);

Output:

B

---

Boolean Expressions

যে expression true বা false return করে, সেটি boolean expression।

Examples:

age >= 18

score == 100

courseName.equals("Java")

isActive && hasPermission

!accountBlocked

Condition statement-এ boolean expression ব্যবহার করা হয়।

---

De Morgan's Laws

Complex logical expression simplify করতে দুটি useful rule আছে।

!(A && B) == !A || !B

এবং:

!(A || B) == !A && !B

Example:

boolean notBothValid =
        !(emailValid && passwordValid);

Equivalent:

boolean notBothValid =
        !emailValid || !passwordValid;

Beginner হিসেবে complex negation-এ parentheses carefully ব্যবহার করুন।

---

Avoid Double Negatives

Hard to read:

boolean allowed =
        !isNotActive;

Better variable design:

boolean isActive;

Then:

boolean allowed = isActive;

Boolean name positive এবং clear রাখার চেষ্টা করুন।

---

Practical Example: Product Order

public class Main {

    static final int DELIVERY_CHARGE = 80;

    public static void main(String[] args) {
        int productPrice = 500;
        int quantity = 3;

        int subtotal =
                productPrice * quantity;

        int total =
                subtotal + DELIVERY_CHARGE;

        boolean freeDelivery =
                subtotal >= 2000;

        System.out.println(
                "Subtotal: " + subtotal
        );

        System.out.println(
                "Delivery charge: "
                + DELIVERY_CHARGE
        );

        System.out.println(
                "Total: " + total
        );

        System.out.println(
                "Free delivery eligible: "
                + freeDelivery
        );
    }
}

Output:

Subtotal: 1500
Delivery charge: 80
Total: 1580
Free delivery eligible: false

---

Practical Example: Student Result

public class Main {

    public static void main(String[] args) {
        int mathematicsMark = 80;
        int englishMark = 90;
        int scienceMark = 85;

        int totalMarks =
                mathematicsMark
                + englishMark
                + scienceMark;

        double averageMark =
                totalMarks / 3.0;

        boolean passedAllSubjects =
                mathematicsMark >= 40
                && englishMark >= 40
                && scienceMark >= 40;

        String result =
                passedAllSubjects
                        ? "Passed"
                        : "Failed";

        System.out.println(
                "Total: " + totalMarks
        );

        System.out.println(
                "Average: " + averageMark
        );

        System.out.println(
                "Result: " + result
        );
    }
}

Output:

Total: 255
Average: 85.0
Result: Passed

---

Practical Example: Enrollment Eligibility

public class Main {

    static final int MINIMUM_AGE = 18;

    public static void main(String[] args) {
        int learnerAge = 20;

        boolean emailVerified = true;
        boolean accountBlocked = false;
        boolean enrollmentOpen = true;

        boolean canEnroll =
                learnerAge >= MINIMUM_AGE
                && emailVerified
                && !accountBlocked
                && enrollmentOpen;

        System.out.println(
                "Can enroll: " + canEnroll
        );
    }
}

Output:

Can enroll: true

---

Practical Example: Login Permission

public class Main {

    public static void main(String[] args) {
        boolean emailValid = true;
        boolean passwordValid = true;
        boolean accountLocked = false;

        boolean canLogin =
                emailValid
                && passwordValid
                && !accountLocked;

        System.out.println(
                "Login allowed: " + canLogin
        );
    }
}

---

Practical Example: Discount Calculation

public class Main {

    public static void main(String[] args) {
        int orderTotal = 5000;
        boolean premiumCustomer = true;

        boolean eligibleForDiscount =
                orderTotal >= 5000
                || premiumCustomer;

        int discount =
                eligibleForDiscount
                        ? 500
                        : 0;

        int finalTotal =
                orderTotal - discount;

        System.out.println(
                "Discount: " + discount
        );

        System.out.println(
                "Final total: " + finalTotal
        );
    }
}

---

Common Error: Assignment Instead of Comparison

Wrong intention:

boolean active = false;

// active = true assigns a value

Condition-এর মধ্যে boolean assignment technically compile করতে পারে:

if (active = true) {
    System.out.println("Active");
}

এখানে comparison হয়নি। true assign হয়েছে।

Better:

if (active) {
    System.out.println("Active");
}

অথবা explicit:

if (active == true) {
    System.out.println("Active");
}

প্রথম version বেশি idiomatic।

---

Common Error: Comparing Boolean with true

Verbose:

if (isAvailable == true) {
}

Better:

if (isAvailable) {
}

For false:

if (!isAvailable) {
}

---

Common Error: Integer Division

Wrong:

double average = 5 / 2;

Result:

2.0

Correct:

double average = 5.0 / 2;

---

Common Error: String Concatenation

Wrong expectation:

System.out.println(
        "Total: " + 10 + 20
);

Output:

Total: 1020

Correct:

System.out.println(
        "Total: " + (10 + 20)
);

---

Common Error: Direct Double Equality

Risky:

double result = 0.1 + 0.2;

if (result == 0.3) {
}

Use tolerance:

boolean equal =
        Math.abs(result - 0.3)
        < 0.000001;

---

Common Error: Wrong Logical Operator

Requirement:

Email এবং password দুটিই valid হতে হবে।

Wrong:

boolean canLogin =
        emailValid || passwordValid;

এতে একটি valid হলেই login allowed হবে।

Correct:

boolean canLogin =
        emailValid && passwordValid;

---

Common Error: Incorrect Range Logic

Requirement:

Mark must be between 0 and 100

Wrong:

boolean valid =
        mark >= 0 || mark <= 100;

এটি প্রায় সব number-এর জন্য true হতে পারে।

Correct:

boolean valid =
        mark >= 0 && mark <= 100;

---

Common Error: Null Check Order

Wrong:

boolean valid =
        !name.isBlank()
        && name != null;

name null হলে error।

Correct:

boolean valid =
        name != null
        && !name.isBlank();

---

Common Error: Increment in Complex Expression

Confusing:

int count = 5;

int result =
        count++ + ++count;

এই code বুঝতে কঠিন এবং bug-prone।

Prefer separate statements।

---

Common Error: Overflow Before long Assignment

Wrong:

long result =
        2_000_000_000
        * 2;

Multiplication int হিসেবে overflow করতে পারে।

Correct:

long result =
        2_000_000_000L
        * 2;

---

Common Error: Extra Semicolon after if

Wrong:

if (age >= 18);
{
    System.out.println("Adult");
}

if statement extra semicolon-এ শেষ হয়েছে।

Block সবসময় execute করবে।

Correct:

if (age >= 18) {
    System.out.println("Adult");
}

---

Common Error: Chained Comparison

Mathematical style:

0 <= mark <= 100

Java-তে invalid:

boolean valid =
        0 <= mark <= 100;

Correct:

boolean valid =
        mark >= 0
        && mark <= 100;

---

Common Error: Confusing ! and !=

!

Boolean negation।

!active

!=

Not equal comparison।

score != 100

এগুলো আলাদা।

---

Common Error: Using & Instead of &&

boolean valid =
        value != null
        & !value.isBlank();

Single & দুই side evaluate করবে।

value null হলে second side error করবে।

Correct:

boolean valid =
        value != null
        && !value.isBlank();

---

Debugging Expressions

Expression debug করার সময় intermediate result variable-এ store করুন।

Hard to inspect:

boolean canEnroll =
        age >= 18
        && emailVerified
        && !accountBlocked
        && enrollmentOpen;

Debug version:

boolean ageValid =
        age >= 18;

boolean accountAllowed =
        !accountBlocked;

System.out.println(
        "ageValid = " + ageValid
);

System.out.println(
        "emailVerified = "
        + emailVerified
);

System.out.println(
        "accountAllowed = "
        + accountAllowed
);

System.out.println(
        "enrollmentOpen = "
        + enrollmentOpen
);

Intermediate value bug identify করতে সাহায্য করে।

---

Important Terms

Operator

এক বা একাধিক value-এর ওপর operation চালানো symbol।

Operand

Operator যে value বা variable-এর ওপর কাজ করে।

Expression

Operator, variable এবং value-এর combination, যা result তৈরি করে।

Arithmetic Operator

Numeric calculation-এর operator।

Assignment Operator

Variable-এ value assign করে।

Compound Assignment

Operation এবং assignment একসঙ্গে করে।

Comparison Operator

দুটি value compare করে boolean result দেয়।

Logical Operator

একাধিক boolean condition combine বা reverse করে।

Unary Operator

একটি operand-এর ওপর কাজ করে।

Increment

Value 1 বাড়ানো।

Decrement

Value 1 কমানো।

Prefix

Value আগে update হয়, তারপর expression-এ ব্যবহৃত হয়।

Postfix

Expression-এ পুরোনো value ব্যবহৃত হয়, তারপর update হয়।

Operator Precedence

Multiple operator-এর evaluation order।

Short-Circuit Evaluation

Result আগে জানা গেলে remaining expression evaluate না করা।

Ternary Operator

Condition অনুযায়ী দুটি value-এর মধ্যে একটি select করে।

Remainder

Division-এর পরে অবশিষ্ট value।

---

Practice Exercise 1: Identify Operators and Operands

Expression:

price * quantity

Identify করুন:

1. Operator
2. First operand
3. Second operand
4. Expression-এর possible result type

---

Practice Exercise 2: Arithmetic Operations

Variables:

int firstNumber = 20;
int secondNumber = 6;

নিচের result বের করুন:

• Addition
• Subtraction
• Multiplication
• Integer division
• Remainder

---

Practice Exercise 3: Predict the Output

System.out.println(10 + 5 * 2);
System.out.println((10 + 5) * 2);
System.out.println(20 / 5 * 2);
System.out.println(20 / (5 * 2));

---

Practice Exercise 4: Integer Division

Output predict করুন:

int firstResult = 7 / 2;
double secondResult = 7 / 2;
double thirdResult = 7.0 / 2;
double fourthResult = 7 / 2.0;

---

Practice Exercise 5: Even or Odd

একটি int variable তৈরি করুন।

% operator ব্যবহার করে determine করুন numberটি even কি না।

Example:

Number: 12
Even: true

---

Practice Exercise 6: Compound Assignment

নিচের code short form-এ লিখুন:

score = score + 10;
balance = balance - 500;
price = price * 2;
total = total / 4;
value = value % 3;

---

Practice Exercise 7: Trace the Variable

int value = 10;

value += 5;
value *= 2;
value -= 4;
value /= 2;

প্রতিটি step-এর পরে value লিখুন।

---

Practice Exercise 8: Comparison Results

Variables:

int firstValue = 10;
int secondValue = 20;

Output predict করুন:

System.out.println(firstValue == secondValue);
System.out.println(firstValue != secondValue);
System.out.println(firstValue > secondValue);
System.out.println(firstValue < secondValue);
System.out.println(firstValue >= 10);
System.out.println(secondValue <= 20);

---

Practice Exercise 9: Valid Mark Range

একটি mark valid হবে যখন:

0 <= mark <= 100

Java boolean expression লিখুন।

---

Practice Exercise 10: Login Validation

Variables:

boolean emailValid = true;
boolean passwordValid = false;

canLogin determine করুন।

Requirement:

Email এবং password দুটিই valid হতে হবে।

---

Practice Exercise 11: Role Access

Variables:

boolean isInstructor = false;
boolean isTeamMember = true;
boolean isAdmin = false;

Requirement:

যেকোনো একটি role থাকলে access দেওয়া হবে।

Boolean expression লিখুন।

---

Practice Exercise 12: Enrollment Eligibility

Requirements:

• Age at least 18
• Email verified
• Account blocked নয়
• Enrollment open

Variables তৈরি করে canEnroll expression লিখুন।

---

Practice Exercise 13: Short-Circuit Safety

নিচের code safe কি না ব্যাখ্যা করুন:

String name = null;

boolean valid =
        name != null
        && !name.isBlank();

তারপর operator order উল্টালে কী হবে, তা লিখুন।

---

Practice Exercise 14: Prefix vs Postfix

Output predict করুন:

int firstValue = 5;
int firstResult = ++firstValue;

int secondValue = 5;
int secondResult = secondValue++;

System.out.println(firstValue);
System.out.println(firstResult);
System.out.println(secondValue);
System.out.println(secondResult);

---

Practice Exercise 15: Increment Counter

একটি variable:

int lessonProgress = 0;

তিনবার increment করুন।

Expected final output:

3

---

Practice Exercise 16: String Concatenation

Output predict করুন:

System.out.println("Total: " + 10 + 20);
System.out.println("Total: " + (10 + 20));
System.out.println(10 + 20 + " Total");
System.out.println("Result " + 10 * 2);

---

Practice Exercise 17: Ternary Operator

Variable:

int score = 75;

Ternary operator ব্যবহার করে:

Passed

অথবা:

Failed

return করুন।

Passing mark:

40

---

Practice Exercise 18: Fix the Range Logic

Wrong:

boolean valid =
        mark >= 0 || mark <= 100;

Codeটি fix করুন এবং explain করুন কেন || ভুল ছিল।

---

Practice Exercise 19: Fix the Addition

Wrong output:

System.out.println(
        "Average: " + 80 + 90 + 85
);

Expected:

Average: 255

Code fix করুন।

---

Practice Exercise 20: Prevent Overflow

Wrong:

long total =
        2_000_000_000
        + 2_000_000_000;

Expected:

4000000000

Code fix করুন।

---

Practice Exercise 21: Product Order Program

Create variables:

productPrice
quantity
deliveryCharge
subtotal
total

Formula:

subtotal = productPrice × quantity
total = subtotal + deliveryCharge

Output format:

Product price: 500
Quantity: 3
Subtotal: 1500
Delivery charge: 80
Total: 1580

---

Practice Exercise 22: Student Result Program

Use:

Mathematics: 80
English: 90
Science: 85

Calculate:

• Total
• Decimal average
• Passed all subjects কি না
• Ternary operator দিয়ে final result

Passing mark per subject:

40

---

Practice Exercise 23: Discount Eligibility

Discount পাওয়া যাবে যদি:

• Order total অন্তত 5000
• অথবা customer premium

Variables:

int orderTotal = 4500;
boolean premiumCustomer = true;

Determine করুন customer eligible কি না।

---

Practice Exercise 24: Fix the Null Check

Wrong:

String courseName = null;

boolean valid =
        !courseName.isBlank()
        && courseName != null;

Safe version লিখুন।

---

Practice Exercise 25: Explain in Your Own Words

নিজের ভাষায় উত্তর দিন:

1. Operator কী?
2. Operand কী?
3. Expression কী?
4. Integer division কী?
5. % operator কী করে?
6. Assignment এবং equality operator-এর পার্থক্য কী?
7. Compound assignment কী?
8. Comparison operator কী return করে?
9. && এবং ||-এর পার্থক্য কী?
10. ! operator কী করে?
11. Short-circuit evaluation কী?
12. Prefix এবং postfix increment-এর পার্থক্য কী?
13. Operator precedence কী?
14. Parentheses কেন ব্যবহার করা হয়?
15. String concatenation-এ parentheses কখন প্রয়োজন?
16. Ternary operator কখন useful?
17. double compare করতে direct == risky কেন?
18. & এবং &&-এর পার্থক্য কী?

---

Knowledge Check

Question 1

Operator কী?

Question 2

Operand কী?

Question 3

Expression কী?

Question 4

Java-এর basic arithmetic operators কী কী?

Question 5

% operator কী return করে?

Question 6

5 / 2-এর result কী?

Question 7

Decimal result পাওয়ার জন্য কী করতে হবে?

Question 8

Assignment operator কোনটি?

Question 9

Equality operator কোনটি?

Question 10

Compound addition assignment কোনটি?

Question 11

Comparison operator কী ধরনের result দেয়?

Question 12

Logical AND operator কোনটি?

Question 13

Logical OR operator কোনটি?

Question 14

Logical NOT operator কোনটি?

Question 15

&& কখন true হয়?

Question 16

|| কখন true হয়?

Question 17

Short-circuit AND কীভাবে কাজ করে?

Question 18

Prefix increment কী?

Question 19

Postfix increment কী?

Question 20

10 + 5 * 2-এর result কী?

Question 21

(10 + 5) * 2-এর result কী?

Question 22

String content compare করতে কোন method ব্যবহার করা উচিত?

Question 23

"Total: " + 10 + 20-এর output কী?

Question 24

Ternary operator-এর syntax কী?

Question 25

value != null && !value.isBlank() safe কেন?

Question 26

& এবং &&-এর প্রধান পার্থক্য কী?

Question 27

byte + byte expression সাধারণত কোন type return করে?

Question 28

Integer overflow assignment-এর আগে ঘটতে পারে কি?

Question 29

Direct floating-point equality risky কেন?

Question 30

Complex expression readable করতে কী ব্যবহার করা উচিত?

---

Knowledge Check Answers

Answer 1

Operator হলো এমন symbol, যা এক বা একাধিক value-এর ওপর operation চালায়।

Answer 2

Operator যে value বা variable-এর ওপর কাজ করে, সেটি operand।

Answer 3

Value, variable এবং operator-এর combination, যা evaluate হয়ে result দেয়, সেটি expression।

Answer 4

+
-
*
/
%

Answer 5

Division-এর remainder return করে।

Answer 6

2

কারণ integer division।

Answer 7

কমপক্ষে একটি operand floating-point করতে হবে।

Example:

5.0 / 2

Answer 8

=

Answer 9

==

Answer 10

+=

Answer 11

boolean result দেয়।

Answer 12

&&

Answer 13

||

Answer 14

!

Answer 15

দুই condition-ই true হলে।

Answer 16

কমপক্ষে একটি condition true হলে।

Answer 17

Left side false হলে right side evaluate হয় না।

Answer 18

Prefix increment আগে value বাড়ায়, তারপর updated value expression-এ ব্যবহার করে।

++value

Answer 19

Postfix increment আগে current value expression-এ ব্যবহার করে, তারপর value বাড়ায়।

value++

Answer 20

20

Answer 21

30

Answer 22

equals()

Answer 23

Total: 1020

Answer 24

condition ? valueIfTrue : valueIfFalse

Answer 25

value null হলে first condition false হয় এবং short-circuit-এর কারণে isBlank() call হয় না।

Answer 26

&& short-circuit করে। & দুই side-ই evaluate করে।

Answer 27

সাধারণত int।

Answer 28

হ্যাঁ। Operands যদি int হয়, result long variable-এ assign হওয়ার আগেই overflow হতে পারে।

Answer 29

Binary floating-point representation সব decimal value exactly store করতে পারে না।

Answer 30

Parentheses, intermediate variables এবং clear formatting ব্যবহার করা উচিত।

---

Lesson Summary

এই lesson-এ আমরা শিখেছি:

• Operator value-এর ওপর operation চালায়
• Operand হলো operator-এর input value
• Expression evaluate হয়ে result তৈরি করে
• Arithmetic operator calculation করে
• +, -, *, / এবং % basic arithmetic operator
• Integer division fractional অংশ বাদ দেয়
• Decimal result-এর জন্য floating-point operand প্রয়োজন
• % remainder return করে
• = assignment এবং == equality comparison
• Compound assignment existing value update করে
• Comparison operator boolean result দেয়
• && দুই condition true হলে true
• || অন্তত একটি condition true হলে true
• ! boolean value reverse করে
• && এবং || short-circuit evaluation ব্যবহার করে
• Null check logical expression-এর শুরুতে রাখা গুরুত্বপূর্ণ
• Increment এবং decrement value 1 করে পরিবর্তন করে
• Prefix আগে update করে
• Postfix পরে update করে
• Complex increment expression avoid করা উচিত
• Operator precedence evaluation order নির্ধারণ করে
• Parentheses calculation এবং logic পরিষ্কার করে
• Multiplication এবং division addition-এর আগে evaluate হয়
• + String concatenation-এও ব্যবহৃত হয়
• String concatenation left-to-right execute হয়
• Ternary operator simple condition অনুযায়ী value select করে
• Floating-point equality-এর জন্য tolerance ব্যবহার করা যায়
• String content comparison-এর জন্য .equals() ব্যবহার করা উচিত
• Integer overflow expression evaluate হওয়ার সময় ঘটতে পারে
• Clear intermediate variables debugging এবং readability improve করে

---

Next Lesson

পরবর্তী lesson-এ আমরা Java type conversion এবং casting শিখব।

আমরা জানব:

• Type conversion কী
• Widening conversion
• Narrowing conversion
• Implicit conversion
• Explicit casting
• Numeric type conversion
• Data loss
• Overflow during casting
• char এবং numeric conversion
• String থেকে number conversion
• Number থেকে String conversion
• Invalid conversion
• Common casting-related error