Programming and Java Fundamentals

Conditional Statements

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Lesson Overview

Program সবসময় একই instruction execute করে না। অনেক সময় data বা condition অনুযায়ী program-কে আলাদা decision নিতে হয়।

উদাহরণ:

User-এর বয়স 18 বা তার বেশি হলে enrollment allow করা
Mark 40 বা তার বেশি হলে passed দেখানো
Password সঠিক হলে login allow করা
Product stock না থাকলে order reject করা
Number positive, negative নাকি zero তা নির্ধারণ করা

Java-তে condition অনুযায়ী program flow control করার জন্য conditional statement ব্যবহার করা হয়।

এই lesson-এ আমরা শিখব:

Boolean condition
if
if-else
else-if
Multiple independent conditions
Nested conditions
Logical operators in conditions
Range validation
String এবং null checks
Common conditional mistakes
Practical decision-making programs

Learning Objectives

এই lesson শেষ করার পর আপনি পারবেন:

Boolean expression ব্যবহার করে decision নিতে
if, if-else এবং else-if লিখতে
Multiple condition combine করতে
Numeric range validate করতে
Nested condition বুঝতে
String safely compare করতে
Invalid input আগে reject করতে
Grade, eligibility এবং login logic তৈরি করতে

What Is Conditional Execution?

Conditional execution হলো condition-এর result অনুযায়ী কোনো code block execute বা skip করা।

int age = 20;

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

Condition:

age >= 18

Result:

true

তাই output:

Adult

যদি age হয় 15, condition false হবে এবং block execute হবে না।

Boolean Conditions

Conditional statement-এর condition অবশ্যই boolean result দিতে হবে।

Valid conditions:

age >= 18

score == 100

isActive

!accountBlocked

emailValid && passwordValid

"ADMIN".equals(role)

প্রতিটি condition-এর result হবে:

true

অথবা:

false

Numbers Are Not Boolean Values

Java-তে number সরাসরি condition হিসেবে ব্যবহার করা যায় না।

Invalid:

int value = 1;

if (value) {
    System.out.println("Valid");
}

Correct:

if (value == 1) {
    System.out.println("Value is one");
}

The `if` Statement

Syntax:

if (condition) {
    // Executes when condition is true
}

Example:

int mark = 75;

if (mark >= 40) {
    System.out.println("Passed");
}

Output:

Passed

যদি condition false হয়, block skip হবে।

Multiple Statements Inside `if`

int age = 20;

if (age >= 18) {
    System.out.println("Age requirement met");
    System.out.println("Registration allowed");
}

Condition true হলে block-এর সব statement top-to-bottom execute হবে।

Always Use Curly Braces

একটি statement হলে braces technically omit করা যায়:

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

তবে সবসময় braces ব্যবহার করা safer।

Wrong and misleading:

if (age >= 18)
    System.out.println("Adult");
    System.out.println("Registration allowed");

এখানে শুধু প্রথম statement condition-এর অংশ।

Actual behaviour:

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

System.out.println("Registration allowed");

Correct:

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

The `if-else` Statement

Condition true হলে একটি block এবং false হলে অন্য block execute করতে if-else ব্যবহার করা হয়।

Syntax:

if (condition) {
    // True branch
} else {
    // False branch
}

Example:

int mark = 35;

if (mark >= 40) {
    System.out.println("Passed");
} else {
    System.out.println("Failed");
}

Output:

Failed

একটি execution-এ শুধু একটি branch execute হবে।

Example: Even or Odd

int number = 17;

if (number % 2 == 0) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

Output:

Odd

The `else-if` Chain

Multiple mutually exclusive condition handle করতে else-if ব্যবহার করা হয়।

Syntax:

if (firstCondition) {
    // First branch
} else if (secondCondition) {
    // Second branch
} else if (thirdCondition) {
    // Third branch
} else {
    // Default branch
}

Java conditionগুলো top-to-bottom check করে।

প্রথম true branch execute হওয়ার পর remaining branches skip হয়।

Example: Positive, Negative, or Zero

int number = -10;

if (number > 0) {
    System.out.println("Positive");
} else if (number < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Zero");
}

Output:

Negative

Condition Order Matters

Wrong order:

int mark = 90;

if (mark >= 40) {
    System.out.println("Passed");
} else if (mark >= 80) {
    System.out.println("Excellent");
}

Output:

Passed

mark >= 40 প্রথমেই true হয়েছে। তাই second branch check হয়নি।

Correct:

if (mark >= 80) {
    System.out.println("Excellent");
} else if (mark >= 40) {
    System.out.println("Passed");
} else {
    System.out.println("Failed");
}

Specific বা higher condition আগে রাখা উচিত।

Grade Calculation

int mark = 85;
String grade;

if (mark < 0 || mark > 100) {
    grade = "Invalid";
} else if (mark >= 80) {
    grade = "A";
} else if (mark >= 70) {
    grade = "B";
} else if (mark >= 60) {
    grade = "C";
} else if (mark >= 50) {
    grade = "D";
} else if (mark >= 40) {
    grade = "E";
} else {
    grade = "F";
}

System.out.println("Grade: " + grade);

Output:

Grade: A

Why Range Checks Are Not Repeated

Consider:

if (mark >= 80) {
    grade = "A";
} else if (mark >= 70) {
    grade = "B";
}

Second condition only check হয় যখন:

mark < 80

তাই:

mark >= 70

effectively means:

70 <= mark < 80

Multiple Independent `if` Statements

Multiple if statement independently evaluate হয়।

int mark = 90;

if (mark >= 40) {
    System.out.println("Passed");
}

if (mark >= 80) {
    System.out.println("Distinction");
}

Output:

Passed
Distinction

দুটি condition true হওয়ায় দুটি block-ই execute হয়েছে।

Multiple `if` vs `else-if`

Multiple `if`

if (conditionA) {
}

if (conditionB) {
}

Conditions independently evaluate হয়
একাধিক block execute হতে পারে

`else-if`

if (conditionA) {
} else if (conditionB) {
}

First matching branch execute হয়
Remaining branches skip হয়

When to Use Multiple `if`

যখন conditions independent।

Example:

if (mark >= 40) {
    System.out.println("Passed");
}

if (mark >= 80) {
    System.out.println("Scholarship eligible");
}

একজন student একইসঙ্গে passed এবং scholarship eligible হতে পারে।

When to Use `else-if`

যখন results mutually exclusive।

Example:

Grade A
Grade B
Grade C
Grade D
Grade F

একটি mark-এর জন্য একটি grade।

Combining Conditions with `&&`

&& ব্যবহার করা হয় যখন সব condition true হতে হবে।

int age = 20;
boolean emailVerified = true;

if (age >= 18 && emailVerified) {
    System.out.println("Enrollment allowed");
}

Block execute হবে যদি:

Age অন্তত 18
Email verified

Combining Conditions with `||`

|| ব্যবহার করা হয় যখন অন্তত একটি condition true হলেই হবে।

boolean isInstructor = false;
boolean isAdmin = true;

if (isInstructor || isAdmin) {
    System.out.println("Access allowed");
}

Output:

Access allowed

Using Logical NOT

! boolean value reverse করে।

boolean accountBlocked = false;

if (!accountBlocked) {
    System.out.println("Account access allowed");
}

Complex Conditions

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

if (
        age >= 18
        && emailVerified
        && !accountBlocked
        && enrollmentOpen
) {
    System.out.println("Enrollment allowed");
}

Long condition multiple line-এ লেখা বেশি readable।

Extracting Conditions into Variables

Complex logic understandable করতে intermediate boolean variable ব্যবহার করা যায়।

boolean ageValid = age >= 18;
boolean accountAllowed = !accountBlocked;

boolean canEnroll =
        ageValid
        && emailVerified
        && accountAllowed
        && enrollmentOpen;

if (canEnroll) {
    System.out.println("Enrollment allowed");
}

Range Validation

Valid mark range:

0 to 100

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

Invalid mark:

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

Wrong Range Logic

Wrong:

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

ধরা যাক:

mark = 500

Evaluation:

500 >= 0 → true
500 <= 100 → false
true || false → true

Wrongly valid হয়ে গেছে।

Correct:

mark >= 0 && mark <= 100

Chained Comparison Is Invalid

Mathematical notation:

0 <= mark <= 100

Java-তে invalid:

if (0 <= mark <= 100) {
}

Correct:

if (mark >= 0 && mark <= 100) {
}

Nested `if`

একটি conditional block-এর মধ্যে আরেকটি conditional statement থাকলে সেটি nested if।

int age = 20;
boolean emailVerified = true;

if (age >= 18) {
    if (emailVerified) {
        System.out.println("Enrollment allowed");
    }
}

Inner condition শুধু outer condition true হলে evaluate হয়।

Nested `if-else`

boolean loggedIn = true;
boolean isAdmin = false;

if (loggedIn) {
    if (isAdmin) {
        System.out.println("Admin dashboard");
    } else {
        System.out.println("User dashboard");
    }
} else {
    System.out.println("Please log in");
}

Nested condition useful যখন inner decision outer context-এর ওপর depend করে।

Avoid Unnecessary Deep Nesting

Hard to read:

if (userExists) {
    if (passwordCorrect) {
        if (!accountLocked) {
            if (emailVerified) {
                System.out.println("Login successful");
            }
        }
    }
}

Simpler:

boolean canLogin =
        userExists
        && passwordCorrect
        && !accountLocked
        && emailVerified;

if (canLogin) {
    System.out.println("Login successful");
}

Showing Specific Failure Reasons

সব condition combine করলে শুধু success বা failure জানা যায়।

Specific reason দেখাতে ordered else-if useful।

if (!userExists) {
    System.out.println("User not found");
} else if (!passwordCorrect) {
    System.out.println("Incorrect password");
} else if (accountLocked) {
    System.out.println("Account is locked");
} else if (!emailVerified) {
    System.out.println("Email verification required");
} else {
    System.out.println("Login successful");
}

Condition order এখানে business flow reflect করে।

Guard Conditions

Invalid input আগে handle করলে main logic সহজ হয়।

int mark = 120;

if (mark < 0 || mark > 100) {
    System.out.println("Invalid mark");
} else {
    System.out.println("Valid mark");
}

Method-এর মধ্যে early return ব্যবহার করা যায়:

static void displayGrade(int mark) {
    if (mark < 0 || mark > 100) {
        System.out.println("Invalid mark");
        return;
    }

    if (mark >= 80) {
        System.out.println("Grade: A");
    } else if (mark >= 70) {
        System.out.println("Grade: B");
    } else {
        System.out.println("Grade: C or below");
    }
}

Variable Scope in Conditional Blocks

Block-এর মধ্যে declare করা variable block-এর বাইরে accessible নয়।

if (mark >= 40) {
    String result = "Passed";

    System.out.println(result);
}

Invalid:

if (mark >= 40) {
    String result = "Passed";
}

System.out.println(result);

result শুধু if block-এর scope-এর মধ্যে আছে।

Assigning a Result in Both Branches

String result;

if (mark >= 40) {
    result = "Passed";
} else {
    result = "Failed";
}

System.out.println(result);

Valid, কারণ দুই possible branch-ই result initialize করে।

Possibly Uninitialized Variable

Wrong:

String result;

if (mark >= 40) {
    result = "Passed";
}

System.out.println(result);

Condition false হলে result initialize হবে না।

Compiler error হতে পারে:

variable result might not have been initialized

Ternary Operator for Simple Decisions

Simple if-else:

String result;

if (mark >= 40) {
    result = "Passed";
} else {
    result = "Failed";
}

Ternary form:

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

Complex logic-এর জন্য if-else বেশি readable।

Comparing Strings in Conditions

String content compare করতে == ব্যবহার করা উচিত নয়।

Wrong:

if (role == "ADMIN") {
    System.out.println("Access allowed");
}

Correct:

if ("ADMIN".equals(role)) {
    System.out.println("Access allowed");
}

Case-insensitive:

if ("ADMIN".equalsIgnoreCase(role)) {
    System.out.println("Access allowed");
}

Null-Safe String Validation

String courseName = null;

if (
        courseName != null
        && !courseName.isBlank()
) {
    System.out.println("Valid course name");
} else {
    System.out.println("Course name is required");
}

courseName null হলে first condition false হবে।

Short-circuit-এর কারণে:

courseName.isBlank()

call হবে না।

Wrong Null Check Order

Wrong:

if (
        !courseName.isBlank()
        && courseName != null
) {
}

courseName null হলে প্রথম method call-এই NullPointerException হবে।

Correct order:

courseName != null
&& !courseName.isBlank()

Checking Multiple String Values

String role = "INSTRUCTOR";

if (
        "ADMIN".equals(role)
        || "INSTRUCTOR".equals(role)
) {
    System.out.println("Access allowed");
}

Invalid approach:

if (role.equals("ADMIN" || "INSTRUCTOR")) {
}

|| boolean expressions combine করে, String values নয়।

Comparing `char`

Primitive char compare করতে == ব্যবহার করা যায়।

char grade = 'A';

if (grade == 'A') {
    System.out.println("Excellent");
}

Floating-Point Conditions

Floating-point value direct equality দিয়ে compare করা risky হতে পারে।

double total = 0.1 + 0.2;

if (total == 0.3) {
    System.out.println("Equal");
}

Condition false হতে পারে।

Approximate comparison:

double tolerance = 0.000001;

if (Math.abs(total - 0.3) < tolerance) {
    System.out.println("Approximately equal");
}

Practical Example: Age Classification

int age = 25;

if (age < 0 || age > 150) {
    System.out.println("Invalid age");
} else if (age < 13) {
    System.out.println("Child");
} else if (age < 18) {
    System.out.println("Teenager");
} else if (age < 60) {
    System.out.println("Adult");
} else {
    System.out.println("Senior");
}

Output:

Adult

Practical Example: Number Classification

int number = -8;

if (number > 0) {
    System.out.println("Positive");
} else if (number < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Zero");
}

if (number % 2 == 0) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

Output:

Negative
Even

Sign এবং even/odd independent property, তাই separate conditional statements ব্যবহার করা হয়েছে।

Practical Example: Course Enrollment

public class Main {

    static final int MINIMUM_AGE = 18;

    public static void main(String[] args) {
        int learnerAge = 20;
        boolean emailVerified = true;
        boolean enrollmentOpen = true;
        boolean alreadyEnrolled = false;
        boolean accountBlocked = false;

        if (learnerAge < 0) {
            System.out.println("Invalid age");
        } else if (learnerAge < MINIMUM_AGE) {
            System.out.println(
                    "Minimum age requirement not met"
            );
        } else if (!emailVerified) {
            System.out.println(
                    "Email verification required"
            );
        } else if (!enrollmentOpen) {
            System.out.println(
                    "Enrollment is closed"
            );
        } else if (alreadyEnrolled) {
            System.out.println(
                    "Learner is already enrolled"
            );
        } else if (accountBlocked) {
            System.out.println(
                    "Blocked account cannot enroll"
            );
        } else {
            System.out.println(
                    "Enrollment allowed"
            );
        }
    }
}

Practical Example: Product Stock

int stockQuantity = 5;
int requestedQuantity = 8;

if (requestedQuantity <= 0) {
    System.out.println(
            "Quantity must be greater than zero"
    );
} else if (stockQuantity == 0) {
    System.out.println(
            "Product is out of stock"
    );
} else if (
        requestedQuantity > stockQuantity
) {
    System.out.println(
            "Not enough stock"
    );
} else {
    System.out.println(
            "Order can be placed"
    );
}

Output:

Not enough stock

Practical Example: Discount Rules

int orderTotal = 6000;
boolean premiumCustomer = true;

int discountPercentage;

if (
        premiumCustomer
        && orderTotal >= 5000
) {
    discountPercentage = 20;
} else if (premiumCustomer) {
    discountPercentage = 10;
} else if (orderTotal >= 5000) {
    discountPercentage = 5;
} else {
    discountPercentage = 0;
}

System.out.println(
        "Discount: "
        + discountPercentage
        + "%"
);

Combined and most specific condition আগে রাখা হয়েছে।

Practical Example: Largest of Three Numbers

int firstNumber = 10;
int secondNumber = 25;
int thirdNumber = 20;

if (
        firstNumber >= secondNumber
        && firstNumber >= thirdNumber
) {
    System.out.println(
            firstNumber + " is largest"
    );
} else if (
        secondNumber >= firstNumber
        && secondNumber >= thirdNumber
) {
    System.out.println(
            secondNumber + " is largest"
    );
} else {
    System.out.println(
            thirdNumber + " is largest"
    );
}

Output:

25 is largest

Interactive Grade Program

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner =
                new Scanner(System.in);

        System.out.print(
                "Enter your mark: "
        );

        int mark =
                Integer.parseInt(
                        scanner
                                .nextLine()
                                .strip()
                );

        if (mark < 0 || mark > 100) {
            System.out.println(
                    "Invalid mark"
            );
        } else if (mark >= 80) {
            System.out.println(
                    "Grade: A"
            );
        } else if (mark >= 70) {
            System.out.println(
                    "Grade: B"
            );
        } else if (mark >= 60) {
            System.out.println(
                    "Grade: C"
            );
        } else if (mark >= 50) {
            System.out.println(
                    "Grade: D"
            );
        } else if (mark >= 40) {
            System.out.println(
                    "Grade: E"
            );
        } else {
            System.out.println(
                    "Grade: F"
            );
        }

        scanner.close();
    }
}

Complete Student Result Program

import java.util.Scanner;

public class Main {

    static final int MINIMUM_MARK = 0;
    static final int MAXIMUM_MARK = 100;
    static final int PASSING_MARK = 40;

    public static void main(String[] args) {
        Scanner scanner =
                new Scanner(System.in);

        System.out.print(
                "Enter student name: "
        );

        String studentName =
                scanner
                        .nextLine()
                        .strip();

        System.out.print(
                "Enter mathematics mark: "
        );

        int mathematicsMark =
                Integer.parseInt(
                        scanner
                                .nextLine()
                                .strip()
                );

        System.out.print(
                "Enter English mark: "
        );

        int englishMark =
                Integer.parseInt(
                        scanner
                                .nextLine()
                                .strip()
                );

        System.out.print(
                "Enter science mark: "
        );

        int scienceMark =
                Integer.parseInt(
                        scanner
                                .nextLine()
                                .strip()
                );

        boolean mathematicsValid =
                mathematicsMark >= MINIMUM_MARK
                && mathematicsMark <= MAXIMUM_MARK;

        boolean englishValid =
                englishMark >= MINIMUM_MARK
                && englishMark <= MAXIMUM_MARK;

        boolean scienceValid =
                scienceMark >= MINIMUM_MARK
                && scienceMark <= MAXIMUM_MARK;

        boolean allMarksValid =
                mathematicsValid
                && englishValid
                && scienceValid;

        if (!allMarksValid) {
            System.out.println(
                    "One or more marks are invalid"
            );

            scanner.close();
            return;
        }

        int totalMarks =
                mathematicsMark
                + englishMark
                + scienceMark;

        double averageMark =
                totalMarks / 3.0;

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

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

        String grade;

        if (!passedAllSubjects) {
            grade = "F";
        } else if (averageMark >= 80) {
            grade = "A";
        } else if (averageMark >= 70) {
            grade = "B";
        } else if (averageMark >= 60) {
            grade = "C";
        } else if (averageMark >= 50) {
            grade = "D";
        } else {
            grade = "E";
        }

        System.out.println();
        System.out.println(
                "Student: " + studentName
        );

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

        System.out.println(
                "Average: "
                + "%.2f".formatted(
                        averageMark
                )
        );

        System.out.println(
                "Grade: " + grade
        );

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

        scanner.close();
    }
}

Common Mistakes

Extra Semicolon After `if`

Wrong:

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

The semicolon ends the if statement।

Correct:

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

Assignment Instead of Comparison

Wrong:

boolean active = false;

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

এখানে true assign হয়েছে।

Preferred:

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

Comparing Boolean with `true`

Verbose:

if (isAvailable == true) {
}

Preferred:

if (isAvailable) {
}

For false:

if (!isAvailable) {
}

Wrong Grade Order

Wrong:

if (mark >= 40) {
    System.out.println("Passed");
} else if (mark >= 80) {
    System.out.println("Grade A");
}

Correct:

if (mark >= 80) {
    System.out.println("Grade A");
} else if (mark >= 40) {
    System.out.println("Passed");
}

Independent `if` for Exclusive Grades

Wrong:

if (mark >= 80) {
    System.out.println("A");
}

if (mark >= 70) {
    System.out.println("B");
}

if (mark >= 60) {
    System.out.println("C");
}

Mark 85 হলে output:

A
B
C

Correct:

if (mark >= 80) {
    System.out.println("A");
} else if (mark >= 70) {
    System.out.println("B");
} else if (mark >= 60) {
    System.out.println("C");
}

Wrong String Comparison

Wrong:

if (role == "ADMIN") {
}

Correct:

if ("ADMIN".equals(role)) {
}

Wrong Null Check Order

Wrong:

if (
        !name.isBlank()
        && name != null
) {
}

Correct:

if (
        name != null
        && !name.isBlank()
) {
}

Impossible Condition

if (
        age < 18
        && age >= 18
) {
}

একটি value একই সময়ে দুই condition satisfy করতে পারে না।

এই block কখনো execute হবে না।

Always-True OR Logic

Wrong:

if (
        !"ADMIN".equals(role)
        || !"INSTRUCTOR".equals(role)
) {
}

কোনো role একইসঙ্গে দুটো value না হওয়ায় expression প্রায় সবসময় true।

Correct “neither role” check:

if (
        !"ADMIN".equals(role)
        && !"INSTRUCTOR".equals(role)
) {
}

Testing Conditional Logic

Conditional logic test করার সময় শুধু normal value নয়, boundary value ব্যবহার করা উচিত।

Age requirement:

age >= 18

Test:

17
18
19

Mark range:

mark >= 0 && mark <= 100

Test:

Grade boundaries:

Boundary tests off-by-one errors identify করতে সাহায্য করে।

Practice Exercises

Exercise 1: Even or Odd

একটি integer নিন এবং determine করুন সেটি even নাকি odd।

Exercise 2: Positive, Negative, or Zero

একটি integer-এর জন্য print করুন:

Positive
Negative
Zero

Exercise 3: Valid Mark

Mark 0 থেকে 100-এর মধ্যে আছে কি না check করুন।

Exercise 4: Grade Calculator

Rules:

80–100 → A
70–79  → B
60–69  → C
50–59  → D
40–49  → E
0–39   → F

Invalid mark handle করুন।

Exercise 5: Login Decision

Variables:

boolean userExists = true;
boolean passwordCorrect = false;
boolean accountLocked = false;
boolean emailVerified = true;

Specific message দেখান:

User not found
Incorrect password
Account locked
Verify email
Login successful

Exercise 6: Enrollment Eligibility

Rules:

Minimum age 18
Email verified
Enrollment open
Account blocked নয়
Already enrolled নয়

Specific failure reason এবং success message দেখান।

Exercise 7: Largest of Three Numbers

তিনটি integer-এর মধ্যে largest value নির্ধারণ করুন।

Exercise 8: Discount Rules

Rules:

Premium এবং total 5000 বা বেশি → 20%
শুধু premium → 10%
শুধু total 5000 বা বেশি → 5%
Otherwise → 0%

Exercise 9: Safe String Validation

একটি name valid হবে যদি:

null না হয়
Blank না হয়

Exercise 10: Interactive Student Result

Scanner ব্যবহার করে:

Student name
Three subject marks

নিন এবং calculate করুন:

Total
Average
Grade
Passed বা failed

Knowledge Check

Question 1

Conditional statement কী?

Question 2

if condition-এর result কোন type হতে হবে?

Question 3

if block কখন execute হয়?

Question 4

else কখন execute হয়?

Question 5

else-if chain-এ কয়টি branch execute হয়?

Question 6

Multiple independent if-এর একাধিক block execute হতে পারে কি?

Question 7

Condition order কেন গুরুত্বপূর্ণ?

Question 8

Valid range 0–100-এর expression কী?

Question 9

Invalid range-এর expression কী?

Question 10

&&, || এবং ! কী করে?

Question 11

Nested if কী?

Question 12

String content compare করতে কী ব্যবহার করা উচিত?

Question 13

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

Question 14

Conditional block-এর local variable বাইরে accessible কি?

Question 15

Boundary testing কেন প্রয়োজন?

Knowledge Check Answers

Answer 1

Condition অনুযায়ী program-এর execution path control করা statement হলো conditional statement।

Answer 2

boolean

Answer 3

Condition true হলে।

Answer 4

Associated if এবং previous else-if conditions false হলে।

Answer 5

সর্বোচ্চ একটি matching branch execute হয়।

Answer 6

হ্যাঁ।

Answer 7

প্রথম matching branch-এর পর remaining branches skip হয়।

Answer 8

mark >= 0 && mark <= 100

Answer 9

mark < 0 || mark > 100

Answer 10

&& সব conditions true require করে
|| অন্তত একটি true require করে
! boolean value reverse করে

Answer 11

এক conditional block-এর মধ্যে আরেকটি conditional statement।

Answer 12

equals()

অথবা case-insensitive comparison-এর জন্য:

equalsIgnoreCase()

Answer 13

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

Answer 14

না।

Answer 15

Condition change হওয়ার boundary-এর আশেপাশে error আছে কি না check করতে।

Lesson Summary

এই lesson-এ আমরা শিখেছি:

Conditional statement program flow control করে
Condition অবশ্যই boolean হতে হয়
if true branch execute করে
else false branch handle করে
else-if multiple exclusive branch তৈরি করে
First matching branch-এর পর remaining branches skip হয়
Condition order result পরিবর্তন করতে পারে
Multiple independent if-এর একাধিক block execute হতে পারে
Mutually exclusive result-এর জন্য else-if appropriate
&&, || এবং ! complex condition তৈরি করে
Numeric range lower এবং upper boundary দিয়ে validate করা হয়
Nested if context-dependent decision handle করে
Deep nesting logical operator দিয়ে simplify করা যায়
Guard condition invalid case আগে handle করে
Block variable বাইরে accessible নয়
String compare করতে .equals() ব্যবহার করা উচিত
Null check method call-এর আগে রাখতে হয়
Extra semicolon conditional logic ভেঙে দিতে পারে
Boundary testing conditional bugs identify করতে সাহায্য করে

Next Lesson

পরবর্তী lesson-এ আমরা switch expressions এবং statements শিখব।

আমরা জানব:

Traditional switch
case
default
break
Fall-through
Multiple case labels
Arrow-style switch
switch expression
yield
String এবং enum values নিয়ে switch
if-else এবং switch নির্বাচন

Lesson Overview

Learning Objectives

What Is Conditional Execution?

Boolean Conditions

Numbers Are Not Boolean Values

The if Statement

Multiple Statements Inside if

Always Use Curly Braces

The if-else Statement

Example: Even or Odd

The else-if Chain

Example: Positive, Negative, or Zero

Condition Order Matters

Grade Calculation

Why Range Checks Are Not Repeated

Multiple Independent if Statements

Multiple if vs else-if

Multiple if

else-if

When to Use Multiple if

When to Use else-if

Combining Conditions with &&

Combining Conditions with ||

Using Logical NOT

Complex Conditions

Extracting Conditions into Variables

Range Validation

Wrong Range Logic

Chained Comparison Is Invalid

Nested if

Nested if-else

Avoid Unnecessary Deep Nesting

Showing Specific Failure Reasons

Guard Conditions

Variable Scope in Conditional Blocks

Assigning a Result in Both Branches

Possibly Uninitialized Variable

Ternary Operator for Simple Decisions

Comparing Strings in Conditions

Null-Safe String Validation

Wrong Null Check Order

Checking Multiple String Values

Comparing char

Floating-Point Conditions

Practical Example: Age Classification

Practical Example: Number Classification

Practical Example: Course Enrollment

Practical Example: Product Stock

Practical Example: Discount Rules

Practical Example: Largest of Three Numbers

Interactive Grade Program

Complete Student Result Program

Common Mistakes

Extra Semicolon After if

Assignment Instead of Comparison

Comparing Boolean with true

Wrong Grade Order

Independent if for Exclusive Grades

Wrong String Comparison

Wrong Null Check Order

Impossible Condition

Always-True OR Logic

Testing Conditional Logic

Practice Exercises

Exercise 1: Even or Odd

Exercise 2: Positive, Negative, or Zero

Exercise 3: Valid Mark

Exercise 4: Grade Calculator

Exercise 5: Login Decision

Exercise 6: Enrollment Eligibility

Exercise 7: Largest of Three Numbers

Exercise 8: Discount Rules

Exercise 9: Safe String Validation

Exercise 10: Interactive Student Result

Knowledge Check

Question 1

Question 2

Question 3

Question 4

Question 5

The `if` Statement

Multiple Statements Inside `if`

The `if-else` Statement

The `else-if` Chain

Multiple Independent `if` Statements

Multiple `if` vs `else-if`

Multiple `if`

`else-if`

When to Use Multiple `if`

When to Use `else-if`

Combining Conditions with `&&`

Combining Conditions with `||`

Nested `if`

Nested `if-else`

Comparing `char`

Extra Semicolon After `if`

Comparing Boolean with `true`

Independent `if` for Exclusive Grades