Programming and Java Fundamentals

Conditional Statements

ReadingPreview

আপনি একটি free preview lesson দেখছেন।

Lesson Overview

A program does not always execute the same instructions.

Often, a program needs to make decisions based on data or conditions.

For example:

Allow enrollment if age is 18 or above
Show passed if a mark is 40 or above
Accept an order if stock is available
Determine whether a number is positive, negative, or zero
Allow login if both email and password are valid

In Java, conditional statements are used to control execution flow based on conditions.

In this lesson, we will learn:

Boolean conditions
if
if-else
else-if
Multiple independent if statements
Logical operators in conditions
Range validation
Nested conditions
Common conditional mistakes

Learning Objectives

After completing this lesson, you will be able to:

Make decisions using boolean expressions
Write if, if-else, and else-if
Distinguish between independent and mutually exclusive conditions
Combine multiple conditions
Validate numeric ranges
Understand nested conditions
Identify common mistakes in conditional logic
Create simple decision-making programs

What Is Conditional Execution?

Conditional execution means executing or skipping a block of code depending on the result of a condition.

int age = 20;

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

Condition:

age >= 18

Result:

true

Therefore, the block executes.

Output:

Adult

If:

int age = 15;

then the condition is false and the block does not execute.

A Condition Must Be Boolean

The result of a condition in a Java conditional statement must be:

true

or:

false

Valid:

age >= 18

score == 100

isActive

!accountBlocked

emailValid && passwordValid

In Java, a number cannot be used directly as a condition.

Invalid:

int value = 1;

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

Correct:

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

`if` Statement

Syntax:

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

Example:

int mark = 75;

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

Output:

Passed

If the condition is false, the block is skipped.

Multiple Statements Inside an `if` Block

int age = 20;

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

If the condition is true, the statements inside the block execute from top to bottom.

Use Curly Braces

Curly braces can technically be omitted for a single statement:

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

However, using braces is safer and clearer.

Preferred:

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

This reduces the risk of accidentally breaking the logic when another statement is added later.

`if-else`

Use if-else when different branches are needed for both the true and false cases.

int mark = 35;

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

Output:

Failed

Only one branch executes during a single execution.

Example: Even or Odd

int number = 17;

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

Output:

Odd

`else-if`

When there are more than two mutually exclusive outcomes, else-if can be used.

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

Java checks conditions from top to bottom.

After the first true branch executes, the remaining branches are skipped.

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 Is Important

Look at this code:

int mark = 90;

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

Output:

Passed

mark >= 40 is already true, so the next condition is never checked.

Better:

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

When conditions overlap, the more specific condition or higher threshold should generally be checked first.

Grade Example

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

Because else-if is used, only one grade is selected for a mark.

Why Do We Not Repeat the Upper Bound?

Look at this section:

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

The second condition is checked only when:

mark < 80

Therefore:

mark >= 70

effectively represents:

70 <= mark < 80

Multiple Independent `if` Statements

Not all conditions are mutually exclusive.

int mark = 90;

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

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

Output:

Passed
Distinction

Both conditions are independently true.

Multiple `if` vs. `else-if`

Multiple `if`

if (conditionA) {
}

if (conditionB) {
}

Multiple blocks may execute.

`else-if`

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

Only the first matching branch executes.

When Should You Use Each?

Independent results:

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

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

Mutually exclusive results:

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

Multiple Conditions with `&&`

When every condition must be true:

int age = 20;
boolean emailVerified = true;

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

Here:

Age must be at least 18
Email must be verified

Both must be true.

Alternative Conditions with `||`

When at least one condition must be true:

boolean isInstructor = false;
boolean isAdmin = true;

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

Output:

Access allowed

Reverse a Condition with `!`

boolean accountBlocked = false;

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

The result of:

!accountBlocked

is:

true

Keep Complex Conditions Readable

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

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

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

A meaningful boolean variable can make a complex condition easier to understand.

Range Validation

Suppose a mark is valid from:

0 to 100

Correct:

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

Invalid mark:

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

Wrong Range Logic

Wrong:

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

Suppose:

mark = 500

Then:

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

500 would incorrectly be considered valid.

Correct:

mark >= 0
&& mark <= 100

Mathematical Chained Comparisons Are Not Valid in Java

In mathematics, you may write:

0 <= mark <= 100

But this is not valid Java:

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

Correct:

if (
        mark >= 0
        && mark <= 100
) {
    System.out.println("Valid mark");
}

Nested `if`

A conditional statement inside another conditional block is called a nested if.

int age = 20;
boolean emailVerified = true;

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

The inner condition is evaluated only if the outer condition is true.

When Is a Nested Condition Useful?

A nested condition can be useful when the second decision depends on the context established by the first decision.

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"
    );
}

However, avoid unnecessary deep nesting.

Showing a Specific Failure Reason

Sometimes it is useful to show a specific failure reason instead of combining everything into one success condition.

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

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

Here, the conditions are ordered according to the user flow.

String Conditions

To compare String content:

String role = "ADMIN";

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

To ignore case:

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

For String content, do not use:

role == "ADMIN"

`null` and String Check

String courseName = null;

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

If courseName is null, the first condition is false.

Because of short-circuit evaluation:

courseName.isBlank()

will not be called.

Common Mistakes

Extra Semicolon After `if`

Wrong:

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

The semicolon has already ended 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");
}

This performs assignment instead of comparison.

Preferred:

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

Comparing a Boolean with `== true`

Verbose:

if (isAvailable == true) {
}

Preferred:

if (isAvailable) {
}

False check:

if (!isAvailable) {
}

Multiple `if` Statements for Exclusive Results

Wrong:

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

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

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

If mark = 85, all three outputs may appear.

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 Null Check Order

Wrong:

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

Correct:

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

Complete Example: Enrollment Decision

public class Main {

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

        if (learnerAge < 0) {
            System.out.println(
                    "Invalid age"
            );
        } else if (learnerAge < 18) {
            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 (accountBlocked) {
            System.out.println(
                    "Blocked account cannot enroll"
            );
        } else {
            System.out.println(
                    "Enrollment allowed"
            );
        }
    }
}

Output:

Enrollment allowed

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();
    }
}

Test Boundary Values

When testing conditional logic, do not test only normal values. Also test values around the boundaries where a condition changes.

If the rule is:

age >= 18

Test:

17
18
19

For a mark range:

0 to 100

Test:

For grade boundaries:

This helps identify off-by-one and incorrect condition-order bugs.

Important Terms

Conditional Statement

Controls execution flow based on a condition.

Condition

A boolean expression.

Branch

A possible execution path selected by a condition.

`if`

Executes a block when its condition is true.

`else`

Executes an alternative block when its associated condition is false.

`else-if`

Checks multiple mutually exclusive conditions.

Nested Condition

A conditional statement inside another conditional block.

Range Validation

Checks whether a value is within minimum and maximum boundaries.

Boundary Value

A test value near the point where a condition changes.

Practice Exercise 1: Even or Odd

Take an integer and print:

Even

or:

Odd

Practice Exercise 2: Positive, Negative, or Zero

For an integer, determine whether it is:

Positive
Negative
Zero

Practice Exercise 3: Valid Mark

A mark is valid from:

0 to 100

Write a boolean expression and print the result.

Practice Exercise 4: Grade Calculator

Rules:

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

Handle invalid marks.

Practice Exercise 5: Login Decision

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

Show a specific result:

User not found
Incorrect password
Account locked
Login successful

Practice Exercise 6: Enrollment Eligibility

Rules:

Minimum age 18
Email verified
Enrollment open
Account is not blocked

Use else-if to show a specific failure reason.

Practice Exercise 7: Multiple Independent Conditions

For a mark, independently check:

Whether the student passed
Whether the student received distinction

Passing mark:

Distinction:

Practice Exercise 8: Boundary Testing

For the grade calculator, test:

Write the expected result for each input.

Knowledge Check

Question 1

What is conditional execution?

Question 2

What type must the result of an if condition be?

Question 3

When does an if block execute?

Question 4

When does else execute?

Question 5

How many branches can execute in an else-if chain?

Question 6

Can multiple independent if blocks execute?

Question 7

Why is condition order important?

Question 8

What expression checks whether a mark is within 0–100?

Question 9

What expression checks whether a mark is outside the valid range?

Question 10

What do &&, ||, and ! do?

Question 11

What is a nested if?

Question 12

Which method should be used to compare String content?

Question 13

Why is name != null && !name.isBlank() safe?

Question 14

Why is boundary value testing useful?

Knowledge Check Answers

Answer 1

Conditional execution means executing or skipping code depending on the result of a condition.

Answer 2

boolean

Answer 3

When the condition is true.

Answer 4

When the associated if and previous else-if conditions are false.

Answer 5

At most one matching branch executes.

Answer 6

Yes.

Answer 7

An else-if chain is evaluated from top to bottom, and after the first matching branch executes, the remaining branches are skipped.

Answer 8

mark >= 0
&& mark <= 100

Answer 9

mark < 0
|| mark > 100

Answer 10

&& — all conditions must be true
|| — at least one condition must be true
! — reverses a boolean value

Answer 11

A conditional statement inside another conditional block.

Answer 12

equals()

For case-insensitive comparison:

equalsIgnoreCase()

Answer 13

If name is null, the first condition becomes false and short-circuit evaluation prevents isBlank() from being called.

Answer 14

Boundary value testing helps detect incorrect comparisons and off-by-one errors near the points where conditions change.

Lesson Summary

In this lesson, we learned:

Conditional statements control a program's execution path
Conditions must produce boolean results
if executes a block when its condition is true
if-else handles two alternative paths
else-if handles multiple mutually exclusive outcomes
After the first matching else-if branch, the remaining branches are skipped
Condition order is important
Multiple if statements can be used for independent conditions
&&, ||, and ! combine or reverse conditions
Numeric ranges can be validated using lower and upper boundaries
Mathematical chained comparisons are not valid Java
Nested if statements can handle context-dependent decisions
.equals() should be used to compare String content
A null check should happen before calling a method on a possibly null value
Extra semicolons and incorrect condition ordering are common bugs
Testing boundary values is important for conditional logic

Lesson Overview

Learning Objectives

What Is Conditional Execution?

A Condition Must Be Boolean

if Statement

Multiple Statements Inside an if Block

Use Curly Braces

if-else

Example: Even or Odd

else-if

Positive, Negative, or Zero

Condition Order Is Important

Grade Example

Why Do We Not Repeat the Upper Bound?

Multiple Independent if Statements

Multiple if vs. else-if

Multiple if

else-if

When Should You Use Each?

Multiple Conditions with &&

Alternative Conditions with ||

Reverse a Condition with !

Keep Complex Conditions Readable

Range Validation

Wrong Range Logic

Mathematical Chained Comparisons Are Not Valid in Java

Nested if

When Is a Nested Condition Useful?

Showing a Specific Failure Reason

String Conditions

null and String Check

Common Mistakes

Extra Semicolon After if

Assignment Instead of Comparison

Comparing a Boolean with == true

Multiple if Statements for Exclusive Results

Wrong Null Check Order

Complete Example: Enrollment Decision

Interactive Grade Program

Test Boundary Values

Important Terms

Conditional Statement

Condition

Branch

if

else

else-if

Nested Condition

Range Validation

Boundary Value

Practice Exercise 1: Even or Odd

Practice Exercise 2: Positive, Negative, or Zero

Practice Exercise 3: Valid Mark

Practice Exercise 4: Grade Calculator

Practice Exercise 5: Login Decision

Practice Exercise 6: Enrollment Eligibility

Practice Exercise 7: Multiple Independent Conditions

Practice Exercise 8: Boundary Testing

Knowledge Check

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Knowledge Check Answers

Answer 1

Answer 2

Answer 3

Answer 4

Answer 5

Answer 6

`if` Statement

Multiple Statements Inside an `if` Block

`if-else`

`else-if`

Multiple Independent `if` Statements

Multiple `if` vs. `else-if`

Multiple `if`

`else-if`

Multiple Conditions with `&&`

Alternative Conditions with `||`

Reverse a Condition with `!`

Nested `if`

`null` and String Check

Extra Semicolon After `if`

Comparing a Boolean with `== true`

Multiple `if` Statements for Exclusive Results

`if`

`else`

`else-if`