Syntax of simple IF statement:

if (boolean-expression) {
    statement(s);
}

Example:

if (radius >= 0) {
    area = PI * radius * radius;
    System.out.println("Area of circle with radius " + 
        radius + " is " + area + ".");
}

Important Notes:

• ▶ The boolean expression must be enclosed in parentheses.
• ▶ Curly braces can be omitted if the block contains only one statement.

SimpleIfDemo Program: Checks if number is divisible by 5 or even.

import java.util.Scanner;

public class SimpleIfDemo {
    public static void main(String[] args) {
        int number;
        Scanner input = new Scanner(System.in);
        
        // Get integer
        System.out.print("Enter an integer: ");
        number = input.nextInt();
        
        // Check divisibility and display result
        if (number % 5 == 0)
            System.out.println("Divisible by 5");
        
        if (number % 2 == 0)
            System.out.println("Even");
    }
}

Syntax of IF-ELSE statement:

if (boolean-expression) {
    statement(s)-for-true-case;
} else {
    statement(s)-for-false-case;
}

Example (checking even/odd):

if (number % 2 == 0)
    System.out.println(number + " is even.");
else
    System.out.println(number + " is odd.");

Syntax of ELSE-IF statement:

if (score >= 90)
    System.out.print("A");
else if (score >= 80)
    System.out.print("B");
else if (score >= 70)
    System.out.print("C");
else if (score >= 60)
    System.out.print("D");
else
    System.out.print("F");

Example with ELSE-IF checking Strings comparison:

import java.util.Scanner;

public class L3ifLab1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String answer = scanner.next();
        
        if (answer.equals("yes")) {
            System.out.print("yes");
        }
        else if (answer.equals("I_don't_know")) {
            System.out.print("I_don't_know");
        }
        else {
            System.out.print("else clause");
        }
    }
}

Lab 1: Checking the Parity of a Number

Objective: Request user to input an integer. Check to see if the number is even. Output the result.

[File name and class name: L3Lab1.java]

Please enter an integer value and press Enter
5
The result: The entered number was not an even number
+++
Please enter an integer value and press Enter
6
The result: The entered number was an even number

Algorithm:

• ▶ It is possible to check a remainder when dividing the number by 2.
• ▶ The (%) or modulus operator returns the remainder of integer division.
• ▶ If the remainder is 0, then the value is able to be divided by 2 with no remainder, which means it is an even number.

/* 
Request user to input an integer. 
Check to see if the number is even. 
Output the result (a phrase "The entered number was an even number" or 
    "The entered number was not an even number")
*/

Lab 2: Body Mass Index (BMI) Calculator

Objective: Write a program that gets weight in kilograms and height in centimeters from user, then displays and interprets BMI (Body Mass Index) using nested if statements.
Note: formula to calculate BMI: bmi = weight / (height_in_meters * height_in_meters):

[File name and class name: L3Lab2.java]

Enter the weight in kilograms:
75
Enter the height in centimeters:
185
bmi = 21.913806
standard

Algorithm:

Task 1: Check Positive or Negative Number

Objective: Request user to input an integer. Check to see if the number is positive or negative. Output the result. 

[File name and class name: L3Task1.java]

Please enter an integer value and press Enter
50
The result: The entered number was positive
+++
Please enter an integer value and press Enter
-7
The result: The entered number was negative

Task 2: Extract Digits from Two-Digit Number

Objective: A two-digit integer is given. Output its right and left digits (use operations %, / to get the digits).

[File name and class name: L3Task2.java]

Please enter a two-digit number:
50
The result: right digit is 0, left digit is 5
---
Please enter a two-digit number:
-76
The result: right digit is 6, left digit is 7

Task 3: Set Middle Digit to Zero in Three-Digit Number

Objective: A three-digit number is given. Set its middle digit to a value of 0.

[File name and class name: L3Task3.java]

Please enter a three-digit number:
523
The digits: 5,2,3; the resulting number: 503

Task 4: Check if Any Two Numbers are Not Equal

Objective: Three integers are given. Output true if any two of them are not equal, and false otherwise.

[File name and class name: L3Task4.java]

Please enter three numbers:
13, -4, 6 
The result: true
+++
Please enter three numbers:
6, -4, 6
The result: false
+++
Please enter three numbers:
13, 13, 6 
The result: false

Task 5: Check Triangle Existence and Calculate Area

Objective: Integers a, b and c are given. Print true if a triangle with matching side lengths can exist, print false otherwise. If triangle exists, print its area.
Note: A triangle exists when the sum of any two of its sides is greater than the third side.

[File name and class name: L3Task5.java]

Please enter three numbers:
4, 8, 6
The result: exists = true, area is 11.61895003

Task 6: Piecewise Function Calculation

Objective: Integer number x is entered. Calculate the value of the function f:
  F(x) = 2x for x ∈ (-∞; -2) ∪ (2; ∞);
 F(x) = -3x for x ∈ [-2; 2]

[File name and class name: L3Task6.java]

Enter an integer:
>>> 4
Result is 8

Task 7: Trigonometric Function Calculation

Objective: Real number x is entered. Calculate the value of the function f:
  F(x) = 2sin(x) for x > 0;
 F(x) = 6 - x for x ≤ 0

Note: use x = Math.toRadians(x);  to convert to radians, and  Math.sin(x)
Expected Output:

Enter a real number:
1.0
Result is 1.682942
+++
Enter a real number:
-2.5
Result is 8.500000

[File name and class name: L3Task7.java]

Task 8: 

Objective: Accept a numeric grade (double) from 0.0 to 100.0 and convert it to a letter grade using type casting to int for comparison:

• A: 90–100;
• B: 80–89;
• C: 70–79;
• D: 60–69;
• F: below 60.

Expected Output:

Enter your grade (0.0–100.0): 
87.5
Your letter grade is B.
+++
Enter your grade (0.0–100.0): 
59.9
Your letter grade is F.

[File name and class name: L3Task8.java]