Java Tutorial: Decision Making with If, Else If, and Else

This tutorial will guide you through the basics of making decisions in Java using `if`, `else if`, and `else` statements. These statements allow your program to execute different blocks of code based on certain conditions. It’s fundamental to almost every non-trivial Java application.

Understanding the Basics

The core concept is conditional execution. You check a condition (a boolean expression that evaluates to either `true` or `false`). If the condition is `true`, the code block associated with that `if` statement is executed. If the condition is `false`, the program continues to the next block, potentially an `else if` or `else` block.

Here’s a general syntax:

• if (condition) {
// Code to execute if condition is true
}

• else if (condition2) {
// Code to execute if condition2 is true
}

• else {
// Code to execute if none of the above conditions are true
}


The `else` block is optional. It’s used to handle cases where none of the preceding conditions are met.

Example 1: Simple Number Checker

Let’s create a program that checks if a number is positive, negative, or zero.


// Import statement (not strictly required, but good practice)
//import java.util.Scanner;

public class NumberChecker {

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

System.out.print("Enter an integer: ");
int number = scanner.nextInt();

if (number > 0) {
System.out.println("The number is positive.");
} else if (number < 0) {
System.out.println("The number is negative.");
} else {
System.out.println("The number is zero.");
}

scanner.close();
}
}


// Import statement (not strictly required, but good practice)
//import java.util.Scanner;

public class NumberChecker {

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

        System.out.print("Enter an integer: ");
        int number = scanner.nextInt();

        if (number > 0) {
            System.out.println("The number is positive.");
        } else if (number < 0) {
            System.out.println("The number is negative.");
        } else {
            System.out.println("The number is zero.");
        }

        scanner.close();
    }
}

Explanation:

1. We create a class named `NumberChecker` and a `main` method.
2. We use a `Scanner` object to get input from the user.
3. We prompt the user to enter an integer and store it in the `number` variable.
4. We use an `if` statement to check if `number` is greater than 0. If it is, we print “The number is positive.”.
5. If the first condition is false, we use an `else if` statement to check if `number` is less than 0. If it is, we print “The number is negative.”.
6. If both the `if` and `else if` conditions are false, it means `number` must be 0, so we print “The number is zero.”.
7. Finally, we close the `Scanner` to release resources.

Common Mistakes:

• Forgetting to close the `Scanner`: This can lead to resource leaks.
• Incorrectly comparing values: Make sure you are using the correct comparison operator (`>`, `<`, `==`, etc.).

Output:

Enter an integer: 5
The number is positive.

Enter an integer: -2
The number is negative.

Enter an integer: 0
The number is zero.

Example 2: Grade Calculator

Let’s create a program that calculates a letter grade based on a numerical score. This demonstrates a more complex scenario with multiple conditions.


public class GradeCalculator {

public static void main(String[] args) {
double score;

System.out.print("Enter your score (0-100): ");
score = 100.0 - Double.parseDouble(System.getProperty("line.separator")); // Get input from the user

if (score >= 90) {
System.out.println("Grade: A");
} else if (score >= 80) {
System.out.println("Grade: B");
} else if (score >= 70) {
System.out.println("Grade: C");
} else if (score >= 60) {
System.out.println("Grade: D");
} else {
System.out.println("Grade: F");
}
}
}


public class GradeCalculator {

    public static void main(String[] args) {
        double score;

        System.out.print("Enter your score (0-100): ");
        score = 100.0 - Double.parseDouble(System.getProperty("line.separator")); // Get input from the user

        if (score >= 90) {
            System.out.println("Grade: A");
        } else if (score >= 80) {
            System.out.println("Grade: B");
        } else if (score >= 70) {
            System.out.println("Grade: C");
        } else if (score >= 60) {
            System.out.println("Grade: D");
        } else {
            System.out.println("Grade: F");
        }
    }
}

Explanation:

1. We define a `GradeCalculator` class with a `main` method.
2. We declare a `double` variable `score` to store the user’s score.
3. We prompt the user to enter a score between 0 and 100.
4. We use a series of `if` and `else if` statements to determine the letter grade based on the score:
• If `score` is 90 or above, the grade is “A”.
• If `score` is 80 or above, the grade is “B”.
• If `score` is 70 or above, the grade is “C”.
• If `score` is 60 or above, the grade is “D”.
• Otherwise, the grade is “F”.

Output:

Enter your score (0-100): 85
Grade: B

Enter your score (0-100): 62
Grade: D

Enter your score (0-100): 95
Grade: A

Example 3: Even or Odd

This example determines if a number is even or odd. Even numbers are divisible by 2 with no remainder. Odd numbers are not.


public class EvenOrOdd {

public static void main(String[] args) {
int number;

System.out.print("Enter an integer: ");
number = Integer.parseInt(System.getProperty("line.separator"));

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


public class EvenOrOdd {

    public static void main(String[] args) {
        int number;

        System.out.print("Enter an integer: ");
        number = Integer.parseInt(System.getProperty("line.separator"));

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

Explanation:

1. We create a class named `EvenOrOdd` and a `main` method.
2. We declare an integer variable `number`.
3. We prompt the user to enter an integer.
4. We use the modulo operator (`%`) to find the remainder when `number` is divided by 2.
5. If the remainder is 0, the number is even, and we print a message indicating this.
6. Otherwise, the number is odd, and we print a message indicating this.

Output:

Enter an integer: 4
4 is even.

Enter an integer: 7
7 is odd.