If–else statement in Java

Introduction

An if-else statement helps your program make choices. It runs one set of instructions if something is true, and another set if it is false.

Deciding if a user is old enough to sign up for a website.

Checking if a number is positive or negative before printing a message.

Choosing what message to show based on the time of day.

Turning on a light if it is dark, or turning it off if it is bright.

Syntax

Java

if (condition) {
    // code to run if condition is true
} else {
    // code to run if condition is false
}

The condition is a question your program asks that can be true or false.

Use curly braces { } to group the code that runs for each case.

Examples

This checks if age is 18 or more. If yes, it prints a voting message. Otherwise, it prints a different message.

Java

if (age >= 18) {
    System.out.println("You can vote.");
} else {
    System.out.println("You are too young to vote.");
}

This decides if the score is above 50 to print a pass or fail message.

Java

if (score > 50) {
    System.out.println("You passed!");
} else {
    System.out.println("Try again.");
}

Sample Program

This program checks if the temperature is above 25 degrees. It prints a message about the weather based on that.

Java

public class Main {
    public static void main(String[] args) {
        int temperature = 30;
        if (temperature > 25) {
            System.out.println("It's hot outside.");
        } else {
            System.out.println("It's not hot today.");
        }
    }
}

Important Notes

Always use == to compare values, not =, which assigns values.

You can add more choices using else if for multiple conditions.

Summary

If-else lets your program pick between two paths.

Use it to run code when a condition is true, and different code when false.

Remember to use curly braces to keep your code clear and organized.

Practice

1. What does an if-else statement do in Java?

easy

A. It defines a new class.

B. It repeats code multiple times.

C. It stores multiple values in a list.

D. It chooses between two actions based on a condition.

Solution

1. Step 1: Understand the purpose of if-else

   An if-else statement lets the program decide between two paths depending on a condition.

2. Step 2: Compare with other options

   Repeating code is done by loops, storing values is done by collections, and defining classes is unrelated.

3. Final Answer:

   It chooses between two actions based on a condition. -> Option D

4. Quick Check:

   If-else = choose between two paths [OK]

Hint: If you want two choices, use if-else [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else stores data
• Mixing if-else with class definitions

2. Which of the following is the correct syntax for an if-else statement in Java?

easy

A. if condition { } else { }

B. if (condition) { } else { }

C. if (condition) then { } else { }

D. if condition then { } else { }

Solution

1. Step 1: Recall Java syntax rules

   Java requires parentheses around the condition and curly braces for code blocks.

2. Step 2: Check each option

   Only if (condition) { } else { } uses parentheses and curly braces correctly without extra keywords like 'then'.

3. Final Answer:

   if (condition) { } else { } -> Option B

4. Quick Check:

   Parentheses + braces = correct syntax [OK]

Hint: Remember: condition in () and code in {} [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using 'then' keyword incorrectly
• Missing curly braces for blocks

3. What will be the output of this Java code?

int x = 10;
if (x > 5) {
    System.out.println("Greater");
} else {
    System.out.println("Smaller or equal");
}

medium

A. Greater

B. Smaller or equal

C. No output

D. Compilation error

Solution

1. Step 1: Evaluate the condition x > 5

   Since x is 10, the condition 10 > 5 is true.

2. Step 2: Determine which block runs

   The if block runs and prints "Greater"; the else block is skipped.

3. Final Answer:

   Greater -> Option A

4. Quick Check:

   10 > 5 is true, so print "Greater" [OK]

Hint: Check condition true? Run if-block, else run else-block [OK]

Common Mistakes:

• Confusing > with <
• Thinking else block runs when if is true
• Assuming no output if condition true

4. Identify the error in this Java code snippet:

int num = 7;
if num > 5 {
    System.out.println("Yes");
} else {
    System.out.println("No");
}

medium

A. Missing curly braces around else block.

B. Missing semicolon after if statement.

C. Missing parentheses around the condition in if statement.

D. Variable num is not declared.

Solution

1. Step 1: Check if statement syntax

   Java requires parentheses around the condition in an if statement.

2. Step 2: Verify other parts

   Semicolon after if is not needed; curly braces are present; num is declared.

3. Final Answer:

   Missing parentheses around the condition in if statement. -> Option C

4. Quick Check:

   If condition needs () [OK]

Hint: Always put condition inside () in if [OK]

Common Mistakes:

• Omitting parentheses in if condition
• Adding unnecessary semicolon after if
• Forgetting to declare variables

5. You want to write a Java program that prints "Even" if a number is even, and "Odd" if it is odd. Which if-else code correctly does this?

int n = 4;
// Your code here

hard

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

B. if (n / 2 == 0) {
System.out.println("Even");
} else {
System.out.println("Odd");
}

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

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

Solution

1. Step 1: Understand how to check even numbers

   A number is even if dividing by 2 leaves remainder 0, so use n % 2 == 0.

2. Step 2: Check each option's condition

   if (n % 2 == 0) { System.out.println("Even"); } else { System.out.println("Odd"); } uses correct equality check with == and modulo operator. if (n / 2 == 0) { System.out.println("Even"); } else { System.out.println("Odd"); } uses division instead of modulo. if (n % 2 = 0) { System.out.println("Even"); } else { System.out.println("Odd"); } uses assignment = instead of comparison ==. if (n % 2 != 0) { System.out.println("Even"); } else { System.out.println("Odd"); } reverses logic.

3. Final Answer:

   if (n % 2 == 0) { System.out.println("Even"); } else { System.out.println("Odd"); } -> Option A

4. Quick Check:

   Use modulo == 0 to check even [OK]

Hint: Use n % 2 == 0 to test even numbers [OK]

Common Mistakes:

• Using = instead of == for comparison
• Using division instead of modulo
• Reversing even and odd logic