Variable declaration and initialization in Java - Mini Project: Build & Apply

Variable declaration and initialization

📖 Scenario: You are creating a simple Java program to store and display information about a book.

🎯 Goal: Learn how to declare variables and assign values to them in Java.

📋 What You'll Learn

Declare variables with correct data types

Initialize variables with given values

Print variable values to the console

💡 Why This Matters

🌍 Real World

Storing and managing information about items like books is common in apps like libraries, bookstores, and inventory systems.

💼 Career

Understanding how to declare and initialize variables is a fundamental skill for any programming job, as it is the basis for storing and working with data.

1

Declare variables for book details

Declare a String variable called title and set it to "The Great Gatsby". Declare an int variable called year and set it to 1925.

Java

public class BookInfo {
    public static void main(String[] args) {
        // Declare the variables 'title' and 'year' with given values
        
    }
}

Hint

Use String for text and int for whole numbers.

2

Add a variable for the author

Declare a String variable called author and set it to "F. Scott Fitzgerald".

Java

public class BookInfo {
    public static void main(String[] args) {
        String title = "The Great Gatsby";
        int year = 1925;
        // Declare the variable 'author' with the given value
        
    }
}

Hint

Remember to use String for text values.

3

Add a variable for the price

Declare a double variable called price and set it to 10.99.

Java

public class BookInfo {
    public static void main(String[] args) {
        String title = "The Great Gatsby";
        int year = 1925;
        String author = "F. Scott Fitzgerald";
        // Declare the variable 'price' with the given value
        
    }
}

Hint

Use double for decimal numbers.

4

Print all book details

Use System.out.println to print the variables title, author, year, and price each on a new line.

Java

public class BookInfo {
    public static void main(String[] args) {
        String title = "The Great Gatsby";
        int year = 1925;
        String author = "F. Scott Fitzgerald";
        double price = 10.99;
        // Print the variables below
        
    }
}

Hint

Use System.out.println for each variable to print on separate lines.

Practice

1. Which of the following is a correct way to declare and initialize an integer variable named age with the value 25 in Java?

easy

A. int age = 25;

B. int age;

C. age = 25;

D. int 25 = age;

Solution

1. Step 1: Understand variable declaration and initialization

   Declaring a variable means telling the computer the type and name. Initializing means giving it a value.

2. Step 2: Check each option for correct syntax

   int age = 25; declares int age and sets it to 25 correctly. int age; declares but does not initialize. age = 25; tries to assign without declaration. int 25 = age; uses invalid syntax.

3. Final Answer:

   int age = 25; -> Option A

4. Quick Check:

   Declaration + initialization = int age = 25; [OK]

Hint: Declare type, name, then assign value with = [OK]

Common Mistakes:

• Forgetting the data type before the variable name
• Assigning a value without declaring the variable first
• Using numbers as variable names

2. Which of the following lines of code will cause a syntax error in Java?

easy

A. double price = 19.99;

B. String name = "John";

C. int 1stNumber = 10;

D. boolean isActive = true;

Solution

1. Step 1: Review Java variable naming rules

   Variable names cannot start with a digit. They must start with a letter, underscore, or dollar sign.

2. Step 2: Check each option for naming errors

   int 1stNumber = 10; uses 1stNumber which starts with a digit, causing a syntax error. Others are valid declarations.

3. Final Answer:

   int 1stNumber = 10; -> Option C

4. Quick Check:

   Variable names cannot start with digits [OK]

Hint: Variable names cannot start with numbers [OK]

Common Mistakes:

• Starting variable names with digits
• Using spaces in variable names
• Missing semicolon at the end

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

int x = 5;
int y = x + 10;
System.out.println(y);

medium

A. 15

B. 10

C. 5

D. Syntax Error

Solution

1. Step 1: Understand variable initialization and addition

   Variable x is set to 5. Variable y is set to x + 10, which is 5 + 10 = 15.

2. Step 2: Determine the printed value

   The System.out.println(y); prints the value of y, which is 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add values after initialization to find output [OK]

Common Mistakes:

• Confusing variable names
• Forgetting to initialize variables before use
• Assuming output is the first variable's value

4. Identify the error in the following Java code snippet:

int number;
number = "10";
System.out.println(number);

medium

A. Variable number is not declared

B. Print statement syntax is incorrect

C. Missing semicolon after declaration

D. Cannot assign a String to an int variable

Solution

1. Step 1: Check variable declaration and assignment types

   The variable number is declared as int, which means it can only hold integer values.

2. Step 2: Identify type mismatch in assignment

   Assigning "10" (a String) to an int variable causes a type mismatch error.

3. Final Answer:

   Cannot assign a String to an int variable -> Option D

4. Quick Check:

   int variables accept only integers [OK]

Hint: Match variable type with assigned value type [OK]

Common Mistakes:

• Assigning text (String) to number variables
• Forgetting to use quotes for Strings
• Confusing variable declaration and assignment

5. You want to declare and initialize three variables in one line: an integer count with 10, a double price with 5.5, and a boolean available with true. Which of the following is the correct way to do this in Java?

hard

A. int count = 10, double price = 5.5, boolean available = true;

B. int count = 10; double price = 5.5; boolean available = true;

C. int count = 10; double price = 5.5, boolean available = true;

D. int count = 10, price = 5.5, available = true;

Solution

1. Step 1: Understand variable declaration rules for different types

   In Java, variables of different types must be declared separately with their own type keyword.

2. Step 2: Analyze each option's syntax

   int count = 10, double price = 5.5, boolean available = true; tries to declare different types separated by commas in one statement, which is invalid. int count = 10; double price = 5.5; boolean available = true; declares each variable with its type and ends each statement with a semicolon, which is correct. int count = 10; double price = 5.5, boolean available = true; mixes semicolon and comma incorrectly. int count = 10, price = 5.5, available = true; declares all variables with one type keyword, which is invalid for different types.

3. Final Answer:

   int count = 10; double price = 5.5; boolean available = true; -> Option B

4. Quick Check:

   Different types need separate declarations [OK]

Hint: Declare each type separately with semicolons [OK]

Common Mistakes:

• Trying to declare different types in one statement with commas
• Missing semicolons between statements
• Assuming one type keyword covers all variables