Variable declaration and initialization in Java

Introduction

Variables store information we want to use later. Declaration tells the computer about the variable, and initialization gives it a starting value.

When you want to save a number to use in calculations.

When you need to remember a word or sentence in your program.

When you want to keep track of a true or false condition.

When you want to store a value that might change later.

When you want to organize data with a name for easy access.

Syntax

Java

type variableName = value;

type is the kind of data (like int for numbers, String for text).

You can declare a variable first, then give it a value later.

Examples

This creates a number variable named age and sets it to 25.

Java

int age = 25;

This creates a text variable named name and sets it to "Alice".

Java

String name = "Alice";

Here, isSunny is declared first, then given the value true later.

Java

boolean isSunny;
isSunny = true;

This creates a decimal number variable named price and sets it to 19.99.

Java

double price = 19.99;

Sample Program

This program declares and initializes three variables with different types, then prints their values.

Java

public class Main {
    public static void main(String[] args) {
        int age = 30;
        String city = "New York";
        boolean isStudent = false;

        System.out.println("Age: " + age);
        System.out.println("City: " + city);
        System.out.println("Is student: " + isStudent);
    }
}

Important Notes

Variable names should be clear and meaningful, like age or city.

Java requires you to specify the type of variable when you declare it.

Once declared, you can change the value of a variable later if needed.

Summary

Declare variables to tell the computer what kind of data you want to store.

Initialize variables to give them a starting value.

Use variables to keep and change information in your program.

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