Variable declaration and initialization in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a variable declaration in Java?

A variable declaration tells the computer to create a space in memory to store a value of a specific type, like int or String.

beginner

What does variable initialization mean?

Initialization means giving a variable its first value when you create it, so it’s ready to use.

beginner

How do you declare and initialize an integer variable named age with the value 25?

You write: int age = 25; This creates the variable and sets its value to 25.

intermediate

Can you declare a variable without initializing it? What happens if you try to use it?

Yes, you can declare a variable without giving it a value. But if you try to use it before setting a value, Java will give an error.

beginner

What is the difference between int x; and int x = 0;?

int x; declares the variable but does not set a value. int x = 0; declares and sets the value to zero.

What does the following code do?
double price = 19.99;

A Creates a method named price

B Declares a variable named price and sets it to 19.99

C Declares a variable named price without a value

D Declares a variable named price and sets it to 1999

The code declares a double variable called price and initializes it with 19.99.

Which of these is a valid variable declaration and initialization in Java?

A int number = 10;

B number int = 10;

C int = 10 number;

D int number 10;

Option A correctly declares an int variable named number and initializes it with 10.

What happens if you try to use a variable before initializing it?

A The variable will store null

B Java will assign a default value automatically

C The program will run fine

D Java will give a compile-time error

Java requires variables to be initialized before use, otherwise it gives a compile-time error.

Which keyword is used to declare a variable in Java?

A declare

B int

C No keyword, just the type

D var

In Java, you declare a variable by specifying its type followed by its name, no special keyword like 'declare' is used.

What is the correct way to declare a String variable named name and set it to "Alice"?

A String name = "Alice";

B string name = 'Alice';

C String name = Alice;

D String name := "Alice";

Option A correctly declares a String variable and initializes it with "Alice" using double quotes.

Explain in your own words what variable declaration and initialization mean in Java.

Think about how you tell the computer to remember a value.

Write an example of declaring and initializing a variable for storing a person's height in centimeters.

Use a type that can hold decimal values if needed.

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