Variable declaration and initialization in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of variable initialization with expressions

What is the output of the following Java code?

Java

public class Main {
    public static void main(String[] args) {
        int a = 5;
        int b = a * 2;
        int c = b - a / 5;
        System.out.println(c);
    }
}

A 9

B 10

C 8

D 7

💡 Hint

Remember the order of operations: division before subtraction.

a = 5, b = 10 (5*2), c = 10 - (5/5) = 10 - 1 = 9

❓ Predict Output

intermediate

Value of variable after multiple declarations

What is the value of variable num after running this code?

Java

public class Main {
    public static void main(String[] args) {
        int num = 10;
        num = num + 5;
        num = num * 2;
        System.out.println(num);
    }
}

A 15

B 20

C 30

D 25

💡 Hint

Follow the operations step by step.

num starts at 10, then 10 + 5 = 15, then 15 * 2 = 30.

❓ Predict Output

advanced

Output with uninitialized local variable

What error or output does this code produce?

Java

public class Main {
    public static void main(String[] args) {
        int x;
        // System.out.println(x); // Uncommenting this line causes error
        x = 5;
        System.out.println(x);
    }
}

A 0

B 5

C Compilation error: variable x might not have been initialized

D Runtime error

💡 Hint

The variable is assigned before printing.

Variable x is declared but not initialized at first. It is assigned 5 before printing, so output is 5.

❓ Predict Output

advanced

Output of variable shadowing in nested blocks

What is the output of this Java program?

Java

public class Main {
    public static void main(String[] args) {
        int val = 10;
        {
            int val = 20;
            System.out.println(val);
        }
        System.out.println(val);
    }
}

A Compilation error due to duplicate variable declaration

B 10\n20

C 20\n10

D 20\n20

💡 Hint

Java allows redeclaring a variable in an inner block (shadowing).

Java does not allow redeclaring a local variable with the same name in an inner block. This causes a compilation error.

🧠 Conceptual

expert

Understanding default values of instance variables

Consider the following Java class. What is the output when new Test().printValues(); is called?

Java

public class Test {
    int a;
    boolean b;
    double c;
    String d;

    void printValues() {
        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
        System.out.println(d);
    }
}

A Compilation error due to uninitialized variables

B Garbage values printed

C 0\ntrue\n0.0\n""

D 0\nfalse\n0.0\nnull

💡 Hint

Instance variables have default values if not initialized.

Instance variables get default values: int=0, boolean=false, double=0.0, String=null.

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