Unboxing in Java - Mini Project: Build & Apply

Unboxing

📖 Scenario: Imagine you have a box that holds a number, but the number is wrapped inside an object. You want to take the number out of the box to use it as a simple number in your program.

🎯 Goal: You will learn how to convert a wrapped number (an Integer object) into a simple number (int) using unboxing in Java.

📋 What You'll Learn

Create an Integer object with a specific value

Create an int variable to hold the unboxed value

Use unboxing to convert the Integer object to int

Print the unboxed int value

💡 Why This Matters

🌍 Real World

Unboxing is useful when you receive numbers as objects but need to use them as simple numbers for calculations or comparisons.

💼 Career

Understanding unboxing helps in writing efficient Java code, especially when working with collections or APIs that use wrapper classes.

1

Create an Integer object

Create an Integer object called boxedNumber and set it to 100.

Java

// Your code here

Hint

Use Integer boxedNumber = 100; to create the object.

2

Create an int variable for unboxing

Create an int variable called unboxedNumber to hold the unboxed value from boxedNumber.

Java

public class Main {
    public static void main(String[] args) {
        Integer boxedNumber = 100;
        // Your code here
    }
}

Hint

Assign boxedNumber directly to unboxedNumber to unbox.

3

Use unboxed value in a calculation

Add 50 to unboxedNumber and store the result in a new int variable called result.

Java

public class Main {
    public static void main(String[] args) {
        Integer boxedNumber = 100;
        int unboxedNumber = boxedNumber;
        // Your code here
    }
}

Hint

Use int result = unboxedNumber + 50; to add 50.

4

Print the result

Print the value of result using System.out.println.

Java

public class Main {
    public static void main(String[] args) {
        Integer boxedNumber = 100;
        int unboxedNumber = boxedNumber;
        int result = unboxedNumber + 50;
        // Your code here
    }
}

Hint

Use System.out.println(result); to display the number.

Practice

1. What does unboxing mean in Java?

easy

A. Converting a primitive type to a String

B. Automatically converting a wrapper object to its primitive type

C. Manually converting a primitive type to a wrapper object

D. Automatically converting a String to a wrapper object

Solution

1. Step 1: Understand wrapper and primitive types

   Wrapper classes like Integer wrap primitive types like int.

2. Step 2: Define unboxing

   Unboxing is the automatic conversion from wrapper object to primitive type.

3. Final Answer:

   Automatically converting a wrapper object to its primitive type -> Option B

4. Quick Check:

   Unboxing = wrapper to primitive [OK]

Hint: Unboxing means wrapper to primitive automatically [OK]

Common Mistakes:

• Confusing unboxing with boxing (primitive to wrapper)
• Thinking unboxing converts to String
• Assuming unboxing requires manual code

2. Which of the following Java statements correctly uses unboxing?

easy

A. Integer num = 10; int val = num;

B. Integer val = new Integer(10);

C. Integer num = int 10;

D. Integer val = Integer.valueOf(10);

Solution

1. Step 1: Check each option for correct syntax and unboxing

   Integer num = 10; int val = num; assigns Integer object to int variable, triggering unboxing automatically.

2. Step 2: Identify errors in other options

   Integer val = new Integer(10); assigns wrapper object to wrapper variable (no unboxing); Integer num = int 10; has invalid syntax; Integer val = Integer.valueOf(10); assigns wrapper object to wrapper variable (no unboxing).

3. Final Answer:

   Integer num = 10; int val = num; -> Option A

4. Quick Check:

   Wrapper to primitive assignment triggers unboxing [OK]

Hint: Assign wrapper object to primitive variable to unbox [OK]

Common Mistakes:

• Assigning wrapper constructors directly to wrapper variables (no unboxing)
• Incorrect syntax like 'Integer num = int 10;'
• Confusing boxing and unboxing syntax

3. What is the output of this Java code?

Integer a = 5;
int b = a + 10;
System.out.println(b);

medium

A. NullPointerException

B. 510

C. Compilation error

D. 15

Solution

1. Step 1: Understand unboxing in expression

   Variable 'a' is Integer object; adding 10 triggers unboxing 'a' to int 5.

2. Step 2: Calculate the sum and print

   5 + 10 = 15, so output is 15 printed.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   Unboxing Integer 5 + 10 = 15 [OK]

Hint: Unboxing happens automatically in arithmetic expressions [OK]

Common Mistakes:

• Thinking it concatenates as string '510'
• Expecting compilation error due to wrapper usage
• Confusing with NullPointerException if variable was null

4. Identify the error in this code snippet:

Integer num = null;
int val = num;
System.out.println(val);

medium

A. Prints 0 without error

B. Compilation error due to null assignment

C. Runtime NullPointerException during unboxing

D. Prints 'null' as output

Solution

1. Step 1: Analyze unboxing of null Integer

   Variable 'num' is null; unboxing null to int causes runtime error.

2. Step 2: Identify exception thrown

   Unboxing null throws NullPointerException at runtime.

3. Final Answer:

   Runtime NullPointerException during unboxing -> Option C

4. Quick Check:

   Unboxing null causes NullPointerException [OK]

Hint: Unboxing null causes runtime NullPointerException [OK]

Common Mistakes:

• Expecting compilation error instead of runtime error
• Assuming null unboxes to zero
• Thinking it prints 'null' string

5. Given this code, what will be the output?

List<Integer> list = List.of(1, 2, null, 4);
int sum = 0;
for (Integer n : list) {
    sum += n; // unboxing happens here
}
System.out.println(sum);

hard

A. Runtime NullPointerException at unboxing null

B. Compilation error due to null in list

C. Sum ignoring null, prints 7

D. 10

Solution

1. Step 1: Understand unboxing in loop with null element

   List contains a null Integer; unboxing null causes error.

2. Step 2: Identify runtime behavior

   When loop reaches null, unboxing triggers NullPointerException.

3. Final Answer:

   Runtime NullPointerException at unboxing null -> Option A

4. Quick Check:

   Unboxing null in loop causes runtime exception [OK]

Hint: Unboxing null in collections causes runtime exception [OK]

Common Mistakes:

• Assuming null is skipped or treated as zero
• Expecting compilation error due to null in list
• Thinking sum is partial ignoring null