Autoboxing in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to autobox the int value into an Integer object.

Java

int num = 10;
Integer boxedNum = [1];

Drag options to blanks, or click blank then click option'

Aint

Bnew Integer(num)

CInteger.valueOf(num)

Dnum

Attempts:

3 left

💡 Hint

Common Mistakes

Using new Integer(num) which is unnecessary and deprecated.

Trying to cast int to Integer explicitly.

In Java, autoboxing automatically converts a primitive int to an Integer object when assigned directly.

2fill in blank

medium

Complete the code to unbox the Integer object to a primitive int.

Java

Integer boxedNum = 20;
int num = [1];

Drag options to blanks, or click blank then click option'

AInteger.parseInt(boxedNum)

BboxedNum

C(int) boxedNum

DboxedNum.intValue()

Attempts:

3 left

💡 Hint

Common Mistakes

Using boxedNum.intValue() which works but is not needed due to unboxing.

Trying to cast Integer to int explicitly.

Unboxing happens automatically when assigning an Integer object to an int variable.

3fill in blank

hard

Fix the error in autoboxing when adding a primitive int to a List of Integer.

Java

List<Integer> list = new ArrayList<>();
int num = 5;
list.add([1]);

Drag options to blanks, or click blank then click option'

Anum

Bnew Integer(num)

CInteger.valueOf(num)

D"num"

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a String instead of an Integer.

Using deprecated new Integer(num) unnecessarily.

Autoboxing allows adding a primitive int directly to a List of Integer objects.

4fill in blank

hard

Fill both blanks to create a Map with Integer keys and values using autoboxing.

Java

Map<Integer, Integer> map = new HashMap<>();
int key = 1;
int value = 100;
map.put([1], [2]);

Drag options to blanks, or click blank then click option'

Akey

Bvalue

C"key"

D"value"

Attempts:

3 left

💡 Hint

Common Mistakes

Using String literals instead of int variables as keys or values.

Trying to manually box int to Integer before putting.

Autoboxing converts primitive int key and value to Integer objects when putting into the Map.

5fill in blank

hard

Fill all three blanks to create a List of Integer squares using autoboxing and a for loop.

Java

List<Integer> squares = new ArrayList<>();
for (int i = 1; i <= 5; i++) {
    squares.add(i [1] [2]);
}
int firstSquare = squares.get([3]);

Drag options to blanks, or click blank then click option'

A*

Bi

C0

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication for squares.

Using wrong index to get the first element.

The code multiplies i by i to get squares, autoboxes the result when adding to the list, and retrieves the first element at index 0.

Practice

1. What is autoboxing in Java?

easy

A. Automatic conversion of primitive types to their wrapper class objects

B. Manual conversion of objects to primitive types

C. A way to box arrays into collections

D. Conversion of strings to numbers

Solution

1. Step 1: Understand primitive and wrapper types

   Java has primitive types like int and wrapper classes like Integer.

2. Step 2: Define autoboxing

   Autoboxing means Java automatically converts a primitive type to its wrapper object when needed.

3. Final Answer:

   Automatic conversion of primitive types to their wrapper class objects -> Option A

4. Quick Check:

   Autoboxing = automatic primitive to object conversion [OK]

Hint: Autoboxing means automatic primitive to object conversion [OK]

Common Mistakes:

• Confusing autoboxing with manual conversion
• Thinking autoboxing converts objects to primitives
• Mixing autoboxing with string parsing

2. Which of the following code snippets correctly uses autoboxing?

easy

A. Integer num = 10;

B. int num = new Integer(10);

C. int num = Integer.parseInt("10");

D. int num = Integer.valueOf(10);

Solution

1. Step 1: Check each option for autoboxing usage

   Autoboxing converts primitive to wrapper automatically, so Integer num = 10; is valid.

2. Step 2: Identify invalid autoboxing

   int num = Integer.parseInt("10"); uses string parsing to primitive (no autoboxing). int num = new Integer(10); uses manual construction followed by unboxing. int num = Integer.valueOf(10); passes primitive to method (internal boxing) then unboxes.

3. Final Answer:

   Integer num = 10; -> Option A

4. Quick Check:

   Assigning primitive to wrapper uses autoboxing [OK]

Hint: Assign primitive to wrapper for autoboxing [OK]

Common Mistakes:

• Using new Integer() instead of autoboxing
• Assigning wrapper object directly to primitive without unboxing
• Confusing parsing methods with autoboxing

3. What is the output of the following code?

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

medium

A. NullPointerException

B. Compilation error

C. 15

D. 5

Solution

1. Step 1: Understand autounboxing in expression

   The Integer a is autounboxed to int to add 10.

2. Step 2: Calculate the sum

   5 + 10 = 15, so b is 15 and printed.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   Autounboxing + addition = 15 [OK]

Hint: Wrapper auto-unboxes in arithmetic expressions [OK]

Common Mistakes:

• Expecting compilation error due to wrapper usage
• Confusing autoboxing with autounboxing
• Thinking NullPointerException occurs without null

4. Identify the error in the following code snippet:

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

medium

A. Compilation error due to null assignment

B. Output is 0

C. Output is 5

D. NullPointerException at runtime

Solution

1. Step 1: Analyze autounboxing with null wrapper

   Variable x is null, so autounboxing to int causes error.

2. Step 2: Identify runtime exception

   Autounboxing null throws NullPointerException at runtime.

3. Final Answer:

   NullPointerException at runtime -> Option D

4. Quick Check:

   Autounboxing null causes NullPointerException [OK]

Hint: Autounboxing null causes NullPointerException [OK]

Common Mistakes:

• Expecting compilation error instead of runtime error
• Assuming null converts to zero
• Ignoring autounboxing behavior

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

List<Integer> list = new ArrayList<>();
for (int i = 1; i <= 3; i++) {
    list.add(i); // autoboxing here
}
int sum = 0;
for (Integer num : list) {
    sum += num; // autounboxing here
}
System.out.println(sum);

hard

A. Compilation error due to autoboxing

B. 6

C. NullPointerException

D. 0

Solution

1. Step 1: Understand autoboxing in list addition

   Primitive int i is autoboxed to Integer when added to the list.

2. Step 2: Understand autounboxing in sum calculation

   Each Integer in list is autounboxed to int for addition to sum.

3. Step 3: Calculate the sum

   Sum of 1 + 2 + 3 = 6.

4. Final Answer:

   6 -> Option B

5. Quick Check:

   Autoboxing + autounboxing sum = 6 [OK]

Hint: Autoboxing in collections, autounboxing in arithmetic sum [OK]

Common Mistakes:

• Expecting errors with autoboxing in collections
• Forgetting autounboxing in sum calculation
• Assuming NullPointerException without null values