Why wrapper classes are used in Java - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an Integer wrapper object from a primitive int.

Java

Integer num = [1];

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

Aint(5)

B5

CInteger.valueOf(5)

Dnew Integer(5)

Attempts:

3 left

💡 Hint

Common Mistakes

Using the primitive value directly without wrapping.

Trying to call int(5) which is invalid in Java.

Wrapper classes like Integer can be created using the new Integer(value) constructor to wrap a primitive int.

2fill in blank

medium

Complete the code to convert a String to an Integer using wrapper class methods.

Java

Integer num = Integer.[1]("123");

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

AparseInt

BvalueOf

CtoString

DintValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using parseInt which returns a primitive int, not an Integer object.

Using toString which converts to String, not Integer.

The valueOf method converts a String to an Integer object.

3fill in blank

hard

Fix the error in the code to correctly convert an Integer to a primitive int.

Java

int value = num.[1]();

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

AintValue

BtoInt

CparseInt

DvalueOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using toInt which does not exist.

Using parseInt which is static and takes a String argument.

The intValue() method returns the primitive int value from an Integer object.

4fill in blank

hard

Fill both blanks to create a list of Integer objects from an array of ints.

Java

int[] arr = {1, 2, 3};
List<Integer> list = new ArrayList<>();
for (int num : arr) {
    list.[1](Integer.[2](num));
}

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

Aadd

BvalueOf

Cappend

DparseInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'append' which is not a List method.

Using 'parseInt' which converts String to int, not int to Integer.

Use add to add elements to the list and Integer.valueOf(num) to convert primitive int to Integer object.

5fill in blank

hard

Fill all three blanks to create a map of String keys to Integer values where values are only added if greater than 10.

Java

Map<String, Integer> map = new HashMap<>();
for (String key : keys) {
    Integer val = Integer.[1](key.length());
    if (val [2] 10) {
        map.[3](key, val);
    }
}

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

AvalueOf

B>

Cput

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add' instead of 'put' for maps.

Using '<' instead of '>' in the condition.

Using 'parseInt' instead of 'valueOf'.

Use Integer.valueOf to convert int to Integer, the comparison operator > to check if value is greater than 10, and put to add entries to the map.

Practice

1. Why are wrapper classes used in Java?

easy

A. To replace all primitive types with objects

B. To improve the speed of primitive operations

C. To allow primitive types to be treated as objects

D. To create new primitive types

Solution

1. Step 1: Understand primitive vs object types

   Primitive types like int or double are not objects, so they can't be used where objects are required.

2. Step 2: Role of wrapper classes

   Wrapper classes like Integer or Double wrap primitive values so they can be used as objects.

3. Final Answer:

   To allow primitive types to be treated as objects -> Option C

4. Quick Check:

   Wrapper classes = primitive as objects [OK]

Hint: Wrapper classes wrap primitives as objects [OK]

Common Mistakes:

• Thinking wrapper classes speed up primitive operations
• Believing wrapper classes replace primitives entirely
• Confusing wrapper classes with new primitive types

2. Which of the following is the correct way to create an Integer wrapper object from an int value 10?

easy

A. Integer obj = new Integer(10);

B. int obj = Integer(10);

C. Integer obj = new int(10);

D. Integer obj = Integer.value(10);

Solution

1. Step 1: Check syntax for creating Integer object

   The constructor Integer(int) creates an Integer object from an int value.

2. Step 2: Validate each option

   Integer obj = new Integer(10); uses new Integer(10), which is correct syntax. int obj = Integer(10); is invalid syntax. Integer obj = new int(10); is invalid as primitive int has no constructor. Integer obj = Integer.value(10); uses a non-existent method value().

3. Final Answer:

   Integer obj = new Integer(10); -> Option A

4. Quick Check:

   Use new Integer(int) to create wrapper [OK]

Hint: Use new Integer(value) to create wrapper object [OK]

Common Mistakes:

• Using invalid method names like Integer.value()
• Trying to instantiate primitives like new int(10)
• Confusing primitive and wrapper syntax

3. What will be the output of the following code?

List<Integer> list = new ArrayList<>();
list.add(5);
list.add(10);
System.out.println(list.get(0) + list.get(1));

medium

A. 510

B. 15

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand autoboxing in collections

   List<Integer> stores Integer objects. Adding int values 5 and 10 autoboxes them to Integer objects.

2. Step 2: Evaluate the addition operation

   list.get(0) and list.get(1) return Integer objects, which unbox to int when added, so 5 + 10 = 15.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   Autoboxing/unboxing sums to 15 [OK]

Hint: Autoboxing lets you add Integer values like ints [OK]

Common Mistakes:

• Thinking list.get returns int directly
• Expecting string concatenation instead of addition
• Assuming compilation or runtime errors

4. Identify the error in the following code snippet:

int a = 5;
Integer b = null;
int c = a + b;

medium

A. No error, code runs fine

B. Compilation error due to incompatible types

C. ArithmeticException at runtime

D. NullPointerException at runtime

Solution

1. Step 1: Analyze autounboxing of Integer b

   Variable b is an Integer object set to null. When used in arithmetic, it is unboxed to int.

2. Step 2: Understand null unboxing behavior

   Unboxing null causes NullPointerException at runtime because there is no int value to extract.

3. Final Answer:

   NullPointerException at runtime -> Option D

4. Quick Check:

   Unboxing null Integer causes NullPointerException [OK]

Hint: Unboxing null wrapper causes NullPointerException [OK]

Common Mistakes:

• Expecting compilation error instead of runtime error
• Assuming code runs without error
• Confusing NullPointerException with ArithmeticException

5. You want to store primitive int values in a collection and perform arithmetic operations easily. Which approach best uses wrapper classes to achieve this?

hard

A. Use List<Integer> and rely on autoboxing/unboxing

B. Use List<int> since it stores primitives directly

C. Convert int to String before adding to collection

D. Use arrays only, wrapper classes are not needed

Solution

1. Step 1: Understand collections and primitives

   Java collections cannot store primitive types like int directly; they require objects.

2. Step 2: Use wrapper classes with autoboxing

   List<Integer> stores Integer objects. Autoboxing converts int to Integer automatically, allowing easy arithmetic with unboxing.

3. Final Answer:

   Use List<Integer> and rely on autoboxing/unboxing -> Option A

4. Quick Check:

   Collections need wrapper classes for primitives [OK]

Hint: Use List<Integer> for primitives with autoboxing [OK]

Common Mistakes:

• Trying to use List<int> which is invalid
• Storing primitives as Strings unnecessarily
• Avoiding wrapper classes in collections