Autoboxing in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of autoboxing with == operator?

Consider the following Java code snippet. What will be printed when it runs?

Java

Integer a = 1000;
Integer b = 1000;
System.out.println(a == b);

A Runtime exception

B true

C Compilation error

D false

💡 Hint

Remember that == compares object references, not values, for wrapper classes.

In Java, Integer objects between -128 and 127 are cached. For 1000, new Integer objects are created, so a and b refer to different objects. Using == compares references, so it returns false.

❓ Predict Output

intermediate

What is the output when adding Integer and int?

What will the following Java code print?

Java

Integer x = 5;
int y = 10;
System.out.println(x + y);

A 15.0

B 15

C Compilation error

D NullPointerException

💡 Hint

Autoboxing converts Integer to int automatically in arithmetic operations.

The Integer x is unboxed to int 5, then added to int y (10), resulting in 15.

🔧 Debug

advanced

Why does this code throw NullPointerException?

Examine the code below. Why does it throw a NullPointerException at runtime?

Java

Integer a = null;
int b = a;
System.out.println(b);

A Because Integer cannot be assigned to int

B Because variable a is not initialized

C Because unboxing null Integer to int causes NullPointerException

D Because printing int b is invalid

💡 Hint

Think about what happens when Java tries to convert null to a primitive type.

Unboxing null Integer to int causes NullPointerException because Java tries to get the int value from a null reference.

🧠 Conceptual

advanced

How does autoboxing affect performance?

Which statement best describes the performance impact of autoboxing in Java?

A Autoboxing can cause performance overhead due to object creation and garbage collection

B Autoboxing always improves performance by caching objects

C Autoboxing has no impact on performance

D Autoboxing reduces memory usage by using primitives only

💡 Hint

Consider what happens behind the scenes when primitives are converted to objects.

Autoboxing creates wrapper objects which can increase memory usage and cause more frequent garbage collection, leading to performance overhead.

❓ Predict Output

expert

What is the output of this tricky autoboxing and caching code?

Analyze the code below and select the output it produces.

Java

Integer i1 = 127;
Integer i2 = 127;
Integer i3 = 128;
Integer i4 = 128;
System.out.println(i1 == i2);
System.out.println(i3 == i4);

A

true
false

B

false
false

C

true
true

D

false
true

💡 Hint

Remember the Integer cache range is from -128 to 127 inclusive.

Integer values between -128 and 127 are cached, so i1 and i2 point to the same object, making == true. For 128, new objects are created, so i3 and i4 are different, making == false.

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