Autoboxing in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is autoboxing in Java?

Autoboxing is the automatic conversion Java does between primitive types (like int) and their corresponding wrapper classes (like Integer). It helps you use primitives as objects without manual conversion.

beginner

Which wrapper class corresponds to the primitive type <code>double</code>?

The wrapper class for the primitive type <code>double</code> is <code>Double</code>.

beginner

What happens when you assign an int value to an Integer variable?

Java automatically converts the int primitive to an Integer object using autoboxing.

beginner

Explain unboxing in Java.

Unboxing is the automatic conversion from a wrapper class object (like <code>Integer</code>) back to its primitive type (like <code>int</code>).

beginner

Why is autoboxing useful in Java?

Autoboxing lets you write cleaner code by automatically converting between primitives and objects. This is helpful when using collections like ArrayList that only work with objects.

What does autoboxing do in Java?

A Converts strings to numbers

B Automatically converts primitives to wrapper objects

C Converts objects to strings

D Automatically converts wrapper objects to primitives

Autoboxing automatically converts primitive types like int to their wrapper classes like Integer.

Which of these is a wrapper class for the primitive type boolean?

A BoolClass

B Bool

C Boolean

D booleanWrapper

The wrapper class for boolean is Boolean.

What happens if you assign an Integer object to an int variable?

A Unboxing converts Integer to int automatically

B Compilation error

C Runtime error

D The Integer object is stored as is

Unboxing automatically converts the Integer object back to an int primitive.

Which of these collections can only hold objects, not primitives?

A LinkedList of primitives

B int[]

C double[]

D ArrayList

ArrayList can only hold objects, so autoboxing helps convert primitives to wrapper objects.

Which Java feature was introduced to simplify working with primitives and wrapper classes?

A Autoboxing and unboxing

B Generics

C Lambda expressions

D Annotations

Autoboxing and unboxing were introduced to automatically convert between primitives and wrapper classes.

Describe autoboxing and unboxing in Java and give an example of each.

Think about how Java converts between primitive types and wrapper objects automatically.

Why is autoboxing important when using Java collections like ArrayList?

Consider what happens if you try to put an int into an ArrayList.

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