What is autoboxing in Java?

Autoboxing is the automatic conversion of primitive types into their corresponding wrapper class objects.

What is unboxing in Java?

Unboxing is the automatic conversion of wrapper class objects into their corresponding primitive values.

Explain autoboxing and unboxing in Java | FullStackPrep.Dev

💡 Concept: Autoboxing & Unboxing

Autoboxing is the automatic conversion of primitive types to their corresponding wrapper classes. Unboxing is the reverse—converting wrapper objects back to primitives.

📘 Quick Intro

Before Java 5, conversions between int and Integer (and other pairs) had to be done manually. Java 5+ introduced autoboxing and unboxing to reduce boilerplate.

🧠 Analogy

Imagine putting a coin (primitive) inside a piggy bank (wrapper object)—that’s autoboxing. Taking it back out to spend it—that’s unboxing.

🔧 Technical Explanation

Autoboxing: int → Integer, char → Character, etc.
Unboxing: Integer → int, Double → double, etc.
Happens automatically in assignments, method calls, or collections.
Boxed types can be null—beware of NullPointerException during unboxing.
Useful for generics which only support objects.

🎯 Use Cases

✅ Using primitives in collections like List<Integer>
✅ Passing primitives to methods expecting wrappers
✅ Interchanging primitives and wrappers with minimal effort

💻 Example: Autoboxing and Unboxing


import java.util.ArrayList;

public class BoxingDemo {
    public static void main(String[] args) {
        int num = 10;

        // Autoboxing: int → Integer
        Integer boxed = num;

        // Unboxing: Integer → int
        int unboxed = boxed;

        // Using with collections
        ArrayList<Integer> list = new ArrayList<>();
        list.add(num); // Autoboxing
        int x = list.get(0); // Unboxing

        System.out.println("Boxed: " + boxed + ", Unboxed: " + unboxed);
    }
}

❓ Interview Q&A

Q1: What is autoboxing?
A: Automatic conversion from a primitive to a wrapper object.

Q2: What is unboxing?
A: Conversion from a wrapper to a primitive.

Q3: When was it introduced?
A: Java 5.

Q4: Can unboxing throw exceptions?
A: Yes, NullPointerException if the wrapper is null.

Q5: Why is autoboxing useful with collections?
A: Collections like List<> only support objects.

Q6: Is boxing explicit or implicit?
A: Implicit (automatic).

Q7: Can you disable autoboxing?
A: No, it’s part of the language.

Q8: Is Integer immutable?
A: Yes.

Q9: What is the wrapper class for double?
A: Double.

Q10: Can you compare primitives and wrappers directly?
A: Yes, due to unboxing.

📝 MCQs

Q1. What is autoboxing?

Object to primitive
Primitive to wrapper
String to int
Class to object

Q2. What is unboxing?

Primitive to wrapper
Wrapper to primitive
Method to value
Loop conversion

Q3. Which version introduced autoboxing?

Java 8
Java 1.3
Java 5
Java 11

Q4. Can unboxing cause exception?

No
Yes, if null
Only in Java 6
Never

Q5. Which is wrapper for int?

IntObj
Integer
IntWrapper
WholeNumber

Q6. What is stored in List<Integer>?

Primitives
Strings
Wrapper objects
Booleans only

Q7. Is boxing done manually?

Yes
No, automatic
Optional
With import

Q8. Can wrapper be null?

No
Yes
Only in arrays
Only static

Q9. What happens on unboxing null?

0 returned
Type error
NullPointerException
Infinity

Q10. Is Double the wrapper for double?

No
Yes
doubleWrapper
DType

💡 Bonus Insight

While autoboxing simplifies syntax, excessive boxing/unboxing can affect performance in loops. Prefer primitives in performance-critical code.

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Explain autoboxing and unboxing in Java