Primitive to object conversion in Java - Time & Space Complexity

Time Complexity: Primitive to object conversion

O(n)

Understanding Time Complexity

When converting primitive values to objects in Java, it's important to understand how the time needed changes as we handle more data.

We want to know how the cost grows when converting many primitives to objects.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int[] numbers = {1, 2, 3, 4, 5};
Integer[] objects = new Integer[numbers.length];
for (int i = 0; i < numbers.length; i++) {
    objects[i] = Integer.valueOf(numbers[i]);
}
    

This code converts each primitive int in an array to an Integer object and stores it in a new array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the array and converting each int to Integer.
• How many times: Once for each element in the input array.

How Execution Grows With Input

Each element requires one conversion operation, so the total work grows directly with the number of elements.

Input Size (n)	Approx. Operations
10	10 conversions
100	100 conversions
1000	1000 conversions

Pattern observation: The work increases evenly as the input size grows.

Final Time Complexity

Time Complexity: O(n)

This means the time needed grows in direct proportion to the number of primitives converted.

Common Mistake

[X] Wrong: "Converting primitives to objects happens instantly and does not depend on input size."

[OK] Correct: Each conversion takes time, so doing many conversions adds up and grows with the number of elements.

Interview Connect

Understanding how simple conversions scale helps you reason about performance in real programs, showing you can think about efficiency clearly.

Self-Check

"What if we used a stream to convert the primitives to objects instead of a loop? How would the time complexity change?"

Practice

1.

What is autoboxing in Java?

easy

A. Manual conversion of objects to primitive types

B. A way to box multiple objects into an array

C. A method to convert strings to numbers

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

Solution

1. Step 1: Understand primitive and wrapper classes

   Java has primitive types like int and wrapper classes like Integer that represent them as objects.

2. Step 2: Define autoboxing

   Autoboxing is the automatic conversion from a primitive type to its corresponding wrapper class object by the compiler.

3. Final Answer:

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

4. Quick Check:

   Autoboxing = automatic primitive to object conversion [OK]

Hint: Autoboxing means Java converts primitives to objects automatically [OK]

Common Mistakes:

• Confusing autoboxing with unboxing
• Thinking autoboxing requires manual code
• Mixing up primitive to object with object to primitive

2.

Which of the following is the correct way to create an Integer object from a primitive int value 5 using autoboxing?

easy

A. Integer num = 5;

B. Integer num = Integer.valueOf("5");

C. Integer num = new Integer(5);

D. Integer num = (Integer)5;

Solution

1. Step 1: Recall autoboxing syntax

   Autoboxing allows assigning a primitive int directly to an Integer object without explicit conversion.

2. Step 2: Analyze options

   Direct assignment Integer num = 5; triggers autoboxing. new Integer(5); uses deprecated constructor (manual). Integer.valueOf("5"); converts from String, not primitive. (Integer)5; is invalid cast as primitives cannot be cast to wrapper objects.

3. Final Answer:

   Integer num = 5; -> Option A

4. Quick Check:

   Autoboxing allows direct assignment int to Integer [OK]

Hint: Assign int directly to Integer for autoboxing [OK]

Common Mistakes:

• Using deprecated Integer constructor
• Trying to cast primitive to wrapper
• Confusing String conversion with autoboxing

3.

What will be the output of the following Java code?

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

medium

A. 15

B. 105

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand autoboxing and unboxing in the code

   Variable x is primitive int 10. Assigning to Integer y autoboxes int to Integer object. In print, y is unboxed to int for addition.

2. Step 2: Calculate the expression

   y + 5 becomes 10 + 5 = 15, so output is 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   Autoboxing and unboxing allow arithmetic: 10 + 5 = 15 [OK]

Hint: Autoboxing/unboxing lets Integer behave like int in math [OK]

Common Mistakes:

• Thinking y + 5 concatenates as string
• Expecting compilation error due to object usage
• Confusing autoboxing with no conversion

4.

Identify the error in the following code snippet:

int a = 7;
Integer b = (Integer) a;
System.out.println(b);

medium

A. No error, code runs fine

B. Cannot cast primitive int to Integer directly

C. Integer cannot hold int values

D. Missing semicolon after declaration

Solution

1. Step 1: Analyze the cast operation

   The code tries to cast primitive int 'a' to Integer using (Integer) a, which is invalid in Java.

2. Step 2: Correct way to convert int to Integer

   Autoboxing allows direct assignment without cast: Integer b = a; Casting primitive to wrapper is not allowed.

3. Final Answer:

   Cannot cast primitive int to Integer directly -> Option B

4. Quick Check:

   Primitive to wrapper cast is invalid, use autoboxing [OK]

Hint: Don't cast primitives to wrappers; assign directly for autoboxing [OK]

Common Mistakes:

• Trying to cast primitive to wrapper type
• Ignoring autoboxing feature
• Assuming Integer can't hold int values

5.

Given the following code, what will be the output?

int[] nums = {1, 2, 3};
Integer[] boxedNums = new Integer[nums.length];
for (int i = 0; i < nums.length; i++) {
    boxedNums[i] = nums[i];
}
System.out.println(java.util.Arrays.toString(boxedNums));

hard

A. Compilation error due to array assignment

B. [null, null, null]

C. [1, 2, 3]

D. [1, 2, 3, null]

Solution

1. Step 1: Understand array and autoboxing in loop

   nums is int array. boxedNums is Integer array. Loop assigns each int to Integer element, autoboxing each value.

2. Step 2: Print the Integer array

   Arrays.toString prints the Integer array elements as their values: [1, 2, 3]. No nulls because all elements assigned.

3. Final Answer:

   [1, 2, 3] -> Option C

4. Quick Check:

   Autoboxing fills Integer array from int array correctly [OK]

Hint: Autoboxing works element-wise when assigning primitives to wrapper arrays [OK]

Common Mistakes:

• Expecting nulls because of object array
• Thinking array assignment causes error
• Confusing array length or extra elements