Primitive to object conversion in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert an int primitive to an Integer object.

Java

int num = 10;
Integer obj = [1];

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

Anew Integer(num)

BInteger.valueOf(num)

Cint.valueOf(num)

DInteger.parseInt(num)

Attempts:

3 left

💡 Hint

Common Mistakes

Using new Integer(num) which is deprecated.

Trying to use int.valueOf(num) which is invalid.

Using Integer.parseInt(num) which converts String to int, not int to Integer.

Use Integer.valueOf(num) to convert an int primitive to an Integer object efficiently.

2fill in blank

medium

Complete the code to convert a double primitive to a Double object.

Java

double val = 5.5;
Double obj = [1];

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

ADouble.parseDouble(val)

Bnew Double(val).doubleValue()

CDouble.valueOf(val)

DDouble.toString(val)

Attempts:

3 left

💡 Hint

Common Mistakes

Using Double.parseDouble(val) which returns primitive double.

Using new Double(val).doubleValue() which returns primitive double, not object.

Using Double.toString(val) which returns String.

Double.valueOf(val) converts a double primitive to a Double object correctly.

3fill in blank

hard

Fix the error in converting a boolean primitive to a Boolean object.

Java

boolean flag = true;
Boolean obj = [1];

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

ABoolean.valueOf(flag)

BBoolean.parseBoolean(flag)

Cnew Boolean(flag).booleanValue()

DBoolean.toString(flag)

Attempts:

3 left

💡 Hint

Common Mistakes

Using Boolean.parseBoolean(flag) which returns primitive boolean.

Using new Boolean(flag).booleanValue() which returns primitive boolean.

Using Boolean.toString(flag) which returns String.

Boolean.valueOf(flag) correctly converts a boolean primitive to a Boolean object.

4fill in blank

hard

Fill both blanks to convert a char primitive to a Character object and print it.

Java

char ch = 'a';
Character obj = [1];
System.out.println([2]);

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

ACharacter.valueOf(ch)

Bch

Cobj

Dnew Character(ch)

Attempts:

3 left

💡 Hint

Common Mistakes

Printing the primitive ch instead of the object.

Using deprecated constructor new Character(ch).

Assigning the primitive ch directly to Character without conversion.

Use Character.valueOf(ch) to convert char to Character object, then print the object obj.

5fill in blank

hard

Fill all three blanks to create a Float object from a float primitive, check equality, and print the result.

Java

float f1 = 3.14f;
Float obj1 = [1];
Float obj2 = [2];
boolean isEqual = obj1.[3](obj2);
System.out.println(isEqual);

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

AFloat.valueOf(f1)

Bnew Float(f1)

Cequals

DFloat.parseFloat("3.14")

Attempts:

3 left

💡 Hint

Common Mistakes

Using Float.parseFloat which returns primitive float, not object.

Using == instead of equals to compare objects.

Mixing up method names or forgetting to create objects properly.

Use Float.valueOf(f1) and new Float(f1) to create Float objects, then use equals to compare them.

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