Primitive to object conversion in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of autoboxing with Integer

What is the output of this Java code snippet?

Java

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

A true

B false

C Compilation error

D Runtime exception

💡 Hint

Remember how Java caches Integer objects for certain values.

Java caches Integer objects for values between -128 and 127. Since 1000 is outside this range, a and b refer to different objects, so '==' returns false.

❓ Predict Output

intermediate

Unboxing and arithmetic with Integer and int

What is the output of this Java code?

Java

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

A 15

B Compilation error

C 5 10

D NullPointerException

💡 Hint

Think about how Java converts Integer to int automatically.

Java automatically unboxes the Integer x to int, then adds it to y, resulting in 15.

🔧 Debug

advanced

Identify the error in primitive to object conversion

Which option will cause a compilation error in this code snippet?

Java

Double d = 10.5;
Integer i = d;

A Double d = 10.5; Integer i = (int) d.doubleValue();

B Double d = 10.5; Integer i = d.intValue();

C Double d = 10.5; int i = d.intValue();

D Double d = 10.5; Integer i = d;

💡 Hint

Check if direct assignment between wrapper classes is allowed.

You cannot assign a Double object directly to an Integer reference. This causes a compilation error.

📝 Syntax

advanced

Correct way to convert int to Integer

Which option correctly converts a primitive int to an Integer object?

A Integer obj = (Integer) 5;

B Integer obj = 5;

C Integer obj = Integer.valueOf(5);

D Integer obj = new Integer(5);

💡 Hint

Consider the recommended way to create Integer objects in modern Java.

Integer.valueOf(5) is preferred because it uses caching and avoids unnecessary object creation.

🚀 Application

expert

Predict the output with mixed primitive and wrapper types

What is the output of this Java code?

Java

Integer a = 127;
Integer b = 127;
Integer c = 128;
Integer d = 128;
System.out.println(a == b);
System.out.println(c == d);

A

true
false

B

false
false

C

true
true

D

false
true

💡 Hint

Remember the Integer cache range in Java.

Integer values between -128 and 127 are cached, so a and b refer to the same object (true). Values 128 are not cached, so c and d are different objects (false).

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