Primitive to object conversion in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is primitive to object conversion in Java?

It is the process of converting a primitive data type (like int, double) into its corresponding wrapper class object (like Integer, Double). This allows primitives to be used where objects are required.

beginner

What is autoboxing in Java?

Autoboxing is the automatic conversion by the Java compiler of a primitive type into its corresponding wrapper class object when needed.

beginner

How do you manually convert an int primitive to an Integer object?

You can use the Integer constructor or the static method Integer.valueOf(int). For example: Integer obj = Integer.valueOf(5);

intermediate

What is the difference between autoboxing and manual boxing?

Autoboxing is done automatically by the compiler, while manual boxing requires explicitly creating wrapper objects using constructors or valueOf methods.

beginner

Why is primitive to object conversion useful in Java?

It allows primitives to be used in collections like ArrayList, which only accept objects, and enables use of methods defined in wrapper classes.

Which of these is the wrapper class for the primitive type 'int'?

A Character

B Int

C Integer

D Double

The wrapper class for 'int' is 'Integer'.

What does autoboxing do in Java?

A Automatically converts primitives to objects

B Converts objects to primitives

C Converts strings to numbers

D Converts objects to strings

Autoboxing automatically converts primitive types to their wrapper class objects.

Which method can be used to manually convert an int to an Integer?

A Integer.parseInt()

B Integer.valueOf()

C int.toInteger()

D Integer.toInt()

Integer.valueOf(int) returns an Integer object representing the int value.

Why can't primitives be added directly to an ArrayList?

A ArrayList only accepts objects

B Primitives are too large

C Primitives are slower

D ArrayList only accepts strings

ArrayList stores objects, so primitives must be converted to wrapper objects first.

What is manual boxing?

A Converting a string to a primitive

B Automatically converting a primitive to an object

C Converting an object to a primitive

D Explicitly converting a primitive to its wrapper object

Manual boxing means explicitly creating a wrapper object from a primitive.

Explain what primitive to object conversion means and why it is important in Java.

Think about how Java treats simple data and objects differently.

Describe the difference between autoboxing and manual boxing with examples.

One is automatic by the compiler, the other requires you to write code.

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