Common wrapper methods in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert the string "123" to an integer using a wrapper method.

Java

int number = Integer.[1]("123");

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

AparseInt

BvalueOf

CtoString

DintValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using toString instead of parseInt

Using valueOf which returns an Integer object, not int

The method parseInt converts a string to a primitive int.

2fill in blank

medium

Complete the code to convert the integer 456 to a string using a wrapper method.

Java

String text = Integer.[1](456);

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

AvalueOf

BtoString

CparseInt

DintValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using parseInt which converts String to int

Using toString which is an instance method, not static

The method valueOf converts an int to a String.

3fill in blank

hard

Fix the error in the code to convert the string "true" to a boolean value.

Java

boolean flag = Boolean.[1]("true");

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

AtoString

BparseBoolean

CvalueOf

DbooleanValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using valueOf which returns a Boolean object, not primitive

Using toString which converts boolean to string

The method parseBoolean converts a string to a primitive boolean.

4fill in blank

hard

Fill both blanks to create a Double object from a string and get its primitive double value.

Java

Double d = new Double("3.14");
double value = d.[1](); // get primitive
boolean check = Double.[2]("3.14") > 3.0;

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

AdoubleValue

BparseDouble

CvalueOf

DtoString

Attempts:

3 left

💡 Hint

Common Mistakes

Using valueOf for the first blank which returns an object, not primitive

Using toString which returns a string, not a number

doubleValue() gets the primitive double from the Double object.
parseDouble() converts a string to a primitive double.

5fill in blank

hard

Fill all three blanks to convert a string to an Integer object, get its int value, and convert it back to string.

Java

Integer obj = Integer.[1]("789");
int num = obj.[2]();
String str = Integer.[3](num);

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

AvalueOf

BintValue

CtoString

DparseInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using parseInt for first blank which returns primitive int, not Integer object

Using valueOf for third blank which returns Integer object, not string

valueOf converts string to Integer object.
intValue gets primitive int from Integer.
toString converts int to string.

Practice

1. Which of the following methods converts a String to a primitive int in Java?

easy

A. Integer.valueOf(String s)

B. Integer.toString(int i)

C. Integer.parseInt(String s)

D. Integer.intValue()

Solution

1. Step 1: Understand method purpose

   parseInt converts a string directly to a primitive int.

2. Step 2: Differentiate from other methods

   valueOf returns an Integer object, not a primitive. toString converts int to string. intValue() is called on an Integer object to get primitive int.

3. Final Answer:

   Integer.parseInt(String s) -> Option C

4. Quick Check:

   parseInt returns int [OK]

Hint: parseInt returns primitive int, valueOf returns Integer object [OK]

Common Mistakes:

• Confusing parseInt and valueOf
• Using intValue() without an Integer object
• Thinking toString converts string to int

2. Which of the following is the correct syntax to convert an int variable num to an Integer object?

easy

A. Integer numObj = Integer.valueOf(num);

B. Integer numObj = Integer.parseInt(num);

C. Integer numObj = num.intValue();

D. Integer numObj = Integer.toString(num);

Solution

1. Step 1: Identify method to get Integer object

   valueOf converts primitive int to Integer object.

2. Step 2: Check other options

   parseInt returns primitive int, not Integer object. intValue() is a method of Integer, not int. toString returns String, not Integer.

3. Final Answer:

   Integer.valueOf(num) -> Option A

4. Quick Check:

   valueOf returns Integer object [OK]

Hint: Use valueOf to get Integer object from int [OK]

Common Mistakes:

• Using parseInt with int input
• Calling intValue() on primitive int
• Confusing toString with valueOf

3. What will be the output of the following code?

String s = "123";
Integer obj = Integer.valueOf(s);
int n = obj.intValue();
System.out.println(n + 10);

medium

A. 133

B. Compilation error

C. NumberFormatException

D. 12310

Solution

1. Step 1: Convert String to Integer object

   Integer.valueOf("123") creates Integer object with value 123.

2. Step 2: Extract primitive int and add 10

   intValue() returns 123 as int. Adding 10 results in 133.

3. Final Answer:

   133 -> Option A

4. Quick Check:

   123 + 10 = 133 [OK]

Hint: valueOf + intValue() converts string to int for math [OK]

Common Mistakes:

• Thinking output is string concatenation
• Expecting NumberFormatException for valid number string
• Confusing intValue() usage

4. Identify the error in the following code snippet:

String s = "456";
int n = Integer.valueOf(s);
System.out.println(n);

medium

A. Missing semicolon after Integer.valueOf(s)

B. String s cannot be converted to Integer

C. Cannot assign Integer object to int directly

D. No error, code runs fine

Solution

1. Step 1: Check return type of Integer.valueOf

   valueOf returns an Integer object.

2. Step 2: Auto-unboxing allows assignment

   Java (5+) automatically unboxes Integer to int, so the code compiles and runs, printing 456.

3. Final Answer:

   No error, code runs fine -> Option D

4. Quick Check:

   valueOf(String) auto-unboxes to int [OK]

Hint: valueOf(String) auto-unboxes to int [OK]

Common Mistakes:

• Forgetting auto-unboxing feature
• Assuming valueOf returns primitive int
• Thinking String can't convert to Integer

5. You have a list of numeric strings: ["10", "20", "30", "40"]. Which code snippet correctly converts this list to a list of Integer objects using wrapper methods?

hard

A. List<Integer> nums = list.stream().map(String::parseInt).collect(Collectors.toList());

B. List<Integer> nums = list.stream().map(Integer::valueOf).collect(Collectors.toList());

C. List<Integer> nums = list.stream().map(s -> s.intValue()).collect(Collectors.toList());

D. List<Integer> nums = list.stream().map(s -> Integer.toString(s)).collect(Collectors.toList());

Solution

1. Step 1: Understand method return types

   Integer.valueOf(String) returns an Integer object, suitable for list of Integer.

2. Step 2: Check other options

   String::parseInt invalid - no parseInt on String. s.intValue() not a method on String. toString returns String, not Integer.

3. Final Answer:

   list.stream().map(Integer::valueOf).collect(Collectors.toList()) -> Option B

4. Quick Check:

   valueOf converts String to Integer object [OK]

Hint: Use valueOf to convert String to Integer object in streams [OK]

Common Mistakes:

• Thinking parseInt is a String method
• Calling intValue() on String
• Using toString instead of valueOf