Common wrapper methods in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Integer wrapper method usage

What is the output of the following Java code snippet using Integer wrapper methods?

Java

public class Main {
    public static void main(String[] args) {
        Integer num = Integer.valueOf("123");
        int result = num.intValue() + Integer.parseInt("77");
        System.out.println(result);
    }
}

A Compilation error

B 100

C 200

D 12377

💡 Hint

Remember Integer.valueOf returns an Integer object and parseInt returns a primitive int.

Integer.valueOf("123") creates an Integer object with value 123. intValue() extracts the primitive int 123. Integer.parseInt("77") returns int 77. Adding 123 + 77 equals 200.

❓ Predict Output

intermediate

Output of Double wrapper method usage

What will be printed by this Java code using Double wrapper methods?

Java

public class Main {
    public static void main(String[] args) {
        Double d1 = Double.valueOf("10.5");
        double d2 = Double.parseDouble("2.5");
        System.out.println(d1.doubleValue() * d2);
    }
}

A 26.25

B 105.0

C 13.0

D Runtime error

💡 Hint

Check the multiplication of 10.5 and 2.5 as doubles.

Double.valueOf("10.5") returns a Double object with value 10.5. doubleValue() extracts 10.5 as primitive double. Double.parseDouble("2.5") returns primitive double 2.5. Multiplying 10.5 * 2.5 equals 26.25.

❓ Predict Output

advanced

Output of Boolean wrapper method usage

What is the output of this Java code snippet using Boolean wrapper methods?

Java

public class Main {
    public static void main(String[] args) {
        Boolean b1 = Boolean.valueOf("true");
        Boolean b2 = Boolean.valueOf("False");
        System.out.println(b1 && b2);
    }
}

A false

B Runtime exception

C Compilation error

D true

💡 Hint

Boolean.valueOf is case insensitive and returns Boolean objects. Logical AND on Boolean objects requires unboxing.

Boolean.valueOf("true") returns Boolean.TRUE. Boolean.valueOf("False") returns Boolean.FALSE. Using && on Boolean objects unboxes them to primitive boolean. true && false is false.

❓ Predict Output

advanced

Output of Character wrapper method usage

What will this Java code print when using Character wrapper methods?

Java

public class Main {
    public static void main(String[] args) {
        Character ch = Character.valueOf('a');
        System.out.println(Character.toUpperCase(ch));
    }
}

A Compilation error

B a

C Runtime exception

D A

💡 Hint

Character.toUpperCase converts a character to uppercase.

Character.valueOf('a') returns a Character object wrapping 'a'. Character.toUpperCase converts 'a' to 'A'.

❓ Predict Output

expert

Output of complex wrapper method chaining

What is the output of this Java code using multiple wrapper methods?

Java

public class Main {
    public static void main(String[] args) {
        String s = "256";
        Integer i = Integer.valueOf(s);
        Double d = Double.valueOf(i.doubleValue() / 4);
        System.out.println(d.intValue() + ":" + d);
    }
}

A 64:64

B 64:64.0

C 64.0:64.0

D Compilation error

💡 Hint

Trace the conversions from String to Integer to Double and back to int.

Integer.valueOf("256") is 256. i.doubleValue() / 4 is 256.0 / 4 = 64.0. Double.valueOf(64.0) is Double object 64.0. d.intValue() is 64. Printing d.intValue() + ":" + d prints "64:64.0".

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