Common wrapper methods in Java - Mini Project: Build & Apply

Common wrapper methods

📖 Scenario: You are working on a simple Java program that processes numbers and text. You will use wrapper class methods to convert between strings and numbers, and to check character types.

🎯 Goal: Build a Java program that uses common wrapper methods to convert strings to numbers, numbers to strings, and check if a character is a digit or letter.

📋 What You'll Learn

Create a String variable with a numeric value

Create an int variable to hold the converted number

Use Integer.parseInt() to convert the String to int

Use Integer.toString() to convert the int back to String

Use Character.isDigit() and Character.isLetter() to check characters

💡 Why This Matters

🌍 Real World

Converting between strings and numbers is common when reading user input or processing text data.

💼 Career

Understanding wrapper methods helps in data validation, parsing, and formatting in many Java applications.

1

Create a String with a numeric value

Create a String variable called numberString and set it to the value "12345".

Java

// Your code here

Hint

Use double quotes to create a String in Java.

2

Convert the String to an int

Add a line to convert numberString to an int called number using Integer.parseInt(numberString).

Java

public class Main {
    public static void main(String[] args) {
        String numberString = "12345";
        // Your code here
    }
}

Hint

Use Integer.parseInt() to convert a numeric String to an int.

3

Convert the int back to a String

Add a line to convert the int number back to a String called convertedString using Integer.toString(number).

Java

public class Main {
    public static void main(String[] args) {
        String numberString = "12345";
        int number = Integer.parseInt(numberString);
        // Your code here
    }
}

Hint

Use Integer.toString() to convert an int to a String.

4

Check if characters are digits or letters

Add code to check if the character '5' is a digit using Character.isDigit('5') and if the character 'a' is a letter using Character.isLetter('a'). Print the results exactly as shown below.

Java

public class Main {
    public static void main(String[] args) {
        String numberString = "12345";
        int number = Integer.parseInt(numberString);
        String convertedString = Integer.toString(number);
        // Your code here
    }
}

Hint

Use System.out.println() to print the results.

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