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

Common string methods

📖 Scenario: You are working on a simple Java program that processes a user's full name to extract useful information.

🎯 Goal: Build a Java program that uses common string methods to analyze and manipulate a full name.

📋 What You'll Learn

Create a String variable with a full name

Create an int variable to store the length of the name

Use string methods to find the first name and last name

Print the length, first name, and last name

💡 Why This Matters

🌍 Real World

Processing user names is common in apps that register users or display profiles.

💼 Career

Knowing string methods helps in data cleaning, user input validation, and text processing tasks.

1

Create the full name variable

Create a String variable called fullName and set it to the exact value "John Doe".

Java

# Create a String variable called fullName and set it to "John Doe"
// Your code here

Hint

Use String fullName = "John Doe"; to create the variable.

2

Find the length of the full name

Add an int variable called nameLength that stores the length of fullName using the length() method.

Java

public class Main {
    public static void main(String[] args) {
        String fullName = "John Doe";
        // Create int variable nameLength to store fullName.length()
        // Your code here
    }
}

Hint

Use int nameLength = fullName.length(); to get the length.

3

Extract the first and last names

Create two String variables: firstName and lastName. Use indexOf(' '), substring() methods on fullName to get the first and last names.

Java

public class Main {
    public static void main(String[] args) {
        String fullName = "John Doe";
        int nameLength = fullName.length();
        // Create firstName and lastName using substring and indexOf
        // Your code here
    }
}

Hint

Use indexOf(' ') to find the space, then substring() to get parts.

4

Print the results

Use System.out.println to print nameLength, firstName, and lastName each on its own line.

Java

public class Main {
    public static void main(String[] args) {
        String fullName = "John Doe";
        int nameLength = fullName.length();
        int spaceIndex = fullName.indexOf(' ');
        String firstName = fullName.substring(0, spaceIndex);
        String lastName = fullName.substring(spaceIndex + 1);
        // Print nameLength, firstName, and lastName each on a new line
        // Your code here
    }
}

Hint

Use System.out.println(variable); to print each value.

Practice

1. Which Java String method checks if a string contains a specific sequence of characters?

easy

A. equals()

B. contains()

C. substring()

D. replace()

Solution

1. Step 1: Understand the purpose of contains()

   The contains() method checks if the string includes a specific sequence of characters and returns true or false.

2. Step 2: Compare with other methods

   equals() checks full equality, substring() extracts part of a string, and replace() changes characters. None check for containment.

3. Final Answer:

   contains() -> Option B

4. Quick Check:

   Check substring presence = contains() [OK]

Hint: Use contains() to check if text is inside a string [OK]

Common Mistakes:

• Confusing contains() with equals()
• Using substring() to check presence
• Thinking replace() checks for text

2. Which of the following is the correct syntax to convert a string str to uppercase in Java?

easy

A. str.toUpperCase();

B. str.uppercase();

C. toUpperCase(str);

D. str.toUpper();

Solution

1. Step 1: Recall the method name for uppercase conversion

   The correct method name in Java String class is toUpperCase(), called on the string object.

2. Step 2: Check syntax correctness

   str.toUpperCase(); uses str.toUpperCase(); which is valid. Others use incorrect method names or syntax.

3. Final Answer:

   str.toUpperCase(); -> Option A

4. Quick Check:

   Uppercase method = toUpperCase() [OK]

Hint: Remember Java String methods use camelCase like toUpperCase() [OK]

Common Mistakes:

• Using uppercase() instead of toUpperCase()
• Calling method without object (toUpperCase(str))
• Using toUpper() which does not exist

3. What is the output of the following Java code?

String text = "Hello World";
int pos = text.indexOf("World");
System.out.println(pos);

medium

A. 6

B. 0

C. -1

D. 5

Solution

1. Step 1: Understand what indexOf() does

   The indexOf() method returns the starting index of the substring if found, or -1 if not found.

2. Step 2: Find the position of "World" in "Hello World"

   "Hello " has 6 characters (index 0 to 5), so "World" starts at index 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Substring start index = 6 [OK]

Hint: indexOf() returns first position of substring or -1 if missing [OK]

Common Mistakes:

• Confusing index with length
• Expecting index 0 for substring
• Thinking indexOf() returns boolean

4. Identify the error in this Java code snippet:

String s = "Java Programming";
String part = s.substring(5, 3);
System.out.println(part);

medium

A. substring() cannot be used on String objects

B. Missing semicolon after substring() call

C. substring() arguments are reversed; start index must be <= end index

D. The variable 'part' is not declared

Solution

1. Step 1: Check substring() method argument rules

   substring(start, end) requires start index less than or equal to end index; here 5 > 3, which is invalid.

2. Step 2: Confirm other syntax correctness

   Semicolons are present, substring() is valid on String, and 'part' is declared properly.

3. Final Answer:

   substring() arguments are reversed; start index must be <= end index -> Option C

4. Quick Check:

   substring(start, end) needs start ≤ end [OK]

Hint: Start index must not be greater than end index in substring() [OK]

Common Mistakes:

• Swapping start and end indexes
• Thinking substring() changes original string
• Ignoring exception thrown at runtime

5. Given the string input = "apple,banana,orange", which code snippet correctly replaces all commas with semicolons and converts the result to uppercase?

hard

A. input.replaceFirst(",", ";").toUpperCase();

B. input.toUpperCase().replaceFirst(",", ";");

C. input.replace(",", ";").toUpper();

D. input.replaceAll(",", ";").toUpperCase();

Solution

1. Step 1: Understand replace() vs replaceAll()

   Both replace() and replaceAll() can replace characters, but replaceAll() uses regex and is safer for multiple replacements.

2. Step 2: Check method chaining and correctness

   A and B use replaceFirst() which only replaces the first comma. C uses invalid toUpper().

3. Step 3: Choose best practice

   Using replaceAll() with chaining to toUpperCase() is correct and clear.

4. Final Answer:

   input.replaceAll(",", ";").toUpperCase(); -> Option D

5. Quick Check:

   Replace commas then uppercase = replaceAll + toUpperCase() [OK]

Hint: Chain replaceAll() then toUpperCase() for clean transformation [OK]

Common Mistakes:

• Using toUpper() instead of toUpperCase()
• Using replaceFirst() which only replaces the first occurrence
• Confusing replace() and replaceAll() usage