Common string methods in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of substring and length methods

What is the output of the following Java code?

Java

public class Main {
    public static void main(String[] args) {
        String s = "HelloWorld";
        System.out.println(s.substring(2, 7) + " " + s.length());
    }
}

A lloWo 9

B lloWo 10

C loWor 10

D loWor 9

💡 Hint

Remember substring(start, end) includes start but excludes end index.

The substring from index 2 to 7 is 'lloWo'. The length of 'HelloWorld' is 10.

❓ Predict Output

intermediate

Result of toUpperCase and trim methods

What will be printed by this Java program?

Java

public class Main {
    public static void main(String[] args) {
        String s = "  java Programming  ";
        System.out.println(s.trim().toUpperCase());
    }
}

A JAVA PROGRAMMING

B GNIMMARGORP AVAJ

C java programming

D AVA PROGRAMMING

💡 Hint

trim() removes spaces at start and end, toUpperCase() converts all letters to uppercase.

trim() removes leading and trailing spaces, then toUpperCase() converts all letters to uppercase.

❓ Predict Output

advanced

Output of replace and equals methods

What is the output of this Java code snippet?

Java

public class Main {
    public static void main(String[] args) {
        String s1 = "apple";
        String s2 = s1.replace('p', 'b');
        System.out.println(s2.equals("abble"));
    }
}

A true

B false

C Compilation error

D Runtime exception

💡 Hint

replace(char oldChar, char newChar) replaces all occurrences of oldChar with newChar.

Replacing 'p' with 'b' in 'apple' results in 'abble'. equals() compares strings for exact match.

❓ Predict Output

advanced

Output of indexOf and charAt methods

What will this Java program print?

Java

public class Main {
    public static void main(String[] args) {
        String s = "Programming";
        int index = s.indexOf('g');
        char c = s.charAt(index + 1);
        System.out.println(c);
    }
}

A i

B m

C r

D a

💡 Hint

indexOf('g') returns the first position of 'g'. charAt gets character at given index.

The first 'g' is at index 3. charAt(4) is 'r'.

❓ Predict Output

expert

Final value of string after chained methods

What is the final output of this Java code?

Java

public class Main {
    public static void main(String[] args) {
        String s = "  Java123 Programming456  ";
        s = s.trim().replaceAll("\\d", "").toLowerCase();
        System.out.println(s);
    }
}

A javaprogramming

B java123 programming456

C gnimmargorp avaj

D java programming

💡 Hint

trim() removes spaces, replaceAll("\\d", "") removes digits, toLowerCase() converts to lowercase.

After trimming spaces, digits are removed, and all letters become lowercase with spaces preserved.

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