StringBuilder and StringBuffer in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of StringBuilder append operations

What is the output of the following Java code snippet?

Java

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");
sb.append('!');
System.out.println(sb.toString());

A !dlroW olleH

B Hello World

C HelloWorld!

D Hello World!

💡 Hint

Remember that append adds to the end of the existing string.

The StringBuilder starts with "Hello". Then " World" is appended, followed by '!'. The final string is "Hello World!".

❓ Predict Output

intermediate

Thread safety difference between StringBuilder and StringBuffer

What will happen if multiple threads modify a StringBuffer and a StringBuilder concurrently without synchronization?

A StringBuffer operations are thread-safe; StringBuilder operations may cause data corruption.

B Neither StringBuffer nor StringBuilder is thread-safe.

C Both StringBuffer and StringBuilder are thread-safe.

D StringBuilder is thread-safe; StringBuffer is not.

💡 Hint

Consider which class was designed for synchronization.

StringBuffer methods are synchronized, making it thread-safe. StringBuilder is not synchronized and can cause data corruption if used by multiple threads concurrently.

🔧 Debug

advanced

Identify the error in StringBuilder usage

What error will the following code produce?

Java

StringBuilder sb = null;
sb.append("Test");

A NullPointerException

B StringIndexOutOfBoundsException

C No error, prints 'Test'

D Compilation error

💡 Hint

Check if the StringBuilder object is initialized before use.

The StringBuilder reference is null, so calling append on it causes a NullPointerException at runtime.

📝 Syntax

advanced

Correct way to convert StringBuilder to String

Which option correctly converts a StringBuilder object to a String?

Java

StringBuilder sb = new StringBuilder("example");

A String s = (String) sb;

B String s = sb.convert();

C String s = sb.toString();

D String s = sb.stringValue();

💡 Hint

Look for the standard method to get a String from StringBuilder.

The toString() method returns the string representation of the StringBuilder content. Other methods do not exist or cause errors.

🚀 Application

expert

Predict the final content after multiple StringBuffer operations

Given the following code, what is the final output?

Java

StringBuffer sb = new StringBuffer("Start");
sb.insert(5, " Middle");
sb.replace(0, 5, "Begin");
sb.delete(6, 13);
System.out.println(sb.toString());

A nigeB

B Begin

C Begin Middle

D Start Middle

💡 Hint

Follow each operation step-by-step on the string.

Initial: "Start" After insert(5, " Middle"): "Start Middle" After replace(0,5,"Begin"): "Begin Middle" After delete(6,13): deletes from index 6 to 12 (7 chars), removing "Middle" Result: "Begin " (with trailing space)

Practice

1.

Which of the following statements about StringBuilder and StringBuffer is true?

easy

A. StringBuffer is faster and not thread-safe, while StringBuilder is thread-safe but slower.

B. StringBuilder is faster but not thread-safe, while StringBuffer is thread-safe but slower.

C. Both StringBuilder and StringBuffer are thread-safe and have the same speed.

D. StringBuilder and StringBuffer cannot be used to modify strings.

Solution

1. Step 1: Understand thread safety difference

   StringBuffer is synchronized, making it thread-safe but slower.

2. Step 2: Understand performance difference

   StringBuilder is not synchronized, so it is faster but not thread-safe.

3. Final Answer:

   StringBuilder is faster but not thread-safe, while StringBuffer is thread-safe but slower. -> Option B

4. Quick Check:

   Thread safety and speed = A [OK]

Hint: Remember: Buffer is safe but slower; Builder is fast but unsafe [OK]

Common Mistakes:

• Confusing which class is thread-safe
• Thinking both have same speed
• Believing they cannot modify strings

2.

Which of the following is the correct way to create a StringBuilder object with initial content "Hello"?

easy

A. StringBuilder sb = new StringBuffer("Hello");

B. StringBuilder sb = StringBuilder("Hello");

C. StringBuilder sb = new StringBuilder(); sb.append("Hello");

D. StringBuilder sb = new StringBuilder("Hello");

Solution

1. Step 1: Check constructor syntax

   The correct way to create a StringBuilder with initial content is using new StringBuilder("Hello").

2. Step 2: Verify other options

   StringBuilder sb = StringBuilder("Hello"); misses new, StringBuilder sb = new StringBuilder(); sb.append("Hello"); is valid but not direct initialization, StringBuilder sb = new StringBuffer("Hello"); uses wrong class.

3. Final Answer:

   StringBuilder sb = new StringBuilder("Hello"); -> Option D

4. Quick Check:

   Correct constructor syntax = D [OK]

Hint: Use 'new' keyword and constructor with string inside parentheses [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using StringBuffer instead of StringBuilder
• Trying to assign without constructor

3.

What is the output of the following code?

StringBuilder sb = new StringBuilder("Java");
sb.append(" Programming");
sb.replace(0, 4, "Python");
System.out.println(sb.toString());

medium

A. PythonJava Programming

B. Java Programming

C. Python Programming

D. ProgrammingPython

Solution

1. Step 1: Append " Programming" to "Java"

   After append, string becomes "Java Programming".

2. Step 2: Replace characters from index 0 to 4 with "Python"

   Characters "Java" (index 0 to 4) are replaced by "Python", resulting in "Python Programming".

3. Final Answer:

   Python Programming -> Option C

4. Quick Check:

   Append + replace result = A [OK]

Hint: Replace changes substring; append adds at end [OK]

Common Mistakes:

• Confusing replace indices
• Thinking append replaces text
• Ignoring toString() output

4.

Identify the error in the following code snippet:

StringBuffer sb = new StringBuffer("Test");
sb.insert(5, "ing");
System.out.println(sb);

medium

A. IndexOutOfBoundsException because insert index is invalid

B. NullPointerException because sb is not initialized

C. Compilation error due to wrong method name

D. No error; output is "Testing"

Solution

1. Step 1: Check insert index validity

   StringBuffer length is 4 ("Test"), but insert index is 5, which is out of bounds.

2. Step 2: Understand exception thrown

   Inserting at index greater than length throws IndexOutOfBoundsException.

3. Final Answer:

   IndexOutOfBoundsException because insert index is invalid -> Option A

4. Quick Check:

   Invalid insert index = B [OK]

Hint: Insert index must be between 0 and length inclusive [OK]

Common Mistakes:

• Assuming insert auto-corrects index
• Confusing insert with append
• Expecting no error on invalid index

5.

You want to build a thread-safe string that appends numbers 1 to 3 separated by commas, resulting in "1,2,3". Which code snippet correctly uses StringBuffer to achieve this?

1. StringBuffer sb = new StringBuffer();
   for (int i = 1; i <= 3; i++) {
       sb.append(i);
       if (i != 3) sb.append(",");
   }
   System.out.println(sb.toString());

2. StringBuffer sb = new StringBuffer();
   for (int i = 1; i <= 3; i++) {
       sb.append(i + ",");
   }
   System.out.println(sb.toString());

3. StringBuffer sb = new StringBuffer();
   sb.append("1,2,3,");
   System.out.println(sb.toString());

4. StringBuffer sb = new StringBuffer();
   for (int i = 1; i < 3; i++) {
       sb.append(i);
       sb.append(",");
   }
   System.out.println(sb.toString());

hard

A. Code snippet 1

B. Code snippet 2

C. Code snippet 3

D. Code snippet 4

Solution

1. Step 1: Analyze snippet 1

   Appends numbers 1 to 3 with commas except after last number, producing "1,2,3" correctly.

2. Step 2: Check other snippets

   Snippet 2 adds extra comma at end; snippet 3 hardcodes string but adds extra comma; snippet 4 misses last number 3.

3. Final Answer:

   Code snippet 1 -> Option A

4. Quick Check:

   Correct comma placement in loop = C [OK]

Hint: Add comma only if not last element in loop [OK]

Common Mistakes:

• Adding extra comma at end
• Missing last number in loop
• Hardcoding string without loop