String vs StringBuilder in Java - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of String concatenation in loop

What is the output of the following Java code?

Java

public class Test {
    public static void main(String[] args) {
        String s = "";
        for (int i = 0; i < 3; i++) {
            s += i;
        }
        System.out.println(s);
    }
}

A null012

B 0 1 2

C 0123

D 012

💡 Hint

Remember that String concatenation with += creates new String objects each time.

The loop appends the numbers 0, 1, and 2 to the empty string. So the output is "012".

❓ Predict Output

intermediate

Output of StringBuilder append in loop

What is the output of this Java code using StringBuilder?

Java

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 3; i++) {
            sb.append(i);
        }
        System.out.println(sb.toString());
    }
}

A 0 1 2

B 012

C null012

D 0123

💡 Hint

StringBuilder appends efficiently without creating new objects each time.

The StringBuilder appends 0, 1, and 2, resulting in "012".

❓ Predict Output

advanced

Performance difference in string concatenation

Consider these two code snippets. Which one is faster for large loops and why?

1) Using String with += in a loop

2) Using StringBuilder with append in a loop

A StringBuilder is faster because it modifies the same object without creating new ones.

B String with += is faster because it is simpler to write.

C Both have the same speed because Java optimizes string concatenation automatically.

D String with += is faster because it uses less memory.

💡 Hint

Think about how many objects are created in each approach.

StringBuilder is faster because it changes the same object, avoiding many temporary String creations that happen with +=.

❓ Predict Output

advanced

Output of StringBuilder reverse method

What is the output of this Java code?

Java

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("hello");
        sb.reverse();
        System.out.println(sb.toString());
    }
}

A olleh

B hello

C null

D HELLO

💡 Hint

StringBuilder has a method to reverse the characters in the string.

The reverse() method reverses the characters, so "hello" becomes "olleh".

🧠 Conceptual

expert

Immutability and thread safety of String vs StringBuilder

Which statement correctly describes the difference between String and StringBuilder in Java regarding immutability and thread safety?

A Both String and StringBuilder are immutable and thread-safe.

B String is mutable and thread-safe; StringBuilder is immutable and not thread-safe.

C String is immutable and thread-safe; StringBuilder is mutable and not thread-safe.

D Both String and StringBuilder are mutable and not thread-safe.

💡 Hint

Think about which class can be changed after creation and which can be safely used by multiple threads.

String objects cannot be changed after creation, making them thread-safe. StringBuilder objects can be changed but are not thread-safe.

Practice

1. Which statement best describes when to use StringBuilder instead of String in Java?

easy

A. Use StringBuilder when you need to change text many times efficiently.

B. Use StringBuilder for fixed text that never changes.

C. Use String when you want faster text modifications.

D. Use String to save memory when changing text repeatedly.

Solution

1. Step 1: Understand the nature of String and StringBuilder

   String objects are immutable, meaning they cannot be changed once created. Changing text creates new objects, which is slow and uses more memory.

2. Step 2: Identify when StringBuilder is better

   StringBuilder allows changing text without creating new objects, making it efficient for many modifications.

3. Final Answer:

   Use StringBuilder when you need to change text many times efficiently. -> Option A

4. Quick Check:

   Many changes = StringBuilder [OK]

Hint: Use StringBuilder for many text changes, String for fixed text [OK]

Common Mistakes:

• Thinking String is better for many changes
• Confusing immutability of String
• Assuming StringBuilder uses more memory always

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

easy

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

B. StringBuilder sb = "Hello";

C. StringBuilder sb = new StringBuilder[] {"Hello"};

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

Solution

1. Step 1: Recall the correct constructor syntax for StringBuilder

   The StringBuilder class requires the new keyword and parentheses with the initial string inside.

2. Step 2: Check each option for syntax correctness

   StringBuilder sb = new StringBuilder("Hello"); uses new StringBuilder("Hello"), which is correct. Options A, B, and D have syntax errors or wrong usage.

3. Final Answer:

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

4. Quick Check:

   Use new + constructor for StringBuilder [OK]

Hint: Always use 'new StringBuilder("text")' to create StringBuilder [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using assignment without constructor
• Using array syntax mistakenly

3. What is the output of this Java code?

StringBuilder sb = new StringBuilder("Hi");
sb.append("!");
sb.append(" How are you?");
System.out.println(sb.toString());

medium

A. Hi!How are you?

B. Hi How are you?

C. Hi! How are you?

D. Hi

Solution

1. Step 1: Understand what append() does on StringBuilder

   Each append() adds the given text to the end of the current StringBuilder content without spaces unless explicitly added.

2. Step 2: Trace the code execution

   Initial text is "Hi". First append adds "!" making "Hi!". Second append adds " How are you?" (with leading space), resulting in "Hi! How are you?".

3. Final Answer:

   Hi! How are you? -> Option C

4. Quick Check:

   Appends add text exactly as given [OK]

Hint: Append adds text exactly; watch spaces carefully [OK]

Common Mistakes:

• Ignoring spaces in appended strings
• Assuming append replaces text
• Printing StringBuilder object directly without toString()

4. Identify the error in this code snippet:

StringBuilder sb = new StringBuilder("Test");
sb = sb + "ing";
System.out.println(sb);

medium

A. Cannot use + operator to concatenate StringBuilder and String.

B. Missing semicolon after StringBuilder initialization.

C. StringBuilder cannot be printed directly with println.

D. StringBuilder must be declared final.

Solution

1. Step 1: Check the use of + operator with StringBuilder

   The + operator cannot be used to add a String to a StringBuilder object directly. This causes a compile-time error.

2. Step 2: Understand correct way to append text to StringBuilder

   Use sb.append("ing") to add text instead of + operator.

3. Final Answer:

   Cannot use + operator to concatenate StringBuilder and String. -> Option A

4. Quick Check:

   Use append(), not +, with StringBuilder [OK]

Hint: Use append() to add text to StringBuilder, not + [OK]

Common Mistakes:

• Trying to use + with StringBuilder
• Thinking println can't print StringBuilder
• Forgetting semicolons

5. You want to build a sentence by adding words one by one in a loop. Which approach is best for performance and why?

Option A:
String sentence = "";
for (String word : words) {
    sentence += word + " ";
}

Option B:
StringBuilder sentence = new StringBuilder();
for (String word : words) {
    sentence.append(word).append(" ");
}

hard

A. Option A is better because String is simpler to use.

B. Option B is better because StringBuilder avoids creating many new objects.

C. Both are equally efficient for building strings in loops.

D. Option A is better because it uses less memory.

Solution

1. Step 1: Analyze Option B is better because String is simpler to use. (String concatenation in loop)

   Each concatenation with += creates a new String object because Strings are immutable, causing slow performance and more memory use.

2. Step 2: Analyze Option B is better because StringBuilder avoids creating many new objects. (StringBuilder in loop)

   StringBuilder modifies the same object without creating new ones, making it faster and more memory efficient for repeated changes.

3. Final Answer:

   Option B is better because StringBuilder avoids creating many new objects. -> Option B

4. Quick Check:

   Use StringBuilder for repeated string changes in loops [OK]

Hint: Use StringBuilder in loops to build strings efficiently [OK]

Common Mistakes:

• Using String concatenation in loops thinking it's fast
• Ignoring immutability of String
• Assuming simpler code is always better