Java Questions on Strings

 Common interview questions related to String, StringBuffer, and StringBuilder in Java

1. String Questions

1. What is a String in Java?

   • A String is an immutable sequence of characters in Java, represented by the java.lang.String class.

2. Why is String immutable in Java?

   • For security, thread safety, caching (String Pool), and performance optimization.

3. What is the String Pool?

   • A special memory area in the heap where Java stores String literals to optimize memory usage.

4. Difference between String s = "hello" and String s = new String("hello")?

   • String s = "hello" checks the String Pool first; new String("hello") creates a new object in heap memory.

5. How to compare two Strings in Java?

   • Using equals() (content comparison) or == (reference comparison).

6. Why should char[] be preferred over String for passwords?

   • Strings are immutable and stay in memory longer, while char[] can be wiped after use.

7. What are the ways to create an immutable class in Java?

   • Make the class final, fields private and final, and avoid setters.

8. How does String concatenation work in Java?

   • Using + operator (internally uses StringBuilder in loops) or concat() method.

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2. StringBuffer Questions

1. What is StringBuffer?

   • A mutable, thread-safe (synchronized) class for string manipulation.

2. Why is StringBuffer thread-safe?

   • Its methods are synchronized.

3. When should you use StringBuffer instead of StringBuilder?

   • In multithreaded environments where thread safety is required.

4. Difference between String and StringBuffer?

   • String is immutable, StringBuffer is mutable and thread-safe.

5. How does StringBuffer improve performance over String?

   • Avoids creating multiple objects during modifications.

6. What is the default capacity of StringBuffer?

   • 16 characters (if not specified).

7. How does StringBuffer increase its capacity?

   • New capacity = (current_capacity + 1) * 2.

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3. StringBuilder Questions

1. What is StringBuilder?

   • A mutable, non-thread-safe alternative to StringBuffer for faster performance.

2. Why is StringBuilder faster than StringBuffer?

   • Because it is not synchronized.

3. When should you use StringBuilder?

   • In single-threaded environments where performance is critical.

4. Difference between StringBuffer and StringBuilder?

   • StringBuffer is thread-safe (synchronized), StringBuilder is not.

5. Which one is better for concatenation in a loop: String, StringBuffer, or StringBuilder?

   • StringBuilder (due to mutability and no synchronization overhead).

6. Is StringBuilder more memory-efficient than String?

   • Yes, because it avoids creating intermediate String objects.

7. Can we convert StringBuilder to String?

   • Yes, using toString() method.

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Comparison Questions

1. String vs StringBuffer vs StringBuilder?

   • String: Immutable, thread-safe (due to immutability).

   • StringBuffer: Mutable, thread-safe (synchronized).

   • StringBuilder: Mutable, not thread-safe, faster.

2. Which is the best choice among String, StringBuffer, and StringBuilder?

   • Depends on use case:

     • Use String for constants.

     • Use StringBuffer for multithreaded scenarios.

     • Use StringBuilder for single-threaded performance-critical cases.

3. How does Java handle String concatenation with + operator?

   • Uses StringBuilder internally in loops (since Java 5).

4. Can we use StringBuffer or StringBuilder as keys in HashMap?

   • Not recommended because they are mutable, and hash codes may change.

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Code-Based Questions

1. Predict the output:

   java

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   String s1 = "Java";
   String s2 = new String("Java");
   System.out.println(s1 == s2); // false (different references)

2. What happens here?

   java

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   String str = "Hello";
   str.concat(" World");
   System.out.println(str); // "Hello" (Strings are immutable)

3. Efficient way to reverse a String?

   java

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   new StringBuilder(str).reverse().toString();

4. How many objects are created here?

   java

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   String s1 = "A";
   String s2 = "A";
   String s3 = new String("A");

   • Answer: 2 (one in String Pool, one in heap via new).

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Advanced Questions

1. How does intern() method work?

   • Returns a canonical representation of the String from the String Pool.

2. Why is String popular as a HashMap key?

   • Because it's immutable, ensuring hashcode consistency.

3. Impact of using + in a loop vs StringBuilder?

   • + in a loop creates multiple intermediate objects, while StringBuilder is efficient.

4. Can we inherit String class?

   • No, because it's final.