String in Java

Short Answer:
String in Java is an immutable sequence of characters stored in the String pool.

String is a final class in java.lang package. It is widely used in Java applications.

1️⃣ Why String is Immutable?

Short: For security, caching, and thread safety.

• Prevents modification after creation.
• Used in String pool.
• Safe for multithreading.
• Used in HashMap keys.

2️⃣ String Creation

Method 1: String Literal (Stored in Pool)

String s1 = "Java";
String s2 = "Java";

Both reference same object from String Pool.

Method 2: Using new Keyword

String s3 = new String("Java");

Creates object in heap memory.

3️⃣ String Pool Explained

Short: Special memory area inside heap for string literals.

• Avoids duplicate objects.
• Improves memory efficiency.
• intern() method forces string into pool.

4️⃣ String vs StringBuilder vs StringBuffer

5️⃣ Important String Methods

• equals()
• length()
• substring()
• contains()
• replace()
• split()
• trim()
• toUpperCase()

Frequently Asked Interview Questions (With Answers)

1️⃣ Difference between == and equals() in String?

Short: == compares reference; equals() compares content.

String a = new String("Java");
String b = new String("Java");

System.out.println(a == b);      // false
System.out.println(a.equals(b)); // true

2️⃣ What is intern() method?

Short: Moves string to String Pool.

String s1 = new String("Java");
String s2 = s1.intern();

3️⃣ Why String is final class?

Answer: To prevent inheritance and ensure immutability.

4️⃣ What happens when we concatenate strings?

Short: New object created.

String s = "Java";
s = s + " Spring";

Creates new object; old one remains unchanged.

5️⃣ Why String is used as HashMap key?

Answer: Because it is immutable and hashCode remains constant.

6️⃣ Difference between StringBuilder and StringBuffer?

Answer: StringBuffer is synchronized; StringBuilder is not.

7️⃣ How String stored internally?

Answer: Stored as char array (before Java 9) and byte array (Java 9+).

8️⃣ What is String constant pool?

Answer: Special heap memory area for string literals.

9️⃣ Is String thread-safe?

Answer: Yes, because it is immutable.

🔟 How to compare two strings ignoring case?

s1.equalsIgnoreCase(s2);