Java Interview Question: Why String is Immutable in Java (With Examples & Benefits)

 🔹 Introduction

One of the most commonly asked Core Java interview questions is:

👉 “Why is String immutable in Java?”

This question tests your understanding of:

• Memory management
• Security
• Performance

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🔹 What Does Immutable Mean?

An immutable object is one whose state cannot be changed after creation.

👉 In Java:

• Once a String is created, it cannot be modified

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🔹 Example

public class Test {
    public static void main(String[] args) {
        String str = "hello";

        str.concat(" world");

        System.out.println(str);
    }
}

🔹 Output

hello

👉 Because original string is not modified

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🔍 How String is Immutable Internally

• String class is final
• Internal value stored in char array
• No setter methods

👉 Any modification creates a new object

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🔹 Example with New Object

String str = "hello";
str = str.concat(" world");

System.out.println(str);

👉 Output:

hello world

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🧠 String Constant Pool Concept

String a = "java";
String b = "java";

👉 Both refer to same memory location
👉 Saves memory

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🔹 Why String is Immutable?

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🥇 1. Security

• Used in:
  • Database URLs
  • File paths
• Prevents modification

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🥈 2. Thread Safety

• Immutable objects are thread-safe
• No synchronization needed

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🥉 3. Performance (String Pool)

• Reuse existing objects
• Saves memory

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🏅 4. HashMap Key Stability

• Used as keys in HashMap
• Ensures consistent hashCode

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🔹 Time Complexity

• Depends on operation
• String creation → O(n)

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🔹 Key Takeaways

✔ String is immutable
✔ Any change creates new object
✔ Improves security and performance
✔ Frequently asked interview question

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🔹 Conclusion

String immutability is a core concept in Java and plays a crucial role in performance, security, and memory optimization.

🔗 Also Read

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