Why String is Immutable or Final in Java?

The term "Mutable" signifies the ability of an object to be modified or altered, while "Immutable" denotes the inability of an object to undergo changes. An Immutable Object, therefore, refers to an object whose state remains unaltered after its creation.

When it comes to Strings in Java, they are immutable, signifying that the content of a String object cannot be modified once it has been instantiated. However, it is important to note that while the String object itself cannot be changed, the reference variable pointing to the String object can be reassigned to a different String object.

To clarify further, changing an object generally entails utilizing its methods to modify its internal fields, or if the fields are public (and not declared as final), they can be updated from external sources without requiring access through methods. However, in the case of Strings, once an object is created, its data or state cannot be modified. Instead, if you attempt to make changes to a String object, a new String object is created with the updated value.

It is crucial to emphasize that although the String object remains immutable, the reference variable that points to the String object is mutable. This implies that you can assign the reference variable to a different String object, effectively altering the reference to point to a new String instance.

String str1="Hello"; System.out.println("str1 is "+a); String str2 = str1; str1 ="World!!"; System.out.println("str1 after modify "+a); System.out.println("str2 is "+b);
str1 is Hello str1 after modify World!! str2 is Hello

Here we see the difference between mutating an object, and changing a reference. str2 still points to the same object as we initially set str1 to point to. Setting str1 to "World!!" only changes the reference, while the String object it originally referred to remains unchanged.


The immutability of strings holds significant advantages in multithreaded applications, primarily due to the inherent thread safety it provides. Immutable types, including String, offer automatic thread safety since their state cannot be modified once created. This characteristic eliminates the possibility of concurrent threads modifying the same string instance and avoids data corruption or inconsistent behavior.

In multithreaded environments, where multiple threads may concurrently access and manipulate shared resources, immutability becomes crucial. By using immutable strings, developers can ensure that string objects remain unchanged and maintain their integrity throughout concurrent operations. This helps prevent race conditions, synchronization issues, and other concurrency-related problems.

Furthermore, the immutability of String has broader implications beyond thread safety. Strings are commonly used as parameters in various Java classes, such as network connections or file operations. If String were mutable, it would pose a serious security threat. For instance, if a network connection or file path could be modified, it would introduce vulnerabilities, allowing unauthorized access, data tampering, or other security breaches.

The immutability of String ensures the stability and security of such critical operations. Once a String object is created and passed as a parameter, its value remains unchanged, guaranteeing that the intended network connection or file path remains intact and unaffected by potential malicious attempts to modify it.


While it is true that immutable types in Java create new instances each time they are modified, resulting in additional resource consumption, it is important to consider the trade-offs and context in which immutability is utilized.

The creation of new instances when modifying immutable objects does incur a certain overhead in terms of memory usage and performance. Each modification involves allocating memory for the new instance and copying the existing data, which can be less efficient compared to in-place modifications possible with mutable objects.

However, the drawbacks of increased resource usage need to be weighed against the benefits that immutability brings. Immutable objects provide several advantages such as thread safety, simplified code reasoning, improved concurrency control, and enhanced security.

In many scenarios, the benefits of immutability outweigh the resource overhead. For example, in multithreaded environments, the thread safety provided by immutable objects eliminates the need for complex synchronization mechanisms, reducing the likelihood of bugs and concurrency issues.

Additionally, immutability simplifies code reasoning and debugging since the state of an object does not change unexpectedly. It enables more predictable behavior, facilitates reasoning about program correctness, and can lead to more maintainable and robust code.

Furthermore, modern JVM optimizations and memory management techniques help mitigate some of the resource usage concerns. JVMs employ various strategies such as string interning, escape analysis, and object pooling to optimize memory utilization and reduce the impact of object creation.

It's worth noting that not all objects need to be immutable. Developers need to carefully consider the requirements of their specific use cases and balance the benefits of immutability against the resource consumption trade-off. In situations where mutability is necessary, mutable objects can be used sensibly, while striving for immutability in critical areas where it offers substantial advantages.

String str=null; for(int i=0;i < =100;i++){ str+="Add this"; }


The above code segment creates 1000 new string variables. In these type of situations you should use the Java StringBuilder class, which allows you to modify the text without making new String class instances. Java StringBuilder is a Mutable type, that means when you alter it , it still holding same data

public class TestClass{ public static void main(String[] args) { StringBuilder sBuilder = new StringBuilder(); for (int i = 0; i < 1000; i++) { sBuilder.append("Add This"); } String str = sBuilder.toString(); } }


String is immutable in Java to ensure thread safety, simplify code reasoning, enhance security, and facilitate predictable behavior in critical operations such as network connections and file paths.