Why are strings considered immutable in Java?

Strings are considered immutable in Java for several important reasons, all contributing to various aspects of performance, security, and efficiency.

When a string is immutable, it means that once a string object is created, its value cannot be changed. If any modification is required, a new string object is created. This immutability conserves memory because it allows Java to optimize memory use: multiple references can point to the same string if they contain the same value, reducing redundancy.

From a security perspective, immutability plays a crucial role. Since strings are widely used in many applications, including those involving sensitive data such as passwords or network communication details, having immutable string objects helps prevent unintended changes. For instance, if a string reference is passed to a method, the method cannot alter the original string, which helps maintain the integrity of critical data.

Furthermore, immutability enhances performance in data structures like hashmaps. When a string is used as a key in a hashmap, its hash code is computed based on its content. If strings were mutable and the content changed after being inserted as a key, it would disrupt the mapping and lead to data retrieval issues. Immutable strings ensure that the hash code remains consistent throughout the usage of the key.

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