Why can't the value of type String in Java be changed?

The value of the String type in Java is immutable because:

• Efficient use of the String Pool: JVM maintains a pool of strings to save memory. Immutable strings allow multiple variables to reference the same object in the pool without risk, as changing one variable won't affect others.
• Thread safety: Since string objects are immutable, they can be safely shared between threads without external synchronization. This simplifies multithreaded programming.
• Use as keys in collections: String is often used as a key in collections like HashMap and HashSet. Immutability guarantees that the object's hash code remains constant throughout its lifetime, which is critical for the correct operation of these data structures. Changing an object used as a key would break their internal structure.
• Security: String objects are used to represent important data such as filenames, network addresses, and passwords. Immutability prevents accidental or malicious modification of these values after creation.

When it seems that a string value is "changing" (for example, during string concatenation), a new String object is actually created.

// Original string in the pool
String s1 = "Hello";

// A new object "Hello World" is created, s1 still references "Hello"
String s2 = s1 + " World";

For operations requiring frequent string modifications, mutable classes like StringBuilder (not thread-safe) or StringBuffer (thread-safe) are used.