Why does reassigning a String variable in Java not change the original string's value?

String is Immutable: Understanding the Concept

In Java, strings are immutable, meaning their contents cannot be directly modified. However, this concept can be confusing when we observe code like:

String a = "a";
System.out.println("a 1-->" + a);
a = "ty";
System.out.println("a 2-->" + a);

Output:

a 1-->a  
a 2-->ty

What's Actually Happening?

Despite assigning a new value to a, the output shows the original value. This apparent contradiction arises from the immutability of strings. When a new string is assigned to a reference variable, a new string object is created and the reference is updated to point to the new object. The original string remains unmodified.

How Strings Work

To understand this better, let's explore the functionality of the String class:

• Creating a string: String str = "knowledge" creates a string object with the value "knowledge" and assigns a reference str to it.
• Assigning a reference: String s = str creates a new reference s that points to the same string object.
• Modification attempt: str = str.concat(" base") conceptually appends " base" to str, but since strings are immutable, a new string object with the value "knowledge base" is created and the reference str is updated to point to it.

Memory Management and the String Constant Pool

String literals are common in applications and can occupy significant memory. To optimize memory usage, the Java Virtual Machine (JVM) maintains a "String constant pool" that stores all string literals. When a string literal is encountered, the compiler checks the pool for an existing match. If found, the reference is directed to the existing string, avoiding the creation of a new object.

Immutability's Importance

The immutability of strings plays a crucial role in memory management. Since references to strings can point to the same underlying object, modifying the contents of one would affect all references. Therefore, immutability ensures that the original string is preserved, even if its reference is modified.

Finality of String Class

To prevent overrides that could break string immutability, the String class is marked final. This means no other class can inherit and modify its behavior.

In Summary

Strings are immutable in Java. When a reference variable pointing to a string object is reassigned, a new string object is created, effectively updating the reference rather than modifying the existing object. Immutability is essential for memory efficiency and ensuring consistency among references pointing to the same string value.

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