Why Java Strings Are Immutable ? – Reasons And Benefits

Hello again, students! In our previous lesson, we learned about the String class in Java. We noticed the concept of immutable string of Java, which might seem unusual at first. In this article, we will delve into the reasons behind this design choice, and discuss its implications.

What Does Immutable String In Java Mean?

When we say a String is immutable, we mean that once a String object is created, its data or state can’t be changed. But you might be thinking, “Wait, I’ve changed string values before!” Here’s where the subtlety lies: when you do something like:

String message = "Hello, ";
message += "world!";

It seems like message changed from “Hello, ” to “Hello, world!”. But in reality, Java created a new String object with the value “Hello, world!” and made message refer to this new object. The original “Hello, ” string still exists in memory until it’s eventually garbage collected.

Why Strings Are Immutable In Java?

There are several reasons why Strings were designed to be immutable in Java.

1. Security: Strings are often used to store sensitive data such as usernames, passwords, and connection URLs. The immutable nature of Strings provides a certain level of security because these values cannot be changed.

2. Thread-Safety: Since Strings can’t be modified, they are inherently thread-safe and can be safely used in a multi-threaded environment without providing explicit synchronization.

3. Hashcode Caching: The String class caches its hashcode after it’s computed for the first time. Since the string can’t change, the hashcode also won’t change. This makes string objects efficient to use as keys in hash-based collections such as HashMap and HashSet.

4. Class Loading Mechanism: Strings are extensively used in the class loading mechanism which requires that String objects be reliably constant and not changeable.

An Example of String Immutability

Let’s consider a simple code snippet:

String greeting = "Hello";
greeting = greeting + ", world!";

When the first line executes, a String object with the value “Hello” is created in memory. Then, greeting is made to point to this object.

When the second line executes, a new String object is created with the value “Hello, world!”. The greeting reference is then updated to point to this new object. The original “Hello” string is still in memory, unchanged.

Here’s a visualization:

Memory: ["Hello"]
greeting ----> "Hello"

Memory: ["Hello", "Hello, world!"]
greeting ------------------> "Hello, world!"


String immutability may seem peculiar, but it is fundamental to Java’s design and promotes security, efficiency, and thread-safety. This does, however, have implications on how we handle strings, especially when performing lots of string manipulations, as each operation creates a new string object. In such cases, other classes such as StringBuilder or StringBuffer can be used, which we’ll explore in upcoming lessons.

Stay tuned for our next lesson where we’ll delve into these alternatives and learn when and how to use them effectively. Happy coding!.

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