Learning Java, 4th Edition

Strings

We’ll start by taking a closer look at the Java String class (or, more specifically, java.lang.String). Because working with Strings is so fundamental, it’s important to understand how they are implemented and what you can do with them. A String object encapsulates a sequence of Unicode characters. Internally, these characters are stored in a regular Java array, but the String object guards this array jealously and gives you access to it only through its own API. This is to support the idea that Strings are immutable; once you create a String object, you can’t change its value. Lots of operations on a String object appear to change the characters or length of a string, but what they really do is return a new String object that copies or internally references the needed characters of the original. Java implementations make an effort to consolidate identical strings used in the same class into a shared-string pool and to share parts of Strings where possible.

The original motivation for all of this was performance. Immutable Strings can save memory and be optimized for speed by the Java VM. The flip side is that a programmer should have a basic understanding of the String class in order to avoid creating an excessive number of String objects in places where performance is an issue. That was especially true in the past, when VMs were slow and handled memory poorly. Nowadays, string usage is not usually an issue in the overall performance of a real application.^[29]

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