Java Performance Tuning, 2nd Edition

Chapter 5. Strings

Everyone has a logger and most of them are string pigs.

—Kirk Pepperdine

String s have a special status in Java. They are the only objects with:

• Their own operators (+ and +=)

• A literal form (characters surrounded by double quotes, e.g., "hello“)

• Their own externally accessible collection in the VM and class files (i.e., string pools, which provide uniqueness of String objects if the string sequence can be determined at compile time)

Strings are immutable and have a special relationship with StringBuffer objects. A String cannot be altered once created. Applying a method that looks like it changes the String (such as String.trim( )) doesn’t actually do so; instead, it returns an altered copy of the String. Strings are also final, and so cannot be subclassed. These points have advantages and disadvantages so far as performance is concerned. For fast string manipulation, the inability to subclass String or access the internal char array can be a serious problem.

The Performance Effects of Strings

Let’s first look at the advantages of the String implementation:

• Compilation creates unique strings. At compile time, strings are resolved as far as possible. This includes applying the concatenation operator and converting other literals to strings. So "hi7" and ("hi"+7) both get resolved at compile time to the same string, and are identical objects in the class string pool (see Section 3.9.1.2). Compilers differ in their ability to achieve this resolution. You can always ...