A String object is called immutable (read-only), because its value cannot be modified after it has been created. Methods that appear to modify a String object actually return a new String object that contains the modification.
Because strings are immutable, string manipulation routines that perform repeated additions or deletions to what appears to be a single string
can exact a significant performance penalty.
You can use the StringBuilder class instead of the String class
for operations that make multiple changes to the value of a string. Unlike instances of the String class, StringBuilder objects are mutable; when you concatenate,
append, or delete substrings from a string, the operations are performed on a single string.
A StringBuilder object maintains a buffer
to accommodate expansions to the string. New data is appended to the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, and the new data is then appended to the new buffer.
Although the StringBuilder class generally
offers better performance than the String class, you should not automatically replace String with StringBuilder whenever you want to manipulate
strings. Performance depends on the size of the string, the amount of memory to be allocated for the new string, the system on which your app is executing, and the type of operation. You should be prepared to test your app to determine whether StringBuilder actually
offers a significant performance improvement.
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