The string
concatenation operator (+) is a convenient way to combine
a few strings into one. It is fine for generating a single line of output or
for constructing the string representation of a small, fixed-size object, but
it does not scale. Using the string concatenation operator repeatedly to concatenate n strings requires
time quadratic in n. It is an
unfortunate consequence of the fact that strings are immutable (Item 15).
When two strings are concatenated, the contents of both are copied.
For example,
consider the following method that constructs a string representation of a
billing statement by repeatedly concatenating a line for each item:
// Inappropriate use of string concatenation -
Performs horribly!
public String statement() {
String result = "";
for (int i = 0; i < numItems(); i++)
result += lineForItem(i); // String concatenation
return result;
}
This method
performs abysmally if the number of items is large. To achieve
acceptable performance, use a StringBuilder
in
place of a String to store the
statement under construction. (The StringBuilder
class,
added in release 1.5, is an unsynchronized replacement for StringBuffer, which is now obsolete.)
public String statement() {
StringBuilder b = new StringBuilder(numItems() *
LINE_WIDTH);
for (int i = 0; i < numItems(); i++)
b.append(lineForItem(i));
return b.toString();
}
The difference
in performance is dramatic. If numItems returns 100
and lineForItem returns a
constant 80-character string, the second method is eightyfive times faster than
the first on my machine. Because the first method is quadratic in the number of
items and the second is linear, the performance difference is even more
dramatic for larger numbers of items. Note that the second method preallocates
a StringBuilder large enough
to hold the result. Even if it is detuned to use a default-sized StringBuilder, it is still fifty times faster.
The moral is
simple: don’t use the string concatenation operator to combine more than a few
strings unless performance is irrelevant. Use StringBuilder’s append method instead. Alternatively, use a
character array, or process the strings one at a time instead of combining
them.
Reference: Effective Java 2nd Edition by Joshua Bloch
