Beware the performance of string concatenation

performance of string concatenation"/>

Beware the performance of string concatenation

运行以下程序，看看两者的区别：

public class Test {String teststr = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";public static void main(String[] args) {new Test().go();}public void go() {long start = System.currentTimeMillis();statement0();long end = System.currentTimeMillis();System.out.println("UsedTime0 = " + (end - start));start = System.currentTimeMillis();statement1();end = System.currentTimeMillis();System.out.println("UsedTime1 = " + (end - start));}// Inappropriate use of string concatenation - Performs horribly!public String statement0() {String result = "";for (int i = 0; i < 100; i++)result += teststr; // String concatenationreturn result;}public String statement1() {StringBuilder b = new StringBuilder(1000 * teststr.length());for (int i = 0; i < 1000; i++)b.append(teststr);return b.toString();}
}

在我机器上结果是：

UsedTime0 = 356
UsedTime1 = 1

字符串连接之所以这么慢是由于字符串是不可变的，每次连接都要复制两者的内容

Using the string concatenation operator repeatedly to concatenate n strings requires time quadratic in n.

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.