Beware the performance of string concatenation

次のプログラムを実行して、両者の違いを確認します.

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 concatenation
		return result;
	}
	
	public String statement1() {
		StringBuilder b = new StringBuilder(1000 * teststr.length());
		for (int i = 0; i < 1000; i++)
			b.append(teststr);
		return b.toString();
	}
}

 
私の機械では
UsedTime0 = 356 UsedTime1 = 1
 
文字列の接続がこんなに遅いのは、文字列が可変ではないためで、接続するたびに両方の内容をコピーします.
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.