Repeated string concatenation is _much_ more expensive in both time and space

out-0256.txt

Randomizing stuff...
Concatenating...
9 [fast]
956 [slow]

out-0512.txt

Randomizing stuff...
Concatenating...
19 [fast]
4841 [slow]

out-1024.txt

Randomizing stuff...
Concatenating...
51 [fast]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
	at java.util.Arrays.copyOf(Arrays.java:2882)
	at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:100)
	at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:390)
	at java.lang.StringBuilder.append(StringBuilder.java:119)
	at StringConcat.goSlow(StringConcat.java:10)
	at StringConcat.main(StringConcat.java:41)

StringConcat.java

import java.util.Random;

public class StringConcat {
	public static String str;

	public static void goSlow(String[] array) {
		final long start = System.currentTimeMillis();
		String s = "";
		for (int i=0; i<array.length; i++) {
			s += array[i];
		}
		str = s;
		final long end = System.currentTimeMillis();
		System.out.println(end - start + " [slow]");
	}

	public static void goFast(String[] array) {
		final long start = System.currentTimeMillis();
		final StringBuilder sb = new StringBuilder();
		for (int i=0; i<array.length; i++) {
			sb.append(array[i]);
		}
		str = sb.toString();
		final long end = System.currentTimeMillis();
		System.out.println(end - start + " [fast]");
	}

	public static void main(final String[] args) throws Exception {
		System.out.println("Randomizing stuff...");
		final String[] array = new String[1024];
		final Random r = new Random();
		for (int i=0; i<array.length; i++) {
			final byte[] bytes = new byte[10384];
			for (int b=0; b<bytes.length; b++) {
				bytes[b] = (byte) r.nextInt(256);
			}
			array[i] = new String(bytes);
		}
		System.out.println("Concatenating...");
		goFast(array);
		goSlow(array);
	}

}