ちゃんと5回呼ばれるやつ(シングルクオートをバッククオートに修正)

HtmlEncoderTest.java

import java.util.HashMap;
import java.util.Map;

public class HtmlEncoderTest {
	static public void main(String[] args) {
		System.out.println(new HtmlEncoder().encode("<script>alert('注意!');</script>"));
		System.out.println(HtmlEncoder.count);
	}
}

abstract class Encoder {
	private Map<Character, String> conversionTable =
			new HashMap<Character, String>();

	protected void addConversion(char c, String s) {
		conversionTable.put(c, s);
	}

	protected void addNoConversion(char c) {
		conversionTable.put(c, String.valueOf(c));
	}

	protected void addNoConversion (char[] collection) {
		for (char c : collection) {
			addNoConversion(c);
		}
	}

	abstract protected String encode(char c);

	public String encode(String s) {
		if (s == null) {
			return null;
		}

		String result = "";
		for (char c  : s.toCharArray()) {
			String t =  conversionTable.get(c);
			if (t == null) {
				t = encode(c);
			}
			result += t;
		}
		return result;
	}
}

class HtmlEncoder extends Encoder {
	private static String ALPHAS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	private static String NUMERICS = "Ø123456789";
	private static String PUNCTUATIONS =
			"!#$%()*+,-.:;=?@[\\]^_`{|}~";

	public HtmlEncoder() {
		addNoConversion(ALPHAS.toCharArray());
		addNoConversion(ALPHAS.toLowerCase().toCharArray());
		addNoConversion(NUMERICS.toCharArray());
		addNoConversion(PUNCTUATIONS.toCharArray());
		addConversion('<', "&lt;");
		addConversion('>', "&gt;");
		addConversion('&', "&amp;");
		addConversion('"', "&quot;");
	}

	public static int count = 0;
	protected String encode(char c) {
		count++;
		return "&#" + (int) c + ";";
	}
}

実行結果載せるとバレバレなので割愛 :)