1) 의미
목적상 변경이 이루어질때 기존 코드를 수정하지 않고
통역 역할을 하는 프로그램을 만들어 실행하는 패턴입니다.
2. 소스
1) Node 추상 클래스 작성
public abstract class Node {
public abstract void parse(Context context) throws ParseException;
}
이를 상속하여 각 문법 하나하나에 대응하는
객체들을 생성한다.
// <program> ::= program <command list>
public class ProgramNode extends Node {
private Node commandListNode;
public void parse(Context context) throws ParseException {
context.skipToken("program");
commandListNode = new CommandListNode();
commandListNode.parse(context);
}
public String toString() {
return "[program " + commandListNode + "]";
}
}
import java.util.Vector;
// <command list> ::= <command>* end
public class CommandListNode extends Node {
private Vector list = new Vector();
public void parse(Context context) throws ParseException {
while (true) {
if (context.currentToken() == null) {
throw new ParseException("Missing 'end'");
} else if (context.currentToken().equals("end")) {
context.skipToken("end");
break;
} else {
Node commandNode = new CommandNode();
commandNode.parse(context);
list.add(commandNode);
}
}
}
public String toString() {
return "" + list;
}
}
// <command list> ::= <command>* end
public class CommandListNode extends Node {
private Vector list = new Vector();
public void parse(Context context) throws ParseException {
while (true) {
if (context.currentToken() == null) {
throw new ParseException("Missing 'end'");
} else if (context.currentToken().equals("end")) {
context.skipToken("end");
break;
} else {
Node commandNode = new CommandNode();
commandNode.parse(context);
list.add(commandNode);
}
}
}
public String toString() {
return "" + list;
}
}
// <command> ::= <repeat command> | <primitive command>
public class CommandNode extends Node {
private Node node;
public void parse(Context context) throws ParseException {
if (context.currentToken().equals("repeat")) {
node = new RepeatCommandNode();
node.parse(context);
} else {
node = new PrimitiveCommandNode();
node.parse(context);
}
}
public String toString() {
return node.toString();
}
}
// <repeat command> ::= repeat <number> <command list>
public class RepeatCommandNode extends Node {
private int number;
private Node commandListNode;
public void parse(Context context) throws ParseException {
context.skipToken("repeat");
number = context.currentNumber();
context.nextToken();
commandListNode = new CommandListNode();
commandListNode.parse(context);
}
public String toString() {
return "[repeat " + number + " " + commandListNode + "]";
}
}
// <primitive command> ::= go | right | left
public class PrimitiveCommandNode extends Node {
private String name;
public void parse(Context context) throws ParseException {
name = context.currentToken();
context.skipToken(name);
if (!name.equals("go") && !name.equals("right") && !name.equals("left")) {
throw new ParseException(name + " is undefined");
}
}
public String toString() {
return name;
}
}
2) 구문 해석에 필요한 Context 클래스를 작성합니다.
그 하단은 exception 처리 클래스 입니다.
import java.util.*;
public class Context {
private StringTokenizer tokenizer;
private String currentToken;
public Context(String text) {
tokenizer = new StringTokenizer(text);
nextToken();
}
public String nextToken() {
if (tokenizer.hasMoreTokens()) {
currentToken = tokenizer.nextToken();
} else {
currentToken = null;
}
return currentToken;
}
public String currentToken() {
return currentToken;
}
public void skipToken(String token) throws ParseException {
if (!token.equals(currentToken)) {
throw new ParseException("Warning: " + token + " is expected, but " + currentToken + " is found.");
}
nextToken();
}
public int currentNumber() throws ParseException {
int number = 0;
try {
number = Integer.parseInt(currentToken);
} catch (NumberFormatException e) {
throw new ParseException("Warning: " + e);
}
return number;
}
}
public class Context {
private StringTokenizer tokenizer;
private String currentToken;
public Context(String text) {
tokenizer = new StringTokenizer(text);
nextToken();
}
public String nextToken() {
if (tokenizer.hasMoreTokens()) {
currentToken = tokenizer.nextToken();
} else {
currentToken = null;
}
return currentToken;
}
public String currentToken() {
return currentToken;
}
public void skipToken(String token) throws ParseException {
if (!token.equals(currentToken)) {
throw new ParseException("Warning: " + token + " is expected, but " + currentToken + " is found.");
}
nextToken();
}
public int currentNumber() throws ParseException {
int number = 0;
try {
number = Integer.parseInt(currentToken);
} catch (NumberFormatException e) {
throw new ParseException("Warning: " + e);
}
return number;
}
}
public class ParseException extends Exception {
public ParseException(String msg) {
super(msg);
}
}
3) 테스트 코드 입니다.
import java.util.*;
import java.io.*;
public class Main {
public static void main(String[] args) {
try {
BufferedReader reader = new BufferedReader(new FileReader("program.txt"));
String text;
while ((text = reader.readLine()) != null) {
System.out.println("text = \"" + text + "\"");
Node node = new ProgramNode();
node.parse(new Context(text));
System.out.println("node = " + node);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
import java.io.*;
public class Main {
public static void main(String[] args) {
try {
BufferedReader reader = new BufferedReader(new FileReader("program.txt"));
String text;
while ((text = reader.readLine()) != null) {
System.out.println("text = \"" + text + "\"");
Node node = new ProgramNode();
node.parse(new Context(text));
System.out.println("node = " + node);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
결과 : text는 실제 내용 node는 해석 결과
node = [program []]text = "program go end"
node = [program [go]]
text = "program go right go right go right go right end"
node = [program [go, right, go, right, go, right, go, right]]
text = "program repeat 4 go right end end"
node = [program [[repeat 4 [go, right]]]]
text = "program repeat 4 repeat 3 go right go left end right end end"
node = [program [[repeat 4 [[repeat 3 [go, right, go, left]], right]]]]
3. 다이어그램
http://zetawiki.com/wiki/Interpreter_%ED%8C%A8%ED%84%B44. 관련패턴
1)
