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Behavioral Pattern
Interpreter Pattern
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Interpreter Pattern
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Define
Interpreter pattern
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Provides a definition of a macro
language or syntax and parsing into objects in a program.
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Where
to use & benefits
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Need your own parser generator.
Translate a specific expression.
Handle a tree-related information.
Related patterns include
- Composite, which is an instance in an interpreter.
- Flyweight, which shows how to share symbols with abstract
context.
- Iterator, which is used to traverse the tree structure.
- Visitor, which is used to maintain behavior of each note in
tree structure.
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Example
of Interpreter pattern
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Given any string expression and
a token, filter out the information you want. The below is a simple
parser program. the myParser method can be used to parse any
expression. The composite, visit and iterator patterns have been used.
import java.util.*;
class Parser{
private String expression;
private String token;
private List result;
private String interpreted;
public Parser(String e, String t) {
expression = e;
token = t;
}
public void myParser() {
StringTokenizer holder = new
StringTokenizer(expression, token);
String[] toBeMatched = new
String[holder.countTokens()];
int idx = 0;
while(holder.hasMoreTokens()) {
String item = holder.nextToken();
int
start = item.indexOf(",");
if(start==0) {
item = item.substring(2);
}
toBeMatched[idx] = item;
idx ++;
}
result =
Arrays.asList(toBeMatched);
}
public List getParseResult() {
return result;
}
public void interpret() {
StringBuffer buffer =
new StringBuffer();
ListIterator list =
result.listIterator();
while (list.hasNext()){
String token = (String)list.next();
if
(token.equals("SFO")){
token = "San
Francisco";
}else if(token.equals("CA")) {
token = "Canada";
}
//...
buffer.append(" " + token);
}
interpreted =
buffer.toString();
}
public String getInterpretedResult() {
return interpreted;
}
public static void main(String[] args) {
String source =
"dest='SFO',origin='CA',day='MON'";
String delimiter =
"=,'";
Parser parser = new
Parser(source, delimiter);
parser.myParser();
parser.interpret();
String result =
parser.getInterpretedResult();
System.out.println(result);
}
}
java Parser
dest San Francisco origin Canada day MON
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