From Wikipedia, the free encyclopedia
In computer programming, the interpreter pattern is a particular design pattern. The basic idea is to have a class for each symbol (terminal or nonterminal) in a specialized computer language, so that the language's syntax tree is an instance of the composite pattern. The interpreter pattern specifies how to evaluate language constructs.
[edit] Structure
[edit] Uses for the Interpreter pattern
- Specialized database query languages such as SQL.
- Specialized computer languages which are often used to describe communication protocols
- Most general-purpose computer languages actually incorporate several specialized languages.
[edit] Examples
The following Java example illustrates how a general purpose language would interpret a more specialized language, here the Reverse Polish notation. The output is:
'42 4 2 - +' equals 44
import java.util.*;
interface Expression {
public void interpret(Stack<Integer> s);
}
class TerminalExpression_Number implements Expression {
private int number;
public TerminalExpression_Number(int number) { this.number = number; }
public void interpret(Stack<Integer> s) { s.push(number); }
}
class TerminalExpression_Plus implements Expression {
public void interpret(Stack<Integer> s) { s.push( s.pop() + s.pop() ); }
}
class TerminalExpression_Minus implements Expression {
public void interpret(Stack<Integer> s) { s.push( - s.pop() + s.pop() ); }
}
class Parser {
private ArrayList<Expression> parseTree = new ArrayList<Expression>(); // only one NonTerminal Expression here
public Parser(String s) {
for (String token : s.split(" ")) {
if (token.equals("+")) parseTree.add( new TerminalExpression_Plus() );
else if (token.equals("-")) parseTree.add( new TerminalExpression_Minus() );
// ...
else parseTree.add( new TerminalExpression_Number(Integer.valueOf(token)) );
}
}
public int evaluate() {
Stack<Integer> context = new Stack<Integer>();
for (Expression e : parseTree) e.interpret(context);
return context.pop();
}
}
class InterpreterExample {
public static void main(String[] args) {
String expression = "42 4 2 - +";
Parser p = new Parser(expression);
System.out.println("'" + expression +"' equals " + p.evaluate());
}
}
[edit] See also
