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| Uses for the Interpreter pattern | |
| 2>
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] Tags:Specialized Computer Language,Sql, | |
| Example | |
| 2>
The following Reverse Polish notation example illustrates the interpreter pattern. The grammar
expression ::= plus | minus | variable | number
plus ::= expression expression '+'
minus ::= expression expression '-'
variable ::= 'a' | 'b' | 'c' | ... | 'z'
digit = '0' | '1' | ... '9'
number ::= digit | digit number
defines a language which contains reverse Polish expressions like:
a b +
a b c + -
a b + c a - -
Following the interpreter pattern there is a class for each grammar rule.
import java.util.Map;
interface Expression {
public int interpret(Map<String,Expression> variables);
}
class Number implements Expression {
private int number;
public Number(int number) { this.number = number; }
public int interpret(Map<String,Expression> variables) { return number; }
}
class Plus implements Expression {
Expression leftOperand;
Expression rightOperand;
public Plus(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return leftOperand.interpret(variables) + rightOperand.interpret(variables);
}
}
class Minus implements Expression {
Expression leftOperand;
Expression rightOperand;
public Minus(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return leftOperand.interpret(variables) - rightOperand.interpret(variables);
}
}
class Variable implements Expression {
private String name;
public Variable(String name) { this.name = name; }
public int interpret(Map<String,Expression> variables) {
if(null==variables.get(name)) return 0; //Either return new Number(0).
return variables.get(name).interpret(variables);
}
}
While the interpreter pattern does not address parsing[2] a parser is provided for completeness.
import java.util.Map;
import java.util.Stack;
class Evaluator implements Expression {
private Expression syntaxTree;
public Evaluator(String expression) {
Stack<Expression> expressionStack = new Stack<Expression>();
for (String token : expression.split(" ")) {
if (token.equals("+")) {
Expression subExpression = new Plus(expressionStack.pop(), expressionStack.pop());
expressionStack.push( subExpression );
}
else if (token.equals("-")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new Minus(left, right);
expressionStack.push( subExpression );
}
else
expressionStack.push( new Variable(token) );
}
syntaxTree = expressionStack.pop();
}
public int interpret(Map<String,Expression> context) {
return syntaxTree.interpret(context);
}
}
Finally evaluating the expression "w x z - +" with w = 5, x = 10, and z = 42.
import java.util.Map;
import java.util.HashMap;
public class InterpreterExample {
public static void main(String[] args) {
String expression = "w x z - +";
Evaluator sentence = new Evaluator(expression);
Map<String,Expression> variables = new HashMap<String,Expression>();
variables.put("w", new Number(5));
variables.put("x", new Number(10));
variables.put("z", new Number(42));
int result = sentence.interpret(variables);
System.out.println(result);
}
}
[edit] Tags:Class, | |
| References | |
| 2>
^ Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. pp. 243 ISBN 0-201-63361-2
^ Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. pp. 247 ISBN 0-201-63361-2
[edit] Tags:Design Pattern,Design Patterns,Isbn 0-201-63361-2, | |
| External links | |
| 2>
The Wikibook Computer Science Design Patterns has a page on the topic of
Interpreterimplementations in various languages
Interpreter implementation in Ruby
SourceMaking tutorial
Interpreter pattern description from the Portland Pattern Repository
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