Interpreter pattern
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In computer programming, the interpreter pattern is a particular design pattern. The basic idea is to implement a specialized computer language to rapidly solve a defined class of problems. Specialized languages often let a problem be solved several to several hundred times more quickly than a general purpose language would permit.
The general idea is to have a class for each symbol (terminal or nonterminal) in the language.
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[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
[edit] Java
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
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