Design Patterns - Interpreter

design-patterns

Design Patterns - Behavioral - Interpreter:

Interpreter pattern provides a way to evaluate language grammar or expression. 
This type of pattern comes under behavioral pattern. This pattern involves 
implementing an expression interface which tells to interpret a particular 
context. This pattern is used in SQL parsing, symbol processing engine etc.

We are going to create an interface Expression and concrete classes implementing 
the Expression interface. A class TerminalExpression is defined which acts as a 
main interpreter of context in question. Other classes OrExpression, 
AndExpression are used to create combinational expressions.

// Step 1: Create an expression interface.
public interface Expression {
   public boolean interpret(String context);
}

// Step 2: Create concrete classes implementing the above interface.
public class TerminalExpression implements Expression {    
   private String data;

   public TerminalExpression(String data){
      this.data = data; 
   }

   public boolean interpret(String context) {   
      if(context.contains(data)){
         return true;
      }
      return false;
   }
}

public class OrExpression implements Expression {
   private Expression expr1 = null;
   private Expression expr2 = null;

   public OrExpression(Expression expr1, Expression expr2) { 
      this.expr1 = expr1;
      this.expr2 = expr2;
   }

   public boolean interpret(String context) {        
      return expr1.interpret(context) || expr2.interpret(context);
   }
}

public class AndExpression implements Expression {
   private Expression expr1 = null;
   private Expression expr2 = null;

   public AndExpression(Expression expr1, Expression expr2) { 
      this.expr1 = expr1;
      this.expr2 = expr2;
   }

   public boolean interpret(String context) {        
      return expr1.interpret(context) && expr2.interpret(context);
   }
}

// Step 3: Uses Expression class to create rules and then parse them.
public class InterpreterPatternDemo {
   //Rule: Robert and John are male
   public static Expression getMaleExpression(){
      Expression robert = new TerminalExpression("Robert");
      Expression john = new TerminalExpression("John");
      return new OrExpression(robert, john);        
   }

   //Rule: Julie is a married women
   public static Expression getMarriedWomanExpression(){
      Expression julie = new TerminalExpression("Julie");
      Expression married = new TerminalExpression("Married");
      return new AndExpression(julie, married);        
   }

   public static void main(String[] args) {
      Expression isMale = getMaleExpression();
      Expression isMarriedWoman = getMarriedWomanExpression();

      System.out.println("John is male? " + isMale.interpret("John"));
      System.out.println("Julie is a married women? " + isMarriedWoman.interpret("Married Julie"));
   }
}
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