Java Design Patterns Tutorial - Java Design Pattern - Interpreter Pattern

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We normally use Interpreter pattern to evaluate language grammar or expression.

This type of pattern is one of the behavioral patterns.

Example

interface Expression {
  public boolean evaluate(String context);
}//  w w w.  ja  v a  2 s . c om

class IsInExpression implements Expression {
  private String data;

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

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

class OrExpression implements Expression {

  private Expression expr1 = null;
  private Expression expr2 = null;

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

  @Override
  public boolean evaluate(String context) {
    return expr1.evaluate(context) || expr2.evaluate(context);
  }
}

class AndExpression implements Expression {

  private Expression expr1 = null;
  private Expression expr2 = null;

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

  @Override
  public boolean evaluate(String context) {
    return expr1.evaluate(context) && expr2.evaluate(context);
  }
}

public class Main {

  public static void main(String[] args) {
    Expression select = new IsInExpression("Select");
    Expression from = new IsInExpression("From");
    Expression isSelectFrom = new AndExpression(select, from);

    Expression insert = new IsInExpression("Insert");
    Expression update = new IsInExpression("Update");
    Expression isInsertOrUpdate = new OrExpression(insert, update);

    System.out.println(isSelectFrom.evaluate("Select"));
    System.out.println(isInsertOrUpdate.evaluate("Insert"));

    System.out.println(isSelectFrom.evaluate("Select From"));
    System.out.println(isInsertOrUpdate.evaluate("Update"));
  }
}

The code above generates the following result.

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