Tutorial

Table of contents:

• Using Built-in evaluator
  • The simplest way
  • Working with variables
• Customizing Built-in evaluator
  • Restricting an evaluator
  • Extending an evaluator
  • Localizing an evaluator
• Creating your own evaluators
  • Creating a simple evaluator from scratch
  • Creating a complex evaluator
  • Working with textual operators

---

Creating your own evaluators

Creating a simple evaluator from scratch

This chapter is not yet available ... but a working sample code is:

package com.fathzer.soft.javaluator.examples;
 
import java.util.Iterator;
 
import com.fathzer.soft.javaluator.*;
 
/** An example of how to implement an evaluator from scratch.
 */
public class SimpleBooleanEvaluator extends AbstractEvaluator<Boolean> {
  /** The negate unary operator.*/
  public static final Operator NEGATE = new Operator("!", 1, Operator.Associativity.RIGHT, 3);
  /** The logical AND operator.*/
  private static final Operator AND = new Operator("&&", 2, Operator.Associativity.LEFT, 2);
  /** The logical OR operator.*/
  public static final Operator OR = new Operator("||", 2, Operator.Associativity.LEFT, 1);
 
  private static final Parameters PARAMETERS;
 
  static {
    // Create the evaluator's parameters
    PARAMETERS = new Parameters();
    // Add the supported operators
    PARAMETERS.add(AND);
    PARAMETERS.add(OR);
    PARAMETERS.add(NEGATE);
  }
 
  public SimpleBooleanEvaluator() {
    super(PARAMETERS);
  }
 
  @Override
  protected Boolean toValue(String literal, Object evaluationContext) {
    return Boolean.valueOf(literal);
  }
 
  @Override
  protected Boolean evaluate(Operator operator, Iterator<Boolean> operands, Object evaluationContext) {
    if (operator == NEGATE) {
      return !operands.next();
    } else if (operator == OR) {
      Boolean o1 = operands.next();
      Boolean o2 = operands.next();
      return o1 || o2;
    } else if (operator == AND) {
      Boolean o1 = operands.next();
      Boolean o2 = operands.next();
      return o1 && o2;
    } else {
      return super.evaluate(operator, operands, evaluationContext);
    }
  }
 
  public static void main(String[] args) {
    SimpleBooleanEvaluator evaluator = new SimpleBooleanEvaluator();
    String expression = "true && false";
    System.out.println (expression+" = "+evaluator.evaluate(expression));
    expression = "true || false";
    System.out.println (expression+" = "+evaluator.evaluate(expression));
    expression = "!true";
    System.out.println (expression+" = "+evaluator.evaluate(expression));
  }
}
 

The output is

true && false = false
true || false = true
!true = false