ComparisonOp.java

package calculator.interpreter.ast.logic;

import calculator.interpreter.Environment;
import calculator.interpreter.EvalException;
import calculator.interpreter.ast.Expression;
import calculator.interpreter.ast.InfixOp;

/**
 * Base class for comparison operators
 */
public abstract class ComparisonOp extends InfixOp {
  @SuppressWarnings("unchecked")
  @Override
  public Object eval(Environment env) {
    Comparable f = checkComparable(env, first);
    Comparable s = checkComparable(env, second);
    if (f.getClass() != s.getClass()) {
      throw new EvalException(this,
          "Only values of the same type could be compared: "
              + f.getClass().getName() + " != " + s.getClass().getName());
    }
    return compared(f.compareTo(s));
  }

  /**
   * Check if the value is comparable
   * 
   * @param env the evaluation environment
   * @param e the expression to evaluate
   * @return the result of expression
   */
  private Comparable<?> checkComparable(Environment env, Expression e) {
    Object o = e.eval(env);
    if (o == null || !(o instanceof Comparable)) {
      throw new EvalException(this, "The expression at "
          + e.location.toShortString() + " has evaluated to " + o + " : "
          + (o == null ? null : o.getClass().getName())
          + " which is not comparable.");
    }
    return (Comparable<?>) o;
  }

  /**
   * Convert result of comparison operation to boolean
   * 
   * @param result the result of {@link Comparable#compareTo(Object)}.
   * @return a boolean value
   */
  protected abstract boolean compared(int result);
}