logicaloperator.java

来自「OpenJMS是一个开源的Java Message Service API 1.」· Java 代码 · 共 131 行

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package org.exolab.jms.selector;

import javax.jms.Message;


/**
 * This class is the base class for all expressions that are boolean binary
 * operators. These return a boolean result when evaluated, or null
 * if the result is undefined.
 *
 * @version     $Revision: 1.1 $ $Date: 2004/11/26 01:50:44 $
 * @author      <a href="mailto:[e-mail]">Tim Anderson</a>
 * @see         Expression
 * @see         SBool
 */
abstract class LogicalOperator extends BinaryOperator {

    /**
     * Construct a new <code>LogicalOperator</code>
     *
     * @param operator the operator
     * @param lhs the left hand side of the expression
     * @param rhs the right hand side of the expression
     */
    protected LogicalOperator(final String operator, final Expression lhs,
                              final Expression rhs) {
        super(operator, lhs, rhs);
    }

    /**
     * Evaluate the expression
     *
     * @param msg the message to use to obtain any header identifier and
     * property values
     * @return the evaluated result, or <code>null</code> if the value of the
     * expression is unknown
     * @throws TypeMismatchException if the expression tries to evaluate
     * mismatched types.
     */
    public SObject evaluate(final Message msg) throws TypeMismatchException {
        SBool result = null;

        SObject lhs = left().evaluate(msg);
        if (lhs != null) {
            SObject rhs = right().evaluate(msg);
            if (rhs != null) {
                checkTypes(lhs.type(), rhs.type());
                result = evaluate(lhs, rhs);
            }
        }
        return result;
    }

    /**
     * Perform a logical comparison
     *
     * @param lhs the left hand side of the expression
     * @param rhs the right hand side of the expression
     * @return the evaluated result
     */
    protected SBool evaluate(final SObject lhs, final SObject rhs) {
        return null;
    }

    /**
     * Verifies that the types of expression can be verified.
     *
     * @param lhs the left hand side of the expression
     * @param rhs the right hand side of the expression
     * @throws TypeMismatchException if the types cannot be compared
     */
    protected void checkTypes(final Type lhs, final Type rhs)
        throws TypeMismatchException {
        if (lhs != rhs) {
            StringBuffer msg = new StringBuffer();
            msg.append("expecting a ");
            msg.append(lhs);
            msg.append(" expression for operator ");
            msg.append(operator());
            msg.append(", found a ");
            msg.append(rhs);
            throw new TypeMismatchException(msg.toString());
        }
    }

} //-- LogicalOperator