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The Java Data Objects API (JDO) : a standard interface-based Java model abstraction of persistence, developed by the JCP.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *     http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software 
 * distributed under the License is distributed on an "AS IS" BASIS, 
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
 * See the License for the specific language governing permissions and 
 * limitations under the License.
 */
package javax.jdo.query;

/**
 * Representation of an expression in a query.
 * 
 * @param  Java type being represented here
 */
public interface Expression
{
    /**
     * Method returning whether this expression equals the other expression.
     * @param expr Other expression
     * @return Whether they are equal
     */
    BooleanExpression eq(Expression expr);

    /**
     * Method returning whether this expression equals the literal.
     * @param t Literal
     * @return Whether they are equal
     */
    BooleanExpression eq(T t);

    /**
     * Method returning whether this expression doesn't equal the other expression.
     * @param expr Other expression
     * @return Whether they are not equal
     */
    BooleanExpression ne(Expression expr);

    /**
     * Method returning whether this expression doesn't equal the literal.
     * @param t literal
     * @return Whether they are not equal
     */
    BooleanExpression ne(T t);

    /**
     * Method to return a numeric expression representing the aggregated count of this expression.
     * @return Numeric expression for the count
     */
    NumericExpression count();

    /**
     * Method to return a numeric expression representing the aggregated (distinct) count of this expression.
     * @return Numeric expression for the distinct count
     */
    NumericExpression countDistinct();

    /**
     * Return an expression for whether this expression is an instanceof the supplied class.
     * @param cls Class to check against
     * @return Whether it is an instanceof
     */
    BooleanExpression instanceOf(Class cls);

    /**
     * Return an expression where this expression is cast to the specified type.
     * @param cls Class to cast to
     * @return The cast expression
     */
    Expression cast(Class cls);
}