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    1. /*
    2.  * Licensed to the Apache Software Foundation (ASF) under one or more
    3.  * contributor license agreements.  See the NOTICE file distributed with
    4.  * this work for additional information regarding copyright ownership.
    5.  * The ASF licenses this file to you under the Apache License, Version 2.0
    6.  * (the "License"); you may not use this file except in compliance with
    7.  * the License.  You may obtain a copy of the License at
    8.  *
    9.  * http://www.apache.org/licenses/LICENSE-2.0
    10.  *
    11.  * Unless required by applicable law or agreed to in writing, software
    12.  * distributed under the License is distributed on an "AS IS" BASIS,
    13.  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    14.  * See the License for the specific language governing permissions and
    15.  * limitations under the License.
    16.  */
    17. package org.apache.calcite.rex;
    19. import org.apache.calcite.rel.type.RelDataType;
    20. import org.apache.calcite.sql.SqlKind;
    22. import java.util.Collection;
    24. /**
    25.  * Row expression.
    26.  *
    27.  * <p>Every row-expression has a type.
    28.  * (Compare with {@link org.apache.calcite.sql.SqlNode}, which is created before
    29.  * validation, and therefore types may not be available.)
    30.  *
    31.  * <p>Some common row-expressions are: {@link RexLiteral} (constant value),
    32.  * {@link RexVariable} (variable), {@link RexCall} (call to operator with
    33.  * operands). Expressions are generally created using a {@link RexBuilder}
    34.  * factory.</p>
    35.  *
    36.  * <p>All sub-classes of RexNode are immutable.</p>
    37.  */
    38. public abstract class RexNode {
    39.   //~ Instance fields --------------------------------------------------------
    41.   // Effectively final. Set in each sub-class constructor, and never re-set.
    42.   // RexNode 的摘要信息
    43.   protected String digest;
    45.   //~ Methods ----------------------------------------------------------------
    47.   public abstract RelDataType getType();
    49.   /**
    50.    * Returns whether this expression always returns true. (Such as if this
    51.    * expression is equal to the literal <code>TRUE</code>.)
    52.    */
    53.   public boolean isAlwaysTrue() {
    54.     return false;
    55.   }
    57.   /**
    58.    * Returns whether this expression always returns false. (Such as if this
    59.    * expression is equal to the literal <code>FALSE</code>.)
    60.    */
    61.   public boolean isAlwaysFalse() {
    62.     return false;
    63.   }
    65.   public boolean isA(SqlKind kind) {
    66.     return getKind() == kind;
    67.   }
    69.   public boolean isA(Collection<SqlKind> kinds) {
    70.     return getKind().belongsTo(kinds);
    71.   }
    73.   /**
    74.    * Returns the kind of node this is.
    75.    *
    76.    * @return Node kind, never null
    77.    */
    78.   public SqlKind getKind() {
    79.     return SqlKind.OTHER;
    80.   }
    82.   public String toString() {
    83.     return digest;
    84.   }
    86.   /**
    87.    * Accepts a visitor, dispatching to the right overloaded
    88.    * {@link RexVisitor#visitInputRef visitXxx} method.
    89.    *
    90.    * <p>Also see {@link RexUtil#apply(RexVisitor, java.util.List, RexNode)},
    91.    * which applies a visitor to several expressions simultaneously.
    92.    */
    93.   public abstract <R> R accept(RexVisitor<R> visitor);
    95.   /**
    96.    * Accepts a visitor with a payload, dispatching to the right overloaded
    97.    * {@link RexBiVisitor#visitInputRef(RexInputRef, Object)} visitXxx} method.
    98.    */
    99.   public abstract <R, P> R accept(RexBiVisitor<R, P> visitor, P arg);
    101.   /** {@inheritDoc}
    102.    *
    103.    * <p>Every node must implement {@link #equals} based on its content
    104.    */
    105.   @Override public abstract boolean equals(Object obj);
    107.   /** {@inheritDoc}
    108.    *
    109.    * <p>Every node must implement {@link #hashCode} consistent with
    110.    * {@link #equals}
    111.    */
    112.   @Override public abstract int hashCode();
    113. }
    115. // End RexNode.java