001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements.  See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache License, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License.  You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the License for the specific language governing permissions and
015     * limitations under the License.
016     */
017    
018    package org.apache.commons.math3.linear;
019    
020    import org.apache.commons.math3.exception.DimensionMismatchException;
021    
022    /**
023     * This class defines a linear operator operating on real ({@code double})
024     * vector spaces. No direct access to the coefficients of the underlying matrix
025     * is provided.
026     *
027     * The motivation for such an interface is well stated by
028     * <a href="#BARR1994">Barrett et al. (1994)</a>:
029     * <blockquote>
030     *  We restrict ourselves to iterative methods, which work by repeatedly
031     *  improving an approximate solution until it is accurate enough. These
032     *  methods access the coefficient matrix A of the linear system only via the
033     *  matrix-vector product y = A &middot; x
034     *  (and perhaps z = A<sup>T</sup> &middot; x). Thus the user need only
035     *  supply a subroutine for computing y (and perhaps z) given x, which permits
036     *  full exploitation of the sparsity or other special structure of A.
037     * </blockquote>
038     * <br/>
039     *
040     * <dl>
041     *  <dt><a name="BARR1994">Barret et al. (1994)</a></dt>
042     *  <dd>
043     *   R. Barrett, M. Berry, T. F. Chan, J. Demmel, J. M. Donato, J. Dongarra,
044     *   V. Eijkhout, R. Pozo, C. Romine and H. Van der Vorst,
045     *   <em>Templates for the Solution of Linear Systems: Building Blocks for
046     *   Iterative Methods</em>, SIAM
047     *  </dd>
048     * </dl>
049     *
050     * @version $Id: RealLinearOperator.java 1416643 2012-12-03 19:37:14Z tn $
051     * @since 3.0
052     */
053    public abstract class RealLinearOperator {
054        /**
055         * Returns the dimension of the codomain of this operator.
056         *
057         * @return the number of rows of the underlying matrix
058         */
059        public abstract int getRowDimension();
060    
061        /**
062         * Returns the dimension of the domain of this operator.
063         *
064         * @return the number of columns of the underlying matrix
065         */
066        public abstract int getColumnDimension();
067    
068        /**
069         * Returns the result of multiplying {@code this} by the vector {@code x}.
070         *
071         * @param x the vector to operate on
072         * @return the product of {@code this} instance with {@code x}
073         * @throws DimensionMismatchException if the column dimension does not match
074         * the size of {@code x}
075         */
076        public abstract RealVector operate(final RealVector x)
077            throws DimensionMismatchException;
078    
079        /**
080         * Returns the result of multiplying the transpose of {@code this} operator
081         * by the vector {@code x} (optional operation). The default implementation
082         * throws an {@link UnsupportedOperationException}. Users overriding this
083         * method must also override {@link #isTransposable()}.
084         *
085         * @param x the vector to operate on
086         * @return the product of the transpose of {@code this} instance with
087         * {@code x}
088         * @throws org.apache.commons.math3.exception.DimensionMismatchException
089         * if the row dimension does not match the size of {@code x}
090         * @throws UnsupportedOperationException if this operation is not supported
091         * by {@code this} operator
092         */
093        public RealVector operateTranspose(final RealVector x)
094            throws DimensionMismatchException, UnsupportedOperationException {
095            throw new UnsupportedOperationException();
096        }
097    
098        /**
099         * Returns {@code true} if this operator supports
100         * {@link #operateTranspose(RealVector)}. If {@code true} is returned,
101         * {@link #operateTranspose(RealVector)} should not throw
102         * {@code UnsupportedOperationException}. The default implementation returns
103         * {@code false}.
104         *
105         * @return {@code false}
106         */
107        public boolean isTransposable() {
108            return false;
109        }
110    }