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diff --git a/src/main/java/org/apache/commons/math3/linear/RealVectorPreservingVisitor.java b/src/main/java/org/apache/commons/math3/linear/RealVectorPreservingVisitor.java
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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 org.apache.commons.math3.linear;
+
+/**
+ * This interface defines a visitor for the entries of a vector. Visitors implementing this
+ * interface do not alter the entries of the vector being visited.
+ *
+ * @since 3.1
+ */
+public interface RealVectorPreservingVisitor {
+    /**
+     * Start visiting a vector. This method is called once, before any entry of the vector is
+     * visited.
+     *
+     * @param dimension the size of the vector
+     * @param start the index of the first entry to be visited
+     * @param end the index of the last entry to be visited (inclusive)
+     */
+    void start(int dimension, int start, int end);
+
+    /**
+     * Visit one entry of the vector.
+     *
+     * @param index the index of the entry being visited
+     * @param value the value of the entry being visited
+     */
+    void visit(int index, double value);
+
+    /**
+     * End visiting a vector. This method is called once, after all entries of the vector have been
+     * visited.
+     *
+     * @return the value returned by {@link
+     *     RealVector#walkInDefaultOrder(RealVectorPreservingVisitor)}, {@link
+     *     RealVector#walkInDefaultOrder(RealVectorPreservingVisitor, int, int)}, {@link
+     *     RealVector#walkInOptimizedOrder(RealVectorPreservingVisitor)} or {@link
+     *     RealVector#walkInOptimizedOrder(RealVectorPreservingVisitor, int, int)}
+     */
+    double end();
+}