Returns the natural logarithm of n!. : Math Functions « Development « Java Tutorial

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import java.io.File;

/* 
 * 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.
 *
 *
 */
public class Main {


  /**
   * Returns the natural logarithm of n!.
   * 
   * <Strong>Preconditions</strong>:
   * <ul>
   * <li> <code>n >= 0</code> (otherwise
   * <code>IllegalArgumentException</code> is thrown)</li>
   * </ul>
   * 
   * @param n argument
   * @return <code>n!</code>
   * @throws IllegalArgumentException if preconditions are not met.
   */
  public static double factorialLog(final int n) {
      if (n < 0) {
          throw new IllegalArgumentException("must have n > 0 for n!");
      }
      double logSum = 0;
      for (int i = 2; i <= n; i++) {
          logSum += Math.log((double)i);
      }
      return logSum;
  }

}