Commons Math example source code file (TestProblem3.java)

This example Commons Math source code file (TestProblem3.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Commons Math tags/keywords

e, e, override, override, testproblem3, testproblem3, testproblemabstract, testproblemabstract

The Commons Math TestProblem3.java source code

/*
 * 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.math.ode;

/**
 * This class is used in the junit tests for the ODE integrators.

 * <p>This specific problem is the following differential equation :
 * <pre>
 *    y1'' = -y1/r^3  y1 (0) = 1-e  y1' (0) = 0
 *    y2'' = -y2/r^3  y2 (0) = 0    y2' (0) =sqrt((1+e)/(1-e))
 *    r = sqrt (y1^2 + y2^2), e = 0.9
 * </pre>
 * This is a two-body problem in the plane which can be solved by
 * Kepler's equation
 * <pre>
 *   y1 (t) = ...
 * </pre>
 * </p>

 */
public class TestProblem3
  extends TestProblemAbstract {

  /** Serializable version identifier. */
  private static final long serialVersionUID = 8567328542728919999L;

  /** Eccentricity */
  double e;

  /** theoretical state */
  private double[] y;

  /**
   * Simple constructor.
   * @param e eccentricity
   */
  public TestProblem3(double e) {
    super();
    this.e = e;
    double[] y0 = { 1 - e, 0, 0, Math.sqrt((1+e)/(1-e)) };
    setInitialConditions(0.0, y0);
    setFinalConditions(20.0);
    double[] errorScale = { 1.0, 1.0, 1.0, 1.0 };
    setErrorScale(errorScale);
    y = new double[y0.length];
  }

  /**
   * Simple constructor.
   */
  public TestProblem3() {
    this(0.1);
  }

  /**
   * Copy constructor.
   * @param problem problem to copy
   */
  public TestProblem3(TestProblem3 problem) {
    super(problem);
    e = problem.e;
    y = problem.y.clone();
  }

  /** {@inheritDoc} */
  @Override
public TestProblem3 copy() {
    return new TestProblem3(this);
  }

  @Override
  public void doComputeDerivatives(double t, double[] y, double[] yDot) {

    // current radius
    double r2 = y[0] * y[0] + y[1] * y[1];
    double invR3 = 1 / (r2 * Math.sqrt(r2));

    // compute the derivatives
    yDot[0] = y[2];
    yDot[1] = y[3];
    yDot[2] = -invR3  * y[0];
    yDot[3] = -invR3  * y[1];

  }

  @Override
  public double[] computeTheoreticalState(double t) {

    // solve Kepler's equation
    double E = t;
    double d = 0;
    double corr = 999.0;
    for (int i = 0; (i < 50) && (Math.abs(corr) > 1.0e-12); ++i) {
      double f2  = e * Math.sin(E);
      double f0  = d - f2;
      double f1  = 1 - e * Math.cos(E);
      double f12 = f1 + f1;
      corr  = f0 * f12 / (f1 * f12 - f0 * f2);
      d -= corr;
      E = t + d;
    }

    double cosE = Math.cos(E);
    double sinE = Math.sin(E);

    y[0] = cosE - e;
    y[1] = Math.sqrt(1 - e * e) * sinE;
    y[2] = -sinE / (1 - e * cosE);
    y[3] = Math.sqrt(1 - e * e) * cosE / (1 - e * cosE);

    return y;
  }

}