Java/Maths/EulerMethod.java at master · ppdouble/Java · GitHub

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package Maths;

import java.util.ArrayList;

import java.util.function.BiFunction;

/**

* In mathematics and computational science, the Euler method (also called forward Euler method) is

* a first-order numerical procedure for solving ordinary differential equations (ODEs) with a given

* initial value. It is the most basic explicit method for numerical integration of ordinary

* differential equations. The method proceeds in a series of steps. At each step the y-value is

* calculated by evaluating the differential equation at the previous step, multiplying the result

* with the step-size and adding it to the last y-value: y_n+1 = y_n + stepSize * f(x_n, y_n).

* (description adapted from https://en.wikipedia.org/wiki/Euler_method ) (see also:

* https://www.geeksforgeeks.org/euler-method-solving-differential-equation/ )

public class EulerMethod {

/** Illustrates how the algorithm is used in 3 examples and prints the results to the console. */

public static void main(String[] args) {

System.out.println("example 1:");

BiFunction<Double, Double, Double> exampleEquation1 = (x, y) -> x;

ArrayList<double[]> points1 = eulerFull(0, 4, 0.1, 0, exampleEquation1);

assert points1.get(points1.size() - 1)[1] == 7.800000000000003;

points1.forEach(

point -> System.out.println(String.format("x: %1$f; y: %2$f", point[0], point[1])));

// example from https://en.wikipedia.org/wiki/Euler_method

System.out.println("\n\nexample 2:");

BiFunction<Double, Double, Double> exampleEquation2 = (x, y) -> y;

ArrayList<double[]> points2 = eulerFull(0, 4, 0.1, 1, exampleEquation2);

assert points2.get(points2.size() - 1)[1] == 45.25925556817596;

points2.forEach(

point -> System.out.println(String.format("x: %1$f; y: %2$f", point[0], point[1])));

// example from https://www.geeksforgeeks.org/euler-method-solving-differential-equation/

System.out.println("\n\nexample 3:");

BiFunction<Double, Double, Double> exampleEquation3 = (x, y) -> x + y + x * y;

ArrayList<double[]> points3 = eulerFull(0, 0.1, 0.025, 1, exampleEquation3);

assert points3.get(points3.size() - 1)[1] == 1.1116729841674804;

points3.forEach(

point -> System.out.println(String.format("x: %1$f; y: %2$f", point[0], point[1])));

}

/**

* calculates the next y-value based on the current value of x, y and the stepSize the console.

* @param xCurrent Current x-value.

* @param stepSize Step-size on the x-axis.

* @param yCurrent Current y-value.

* @param differentialEquation The differential equation to be solved.

* @return The next y-value.

public static double eulerStep(

double xCurrent,

double stepSize,

double yCurrent,

BiFunction<Double, Double, Double> differentialEquation) {

if (stepSize <= 0) {

throw new IllegalArgumentException("stepSize should be greater than zero");

}

double yNext = yCurrent + stepSize * differentialEquation.apply(xCurrent, yCurrent);

return yNext;

}

/**

* Loops through all the steps until xEnd is reached, adds a point for each step and then returns

* all the points

* @param xStart First x-value.

* @param xEnd Last x-value.

* @param stepSize Step-size on the x-axis.

* @param yStart First y-value.

* @param differentialEquation The differential equation to be solved.

* @return The points constituting the solution of the differential equation.

public static ArrayList<double[]> eulerFull(

double xStart,

double xEnd,

double stepSize,

double yStart,

BiFunction<Double, Double, Double> differentialEquation) {

if (xStart >= xEnd) {

throw new IllegalArgumentException("xEnd should be greater than xStart");

}

if (stepSize <= 0) {

throw new IllegalArgumentException("stepSize should be greater than zero");

}

ArrayList<double[]> points = new ArrayList<double[]>();

double[] firstPoint = {xStart, yStart};

points.add(firstPoint);

double yCurrent = yStart;

double xCurrent = xStart;

while (xCurrent < xEnd) {

// Euler method for next step

yCurrent = eulerStep(xCurrent, stepSize, yCurrent, differentialEquation);

xCurrent += stepSize;

double[] point = {xCurrent, yCurrent};

points.add(point);

}

return points;

}

Java/Maths/EulerMethod.java at master · ppdouble/Java · GitHub

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