Add tests for EulerMethod (#5769)

Author: Hardvan

DIRECTORY.md

@@ -945,6 +945,7 @@
             * [DigitalRootTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/DigitalRootTest.java)
             * [DistanceFormulaTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/DistanceFormulaTest.java)
             * [DudeneyNumberTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/DudeneyNumberTest.java)
+            * [EulerMethodTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/EulerMethodTest.java)
             * [EulersFunctionTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/EulersFunctionTest.java)
             * [FactorialRecursionTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/FactorialRecursionTest.java)
             * [FactorialTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/FactorialTest.java)

src/test/java/com/thealgorithms/maths/EulerMethodTest.java

@@ -0,0 +1,79 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.ArrayList;
+import java.util.function.BiFunction;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class EulerMethodTest {
+
+    private static class EulerFullTestCase {
+        double[] params;
+        BiFunction<Double, Double, Double> equation;
+        int expectedSize;
+        double[] expectedFirstPoint;
+        double[] expectedLastPoint;
+
+        EulerFullTestCase(double[] params, BiFunction<Double, Double, Double> equation, int expectedSize, double[] expectedFirstPoint, double[] expectedLastPoint) {
+            this.params = params;
+            this.equation = equation;
+            this.expectedSize = expectedSize;
+            this.expectedFirstPoint = expectedFirstPoint;
+            this.expectedLastPoint = expectedLastPoint;
+        }
+    }
+
+    @ParameterizedTest
+    @MethodSource("eulerStepTestCases")
+    void testEulerStep(double x, double h, double y, BiFunction<Double, Double, Double> equation, double expected) {
+        double result = EulerMethod.eulerStep(x, h, y, equation);
+        assertEquals(expected, result, 1e-9, "Euler step failed for the given equation.");
+    }
+
+    static Stream<Arguments> eulerStepTestCases() {
+        return Stream.of(Arguments.of(0.0, 0.1, 1.0, (BiFunction<Double, Double, Double>) ((x, y) -> x + y), 1.1));
+    }
+
+    @ParameterizedTest
+    @MethodSource("eulerStepInvalidCases")
+    void testEulerStepInvalidInput(double x, double h, double y, BiFunction<Double, Double, Double> equation, Class<? extends Exception> expectedExceptionClass) {
+        assertThrows(expectedExceptionClass, () -> EulerMethod.eulerStep(x, h, y, equation));
+    }
+
+    static Stream<Arguments> eulerStepInvalidCases() {
+        BiFunction<Double, Double, Double> dummyEquation = (x, y) -> x + y;
+        return Stream.of(Arguments.of(0.0, -0.1, 1.0, dummyEquation, IllegalArgumentException.class), Arguments.of(0.0, 0.0, 1.0, dummyEquation, IllegalArgumentException.class));
+    }
+
+    @ParameterizedTest
+    @MethodSource("eulerFullTestCases")
+    void testEulerFull(EulerFullTestCase testCase) {
+        ArrayList<double[]> result = EulerMethod.eulerFull(testCase.params[0], testCase.params[1], testCase.params[2], testCase.params[3], testCase.equation);
+        assertEquals(testCase.expectedSize, result.size(), "Incorrect number of points in the result.");
+        assertArrayEquals(testCase.expectedFirstPoint, result.get(0), 1e-9, "Incorrect first point.");
+        assertArrayEquals(testCase.expectedLastPoint, result.get(result.size() - 1), 1e-9, "Incorrect last point.");
+    }
+
+    static Stream<Arguments> eulerFullTestCases() {
+        return Stream.of(Arguments.of(new EulerFullTestCase(new double[] {0.0, 1.0, 0.5, 0.0}, (x, y) -> x, 3, new double[] {0.0, 0.0}, new double[] {1.0, 0.25})),
+            Arguments.of(new EulerFullTestCase(new double[] {0.0, 1.0, 0.1, 1.0}, (x, y) -> y, 12, new double[] {0.0, 1.0}, new double[] {1.0999999999999999, 2.8531167061100002})),
+            Arguments.of(new EulerFullTestCase(new double[] {0.0, 0.1, 0.1, 1.0}, (x, y) -> x + y, 2, new double[] {0.0, 1.0}, new double[] {0.1, 1.1})));
+    }
+
+    @ParameterizedTest
+    @MethodSource("eulerFullInvalidCases")
+    void testEulerFullInvalidInput(double xStart, double xEnd, double stepSize, double yInitial, BiFunction<Double, Double, Double> equation, Class<? extends Exception> expectedExceptionClass) {
+        assertThrows(expectedExceptionClass, () -> EulerMethod.eulerFull(xStart, xEnd, stepSize, yInitial, equation));
+    }
+
+    static Stream<Arguments> eulerFullInvalidCases() {
+        BiFunction<Double, Double, Double> dummyEquation = (x, y) -> x + y;
+        return Stream.of(Arguments.of(1.0, 0.0, 0.1, 1.0, dummyEquation, IllegalArgumentException.class), Arguments.of(0.0, 1.0, 0.0, 1.0, dummyEquation, IllegalArgumentException.class), Arguments.of(0.0, 1.0, -0.1, 1.0, dummyEquation, IllegalArgumentException.class));
+    }
+}