Add tests for EulerMethod

DIRECTORY.md

@@ -912,6 +912,7 @@
             * [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 +1,80 @@
+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 org.junit.jupiter.api.Test;
+
+public class EulerMethodTest {
+
+    @Test
+    public void testEulerStepBasic() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x + y;
+        double result = EulerMethod.eulerStep(0.0, 0.1, 1.0, equation);
+        assertEquals(1.1, result, 1e-9, "Euler step failed for basic equation.");
+    }
+
+    @Test
+    public void testEulerStepNegativeStepSize() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x + y;
+        assertThrows(IllegalArgumentException.class, () -> EulerMethod.eulerStep(0.0, -0.1, 1.0, equation), "Expected IllegalArgumentException for negative step size.");
+    }
+
+    @Test
+    public void testEulerStepZeroStepSize() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x + y;
+        assertThrows(IllegalArgumentException.class, () -> EulerMethod.eulerStep(0.0, 0.0, 1.0, equation), "Expected IllegalArgumentException for zero step size.");
+    }
+
+    @Test
+    public void testEulerFullBasic() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x;
+        ArrayList<double[]> result = EulerMethod.eulerFull(0, 1, 0.5, 0, equation);
+
+        assertEquals(3, result.size(), "Incorrect number of points in the result.");
+        assertArrayEquals(new double[] {0.0, 0.0}, result.get(0), 1e-9, "Incorrect first point.");
+        assertArrayEquals(new double[] {0.5, 0.0}, result.get(1), 1e-9, "Incorrect second point.");
+        assertArrayEquals(new double[] {1.0, 0.25}, result.get(2), 1e-9, "Incorrect third point.");
+    }
+
+    @Test
+    public void testEulerFullWithExponentialEquation() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> y;
+        ArrayList<double[]> result = EulerMethod.eulerFull(0, 1, 0.1, 1, equation);
+
+        assertEquals(12, result.size(), "Incorrect number of points in the result.");
+        double[] lastPoint = result.get(result.size() - 1);
+        assertEquals(1.0999999999999999, lastPoint[0], 1e-9, "Incorrect x-value of the last point.");
+        assertEquals(2.8531167061100002, lastPoint[1], 1e-9, "Incorrect y-value of the last point.");
+    }
+
+    @Test
+    public void testEulerFullInvalidRange() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x + y;
+        assertThrows(IllegalArgumentException.class, () -> EulerMethod.eulerFull(1, 0, 0.1, 1, equation), "Expected IllegalArgumentException for invalid range (xStart >= xEnd).");
+    }
+
+    @Test
+    public void testEulerFullZeroStepSize() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x + y;
+        assertThrows(IllegalArgumentException.class, () -> EulerMethod.eulerFull(0, 1, 0.0, 1, equation), "Expected IllegalArgumentException for zero step size.");
+    }
+
+    @Test
+    public void testEulerFullNegativeStepSize() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x + y;
+        assertThrows(IllegalArgumentException.class, () -> EulerMethod.eulerFull(0, 1, -0.1, 1, equation), "Expected IllegalArgumentException for negative step size.");
+    }
+
+    @Test
+    public void testEulerFullSingleStep() {
+        BiFunction<Double, Double, Double> equation = (x, y) -> x + y;
+        ArrayList<double[]> result = EulerMethod.eulerFull(0, 0.1, 0.1, 1, equation);
+        assertEquals(2, result.size(), "Incorrect number of points for single step.");
+        assertArrayEquals(new double[] {0.0, 1.0}, result.get(0), 1e-9, "Incorrect first point.");
+        assertArrayEquals(new double[] {0.1, 1.1}, result.get(1), 1e-9, "Incorrect second point.");
+    }
+}