#6219: Adding input validation

Author: MuhammadEzzatHBK

src/main/java/com/thealgorithms/randomized/MonteCarloIntegration.java

@@ -61,12 +61,22 @@ public static double approximate(Function<Double, Double> fx, double a, double b
     }
 
     private static double doApproximate(Function<Double, Double> fx, double a, double b, int n, Random generator) {
+        if (!validate(fx, a, b, n)) {
+            throw new IllegalArgumentException("Invalid input parameters");
+        }
         double totalArea = 0.0;
-        double range = b - a;
+        double interval = b - a;
         for (int i = 0; i < n; i++) {
-            double x = a + generator.nextDouble() * range;
+            double x = a + generator.nextDouble() * interval;
             totalArea += fx.apply(x);
         }
-        return range * totalArea / n;
+        return interval * totalArea / n;
+    }
+
+    private static boolean validate(Function<Double, Double> fx, double a, double b, int n) {
+        boolean isFunctionValid = fx != null;
+        boolean isIntervalValid = a < b;
+        boolean isSampleSizeValid = n > 0;
+        return isFunctionValid && isIntervalValid && isSampleSizeValid;
     }
 }

src/test/java/com/thealgorithms/randomized/MonteCarloIntegrationTest.java

@@ -1,6 +1,9 @@
 package com.thealgorithms.randomized;
 
+import static com.thealgorithms.randomized.MonteCarloIntegration.approximate;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.function.Function;
 import org.junit.jupiter.api.Test;
@@ -13,47 +16,76 @@ class MonteCarloIntegrationTest {
     void testConstantFunction() {
         // Integral of f(x) = 2 from 0 to 1 is 2
         Function<Double, Double> constant = x -> 2.0;
-        double result = MonteCarloIntegration.approximate(constant, 0, 1, 10000);
+        double result = approximate(constant, 0, 1, 10000);
         assertEquals(2.0, result, EPSILON);
     }
 
     @Test
     void testLinearFunction() {
         // Integral of f(x) = x from 0 to 1 is 0.5
         Function<Double, Double> linear = Function.identity();
-        double result = MonteCarloIntegration.approximate(linear, 0, 1, 10000);
+        double result = approximate(linear, 0, 1, 10000);
         assertEquals(0.5, result, EPSILON);
     }
 
     @Test
     void testQuadraticFunction() {
         // Integral of f(x) = x^2 from 0 to 1 is 1/3
         Function<Double, Double> quadratic = x -> x * x;
-        double result = MonteCarloIntegration.approximate(quadratic, 0, 1, 10000);
+        double result = approximate(quadratic, 0, 1, 10000);
         assertEquals(1.0 / 3.0, result, EPSILON);
     }
 
     @Test
     void testLargeSampleSize() {
         // Integral of f(x) = x^2 from 0 to 1 is 1/3
         Function<Double, Double> quadratic = x -> x * x;
-        double result = MonteCarloIntegration.approximate(quadratic, 0, 1, 50000000);
+        double result = approximate(quadratic, 0, 1, 50000000);
         assertEquals(1.0 / 3.0, result, EPSILON / 2); // Larger sample size, smaller error margin
     }
 
     @Test
     void testReproducibility() {
         Function<Double, Double> linear = Function.identity();
-        double result1 = MonteCarloIntegration.approximate(linear, 0, 1, 10000, 42L);
-        double result2 = MonteCarloIntegration.approximate(linear, 0, 1, 10000, 42L);
+        double result1 = approximate(linear, 0, 1, 10000, 42L);
+        double result2 = approximate(linear, 0, 1, 10000, 42L);
         assertEquals(result1, result2, 0.0); // Exactly equal
     }
 
     @Test
     void testNegativeInterval() {
         // Integral of f(x) = x from -1 to 1 is 0
         Function<Double, Double> linear = Function.identity();
-        double result = MonteCarloIntegration.approximate(linear, -1, 1, 10000);
+        double result = approximate(linear, -1, 1, 10000);
         assertEquals(0.0, result, EPSILON);
     }
+
+    @Test
+    void testNullFunction() {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> approximate(null, 0, 1, 1000));
+        assertNotNull(exception);
+    }
+
+    @Test
+    void testInvalidInterval() {
+        Function<Double, Double> linear = Function.identity();
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> {
+            approximate(linear, 2, 1, 1000); // b <= a
+        });
+        assertNotNull(exception);
+    }
+
+    @Test
+    void testZeroSampleSize() {
+        Function<Double, Double> linear = Function.identity();
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> approximate(linear, 0, 1, 0));
+        assertNotNull(exception);
+    }
+
+    @Test
+    void testNegativeSampleSize() {
+        Function<Double, Double> linear = Function.identity();
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> approximate(linear, 0, 1, -100));
+        assertNotNull(exception);
+    }
 }