Add Standard Deviation (#3039)

src/main/java/com/thealgorithms/maths/StandardDeviation.java

@@ -0,0 +1,22 @@
+package com.thealgorithms.maths;
+
+public class StandardDeviation {
+	
+  public static double stdDev(double[] data)
+	{
+		double var = 0;
+		double avg = 0;
+		for (int i = 0; i < data.length; i++)
+		{
+			avg += data[i];
+		}
+		avg /= data.length;
+		for (int j = 0; j < data.length; j++)
+		{
+			var += Math.pow((data[j] - avg), 2);
+		}
+		var /= data.length;
+		return Math.sqrt(var);
+	}
+  
+}

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

@@ -0,0 +1,32 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class StandardDeviationTest{
+  @Test
+  void test1()
+  {
+    double[] t1 = new double[]{1,1,1,1,1};
+    Assertions.assertEquals(StandardDeviation.stdDev(t1), 0.0);
+  }
+  @Test
+  void test2()
+  {
+    double[] t2 = new double[]{1,2,3,4,5,6,7,8,9,10};
+    Assertions.assertEquals(StandardDeviation.stdDev(t2), 2.8722813232690143);
+  }
+  @Test
+  void test3()
+  {
+    double[] t3= new double[]{1.1, 8.5, 20.3, 2.4, 6.2};
+    Assertions.assertEquals(StandardDeviation.stdDev(t3), 6.8308125431752265);
+  }
+  @Test
+  void test4()
+  {
+    double[] t4 = new double[]{3.14, 2.22222, 9.89898989, 100.00045, 56.7};
+     Assertions.assertEquals(StandardDeviation.stdDev(t4), 38.506117353865775);
+  }
+}
+