Added Factory pattern implementation in Java and its test

src/main/java/com/designpatterns/factorypattern/Pentagon.java

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+package src.main.java.com.designpatterns.factorypattern;
+
+public class Pentagon implements Polygon {
+    @Override
+    public String getType() {
+        return "Pentagon";
+    }
+
+    @Override
+    public double area(double side) {
+        return 3.847104 * side * side;
+    }
+}

src/main/java/com/designpatterns/factorypattern/Polygon.java

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+package src.main.java.com.designpatterns.factorypattern;
+
+public interface Polygon {
+    /**
+     * Should be overriden to describe the type of each polygon
+     *
+     * @return a String value describing the name of the polygon
+     */
+    String getType();
+
+    /**
+     * Calculates the area of the regular polygon
+     *
+     * @param side The length of the side of regular polygon
+     * @return area of the polygon
+     */
+    double area(double side);
+}
+
+

src/main/java/com/designpatterns/factorypattern/PolygonFactory.java

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+package src.main.java.com.designpatterns.factorypattern;
+
+/**
+ * In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal
+ * with the problem of creating objects without having to specify the exact class of the object that will be created.
+ * This is done by creating objects by calling a factory method—either specified in an interface and implemented by
+ * child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling
+ * a constructor.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Factory_method_pattern">Factory Pattern</a>
+ */
+public class PolygonFactory {
+    /**
+     * Factory pattern implementation for the Polygon Interface to return the correct regular polygon object
+     * depending on the number of sides it has.
+     *
+     * @param numberOfSides in the polygon to initialize.
+     * @return the object having the respective number of sides
+     */
+    public Polygon getPolygon(int numberOfSides) {
+        switch (numberOfSides) {
+            case 3:
+                return new Triangle();
+            case 4:
+                return new Square();
+            case 5:
+                return new Pentagon();
+            default:
+                return null;
+        }
+    }
+}

src/main/java/com/designpatterns/factorypattern/Square.java

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+package src.main.java.com.designpatterns.factorypattern;
+
+public class Square implements Polygon {
+
+    @Override
+    public String getType() {
+        return "Square";
+    }
+
+    @Override
+    public double area(double side) {
+        return side * side;
+    }
+}

src/main/java/com/designpatterns/factorypattern/Triangle.java

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+package src.main.java.com.designpatterns.factorypattern;
+
+public class Triangle implements Polygon {
+    @Override
+    public String getType() {
+        return "Triangle";
+    }
+
+    @Override
+    public double area(double side) {
+        return 0.433013 * side * side;
+    }
+}

src/test/java/com/designpatterns/factorypattern/PolygonFactoryTest.java

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+package src.test.java.com.designpatterns.factorypattern;
+
+import org.junit.Assert;
+import org.junit.Test;
+import src.main.java.com.designpatterns.factorypattern.Polygon;
+import src.main.java.com.designpatterns.factorypattern.PolygonFactory;
+
+public class PolygonFactoryTest {
+    @Test
+    public void testPolygonFactory() {
+        String failReason = "";
+        PolygonFactory polFactory = new PolygonFactory();
+
+        // Test for triangle
+        Polygon triangle = polFactory.getPolygon(3);
+        if (!"Triangle".equals(triangle.getType())) {
+            failReason += "Polygon Factory failed for Triangle.";
+        }
+        if (triangle.area(4) != 6.928208) {
+            failReason += "Triangle area is incorrect!";
+        }
+
+
+        // Test for square
+        Polygon square = polFactory.getPolygon(4);
+        if (!"Square".equals(square.getType())) {
+            failReason += "Polygon Factory failed for Square.";
+        }
+        if (square.area(5) != 25) {
+            failReason += "Square area is incorrect!";
+        }
+
+
+        // Test for pentagon
+        Polygon pentagon = polFactory.getPolygon(5);
+        if (!"Pentagon".equals(pentagon.getType())) {
+            failReason += "Polygon Factory failed for Pentagon.";
+        }
+        if (pentagon.area(9) != 311.615424) {
+            failReason += "Pentagon area is incorrect!";
+        }
+
+        Assert.assertEquals(failReason, failReason, "");
+    }
+}