added some tests

server/src/test/java/server/PortCheckerTest.java

@@ -0,0 +1,96 @@
+package server;
+
+import org.junit.jupiter.api.Test;
+import java.io.IOException;
+import java.net.ServerSocket;
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class PortCheckerTest {
+    @Test
+    void portAvailable() {
+        PortChecker checker = new PortChecker();
+
+        int port = 0; //will find a random available port
+        boolean available = checker.isPortAvailable(port);
+
+        assertTrue(available);
+    }
+    @Test
+    void portNotAvailable(){
+        try  {
+            ServerSocket socket = new ServerSocket(0);
+            int usedPort = socket.getLocalPort();
+
+            PortChecker checker = new PortChecker();
+            boolean notFree = checker.isPortAvailable(usedPort);
+            assertFalse(notFree);
+        }
+        catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+    @Test
+    void invalidPort(){
+        PortChecker checker = new PortChecker();
+
+        assertThrows(IllegalArgumentException.class, ()-> {
+            checker.isPortAvailable(-1);
+                }
+        );
+        assertThrows(IllegalArgumentException.class, ()-> {
+            checker.isPortAvailable(65536);
+                }
+        );
+    }
+    @Test
+    void findFreePort() throws IOException {
+        PortChecker checker = new PortChecker();
+
+        int port = checker.findFreePort();
+        int defaultPort = 8080;
+        int lastPort = 8090;
+
+        boolean greaterOrEqual = port >= defaultPort;
+        boolean lessOrEqual = port <= lastPort;
+        boolean inRange = greaterOrEqual && lessOrEqual;
+        boolean isItFree = checker.isPortAvailable(port);
+
+        assertTrue(inRange);
+    }
+    @Test
+    void noFreePort() {
+        List<ServerSocket> notFreePorts = new ArrayList<>();
+        int defaultPort = 8080;
+        int lastPort = 8090;
+
+        try {
+            for (int i = defaultPort; i <= lastPort; i++) {
+                try {
+                    notFreePorts.add(new ServerSocket(i));
+                }
+                catch (IOException e) {
+                }
+            }
+
+            PortChecker checker = new PortChecker();
+            assertThrows(IOException.class, () -> {
+                checker.findFreePort();
+                    }
+            );
+        }
+        catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+
+        for (ServerSocket s : notFreePorts) {
+            try {
+                s.close();
+            }
+            catch (IOException e) {
+            }
+        }
+    }
+}