diff --git a/src/test/java/ExchangeTest.java b/src/test/java/ExchangeTest.java
index f80b1d9..9261bd6 100644
--- a/src/test/java/ExchangeTest.java
+++ b/src/test/java/ExchangeTest.java
@@ -1,8 +1,13 @@
-import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import Model.Exchange;
+import Model.Player;
+import Model.Share;
 import Model.Stock;
+import Model.Transaction;
+import Model.Purchase;
+import Model.Sale;
 
 import static org.junit.jupiter.api.Assertions.*;
 
@@ -15,9 +20,10 @@ public class ExchangeTest {
     private Exchange exchange;
     private Stock apple;
     private Stock google;
+    private Player player;
 
-    @BeforeAll
-    public void setUp() {
+    @BeforeEach
+    void setUp() {
         apple = new Stock("AAPL", "Apple", new BigDecimal("100"));
         google = new Stock("GOOGL", "Google", new BigDecimal("200"));
 
@@ -25,29 +31,235 @@ public void setUp() {
         stocks.add(apple);
         stocks.add(google);
 
-        exchange = new Exchange("ABC", stocks);
+        exchange = new Exchange("TestExchange", stocks);
+        player = new Player("Jane", new BigDecimal("500000"));
     }
 
+    // ---- Positive tests ----
+
     @Test
-    public void testFindStocksBySymbol() {
-        List<Stock> result = exchange.findStocks("AAPL");
+    void testGetName() {
+        assertEquals("TestExchange", exchange.getName());
+    }
+
+    @Test
+    void testInitialWeekIsOne() {
+        assertEquals(1, exchange.getWeek());
+    }
+
+    @Test
+    void testAdvanceIncrementsWeek() {
+        exchange.advance();
+        assertEquals(2, exchange.getWeek());
+    }
+
+    @Test
+    void testHasStockReturnsTrue() {
+        assertTrue(exchange.hasStock("AAPL"));
+    }
 
+    @Test
+    void testHasStockReturnsFalse() {
+        assertFalse(exchange.hasStock("MSFT"));
+    }
+
+    @Test
+    void testGetStockReturnsCorrectStock() {
+        assertEquals(apple, exchange.getStock("AAPL"));
+    }
+
+    @Test
+    void testGetStockReturnsNullForUnknown() {
+        assertNull(exchange.getStock("MSFT"));
+    }
+
+    @Test
+    void testFindStocksBySymbol() {
+        List<Stock> result = exchange.findStocks("AAPL");
         assertEquals(1, result.size());
         assertEquals("AAPL", result.get(0).getSymbol());
     }
 
     @Test
-    public void testFindStocksByCompanyNames() {
+    void testFindStocksByCompanyName() {
         List<Stock> result = exchange.findStocks("e");
-
         assertEquals(2, result.size());
     }
 
     @Test
-    public void testFindStocksNotInList() {
+    void testFindStocksNoMatch() {
         List<Stock> result = exchange.findStocks("Samsung");
-
         assertEquals(0, result.size());
     }
-    
+
+    @Test
+    void testFindStocksEmptyTermReturnsAll() {
+        List<Stock> result = exchange.findStocks("");
+        assertEquals(2, result.size());
+    }
+
+    @Test
+    void testBuyReturnsCommittedPurchase() {
+        Transaction t = exchange.buy("AAPL", new BigDecimal("5"), player);
+        assertNotNull(t);
+        assertTrue(t.isCommitted());
+        assertInstanceOf(Purchase.class, t);
+    }
+
+    @Test
+    void testBuyDeductsMoneyFromPlayer() {
+        BigDecimal before = player.getMoney();
+        exchange.buy("AAPL", new BigDecimal("5"), player);
+        assertTrue(player.getMoney().compareTo(before) < 0);
+    }
+
+    @Test
+    void testBuyAddsShareToPortfolio() {
+        exchange.buy("AAPL", new BigDecimal("5"), player);
+        assertFalse(player.getPortfolio().getShares("AAPL").isEmpty());
+    }
+
+    @Test
+    void testSellReturnsCommittedSale() {
+        exchange.buy("AAPL", new BigDecimal("5"), player);
+        Share share = player.getPortfolio().getShares("AAPL").get(0);
+
+        Transaction t = exchange.sell(share, player);
+        assertNotNull(t);
+        assertTrue(t.isCommitted());
+        assertInstanceOf(Sale.class, t);
+    }
+
+    @Test
+    void testSellAddsMoney() {
+        exchange.buy("AAPL", new BigDecimal("5"), player);
+        Share share = player.getPortfolio().getShares("AAPL").get(0);
+        BigDecimal before = player.getMoney();
+
+        exchange.sell(share, player);
+        assertTrue(player.getMoney().compareTo(before) > 0);
+    }
+
+    @Test
+    void testSellRemovesShareFromPortfolio() {
+        exchange.buy("AAPL", new BigDecimal("5"), player);
+        Share share = player.getPortfolio().getShares("AAPL").get(0);
+
+        exchange.sell(share, player);
+        assertTrue(player.getPortfolio().getShares("AAPL").isEmpty());
+    }
+
+    @Test
+    void testGetGainersReturnsRequestedLimit() {
+        exchange.advance(); // prices must have changed at least once for a meaningful sort
+        List<Stock> gainers = exchange.getGainers(1);
+        assertEquals(1, gainers.size());
+    }
+
+    @Test
+    void testGetLosersReturnsRequestedLimit() {
+        exchange.advance();
+        List<Stock> losers = exchange.getLosers(1);
+        assertEquals(1, losers.size());
+    }
+
+    // ---- Negative tests ----
+
+    @Test
+    void testConstructorNullNameThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            new Exchange(null, List.of(apple))
+        );
+    }
+
+    @Test
+    void testConstructorBlankNameThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            new Exchange("  ", List.of(apple))
+        );
+    }
+
+    @Test
+    void testConstructorNullStockListThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            new Exchange("X", null)
+        );
+    }
+
+    @Test
+    void testConstructorNullStockEntryThrows() {
+        List<Stock> withNull = new ArrayList<>();
+        withNull.add(apple);
+        withNull.add(null);
+        assertThrows(IllegalArgumentException.class, () ->
+            new Exchange("X", withNull)
+        );
+    }
+
+    @Test
+    void testBuyNullSymbolThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            exchange.buy(null, new BigDecimal("5"), player)
+        );
+    }
+
+    @Test
+    void testBuyUnknownSymbolThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            exchange.buy("MSFT", new BigDecimal("5"), player)
+        );
+    }
+
+    @Test
+    void testBuyZeroQuantityThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            exchange.buy("AAPL", BigDecimal.ZERO, player)
+        );
+    }
+
+    @Test
+    void testBuyNegativeQuantityThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            exchange.buy("AAPL", new BigDecimal("-1"), player)
+        );
+    }
+
+    @Test
+    void testBuyNullPlayerThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            exchange.buy("AAPL", new BigDecimal("5"), null)
+        );
+    }
+
+    @Test
+    void testBuyInsufficientFundsThrows() {
+        Player poorPlayer = new Player("Broke", new BigDecimal("1"));
+        assertThrows(IllegalStateException.class, () ->
+            exchange.buy("AAPL", new BigDecimal("5"), poorPlayer)
+        );
+    }
+
+    @Test
+    void testSellNullPlayerThrows() {
+        exchange.buy("AAPL", new BigDecimal("5"), player);
+        Share share = player.getPortfolio().getShares("AAPL").get(0);
+        assertThrows(IllegalArgumentException.class, () ->
+            exchange.sell(share, null)
+        );
+    }
+
+    @Test
+    void testSellShareNotInPortfolioThrows() {
+        Share unowned = new Share(apple, new BigDecimal("1"), new BigDecimal("100"));
+        assertThrows(IllegalStateException.class, () ->
+            exchange.sell(unowned, player)
+        );
+    }
+
+    @Test
+    void testAddNullObserverThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            exchange.addObserver(null)
+        );
+    }
 }
diff --git a/target/test-classes/ExchangeTest.class b/target/test-classes/ExchangeTest.class
index 69ecbef..238ca9d 100644
Binary files a/target/test-classes/ExchangeTest.class and b/target/test-classes/ExchangeTest.class differ