diff --git a/src/test/java/PortfolioTest.java b/src/test/java/PortfolioTest.java
index d921601..34e46aa 100644
--- a/src/test/java/PortfolioTest.java
+++ b/src/test/java/PortfolioTest.java
@@ -1,5 +1,6 @@
 import static org.junit.jupiter.api.Assertions.*;
 
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import Model.Portfolio;
@@ -7,82 +8,148 @@
 import Model.Stock;
 
 import java.math.BigDecimal;
+import java.util.List;
 
 public class PortfolioTest {
 
-    @Test
-    void testAddSharePortfolio() {
-        Portfolio portfolio = new Portfolio();
+    private Portfolio portfolio;
+    private Stock stock;
+    private Share share;
 
-        Stock stock = new Stock("AAPL", "Apple", new BigDecimal("150"));
-        Share share = new Share(stock, new BigDecimal("10"), new BigDecimal("140"));
+    @BeforeEach
+    void setUp() {
+        portfolio = new Portfolio();
+        stock = new Stock("AAPL", "Apple", new BigDecimal("150"));
+        share = new Share(stock, new BigDecimal("10"), new BigDecimal("140"));
+    }
 
-        boolean result = portfolio.addShare(share);
+    // ---- Positive tests ----
 
+    @Test
+    void testAddShare() {
+        boolean result = portfolio.addShare(share);
         assertTrue(result);
         assertTrue(portfolio.contains(share));
     }
 
     @Test
-    void testRemoveSharePortfolio() {
-        Portfolio portfolio = new Portfolio();
-
-        Stock stock = new Stock("AAPL", "Apple", new BigDecimal("150"));
-        Share share = new Share(stock, new BigDecimal("10"), new BigDecimal("140"));
+    void testAddMultipleShares() {
+        Stock stock2 = new Stock("GOOGL", "Google", new BigDecimal("200"));
+        Share share2 = new Share(stock2, new BigDecimal("5"), new BigDecimal("190"));
 
         portfolio.addShare(share);
+        portfolio.addShare(share2);
 
-        boolean result = portfolio.removeShare(share);
+        assertEquals(2, portfolio.getShares().size());
+    }
 
+    @Test
+    void testRemoveShare() {
+        portfolio.addShare(share);
+        boolean result = portfolio.removeShare(share);
         assertTrue(result);
         assertFalse(portfolio.contains(share));
     }
 
     @Test
-    void testGetSharesPortfolio() {
-        Portfolio portfolio = new Portfolio();
-
-        Stock stock = new Stock("AAPL", "Apple", new BigDecimal("150"));
-        Share share = new Share(stock, new BigDecimal("10"), new BigDecimal("140"));
+    void testRemoveShareNotInPortfolioReturnsFalse() {
+        boolean result = portfolio.removeShare(share);
+        assertFalse(result);
+    }
 
+    @Test
+    void testGetSharesReturnsAll() {
         portfolio.addShare(share);
-
-        var shares = portfolio.getShares();
-
+        List<Share> shares = portfolio.getShares();
         assertEquals(1, shares.size());
         assertTrue(shares.contains(share));
     }
 
     @Test
-    void testGetSharesBySymbolPortfolio() {
-        Portfolio portfolio = new Portfolio();
+    void testGetSharesIsDefensiveCopy() {
+        portfolio.addShare(share);
+        List<Share> copy = portfolio.getShares();
+        copy.clear();
+        assertEquals(1, portfolio.getShares().size());
+    }
 
-        Stock stock1 = new Stock("AAPL", "Apple", new BigDecimal("150"));
+    @Test
+    void testGetSharesBySymbol() {
         Stock stock2 = new Stock("GOOGL", "Google", new BigDecimal("200"));
+        Share share2 = new Share(stock2, new BigDecimal("5"), new BigDecimal("190"));
 
-        Share share1 = new Share(stock1, new BigDecimal("10"), new BigDecimal("140"));
-        Share share2 = new Share(stock2, new BigDecimal("10"), new BigDecimal("190"));
-
-        portfolio.addShare(share1);
+        portfolio.addShare(share);
         portfolio.addShare(share2);
 
-        var result = portfolio.getShares("AAPL");
-
+        List<Share> result = portfolio.getShares("AAPL");
         assertEquals(1, result.size());
-        assertTrue(result.contains(share1));
+        assertTrue(result.contains(share));
+    }
+
+    @Test
+    void testGetSharesBySymbolNoMatch() {
+        portfolio.addShare(share);
+        List<Share> result = portfolio.getShares("MSFT");
+        assertTrue(result.isEmpty());
+    }
+
+    @Test
+    void testContainsReturnsTrueWhenPresent() {
+        portfolio.addShare(share);
+        assertTrue(portfolio.contains(share));
     }
 
     @Test
-    void testContainsPortfolio() {
-        Portfolio portfolio = new Portfolio();
+    void testContainsReturnsFalseWhenAbsent() {
+        assertFalse(portfolio.contains(share));
+    }
 
-        Stock stock = new Stock("AAPL", "Apple", new BigDecimal("150"));
-        Share share = new Share(stock, new BigDecimal("10"), new BigDecimal("140"));
+    @Test
+    void testGetNetWorthEmptyPortfolio() {
+        assertEquals(BigDecimal.ZERO, portfolio.getNetWorth());
+    }
 
+    @Test
+    void testGetNetWorthWithShares() {
         portfolio.addShare(share);
+        // net worth should be positive when portfolio has holdings
+        assertTrue(portfolio.getNetWorth().compareTo(BigDecimal.ZERO) > 0);
+    }
 
-        assertTrue(portfolio.contains(share));
+    // ---- Negative tests ----
+
+    @Test
+    void testAddNullShareThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            portfolio.addShare(null)
+        );
     }
 
-    
+    @Test
+    void testRemoveNullShareThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            portfolio.removeShare(null)
+        );
+    }
+
+    @Test
+    void testContainsNullShareThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            portfolio.contains(null)
+        );
+    }
+
+    @Test
+    void testGetSharesByNullSymbolThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            portfolio.getShares((String) null)
+        );
+    }
+
+    @Test
+    void testGetSharesByBlankSymbolThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+            portfolio.getShares("  ")
+        );
+    }
 }
diff --git a/target/test-classes/PortfolioTest.class b/target/test-classes/PortfolioTest.class
index 9be31bc..679e0c4 100644
Binary files a/target/test-classes/PortfolioTest.class and b/target/test-classes/PortfolioTest.class differ