Added Portfolio class

src/main/java/Portfolio.java

@@ -0,0 +1,21 @@
+import java.util.ArrayList;
+import java.util.List;
+
+public class Portfolio {
+  List<Share> shares;
+  public Portfolio() {
+    shares = new ArrayList<>();
+  }
+  public boolean addShare(Share share) {
+    return this.shares.add(share);
+  }
+  public boolean removeShare(Share share) {
+    return this.shares.remove(share);
+  }
+  public List<Share> getShares() {
+    return this.shares;
+  }
+  public boolean contains(Share share) {
+    return this.shares.contains(share);
+  }
+}