Implement the find feature

Users can now input the find command followed by any number of keywords to search for Tasks that have names containing the search target keywords

src/main/java/duke/Duke.java

@@ -57,6 +57,12 @@ void run() {
                 } catch (DukeException e) {
                     ui.displayMessage(e.getMessage());
                 }
+            } else if (tokens[0].equals("find")) {
+                try {
+                    taskList.findItemInList(tokens, ui);
+                } catch (DukeException e) {
+                    ui.displayMessage(e.getMessage());
+                }
             } else {
                 ui.displayMessage("unknown command\n");
             }

src/main/java/duke/TaskList.java

@@ -166,6 +166,32 @@ void deleteItem(String[] tokens, Ui ui) throws DukeException {
         }
     }
 
+    /**
+     * Method to find all items containing keyword in the list, and invokes the associated Ui event to
+     * display matching items
+     * @param tokens <code>String[]</code> provided by <code>Parser</code>.
+     * @param ui Instance of <code>Ui</code> belonging to the calling instance of <code>Duke</code>.
+     * @throws DukeException In the event that no keyword is specified.
+     */
+    void findItemInList(String[] tokens, Ui ui) throws DukeException {
+        if (tokens.length == 1) {
+            throw new DukeException("please provide a keyword or keywords to search for");
+        }
+        StringBuilder sb = new StringBuilder();
+        for (int i = 1; i < tokens.length; i++) {
+            sb.append(tokens[i]).append(' ');
+        }
+        sb.deleteCharAt(sb.length()-1);
+        List<Integer> indices = new ArrayList<>();
+        for (int i = 0; i < tasks.size(); i++) {
+            Task task = tasks.get(i);
+            if (task.getName().matches(".*" + sb + ".*")) {
+                indices.add(i);
+            }
+        }
+        ui.displayItemsAtIndices(this, indices);
+    }
+
     public int size() {
         return tasks.size();
     }

src/main/java/duke/Ui.java

@@ -1,5 +1,7 @@
 package duke;
 
+import java.util.List;
+
 public class Ui {
     void displayMessage(String message) {
         System.out.println(
@@ -28,4 +30,22 @@ void displayItemList(TaskList tasks) {
         }
         displayMessage(sb.toString());
     }
+
+    /**
+     * Method to display the <code>Task</code> items at the indices specified.
+     * @param tasks Instance of <code>TaskList</code> belonging to the calling instance of <code>Duke</code>.
+     * @param indices <code>List<Integer></code> containing desired indices to be displayed
+     */
+    void displayItemsAtIndices(TaskList tasks, List<Integer> indices) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("Here are the wanted tasks in your list:\n");
+        for (int i : indices) {
+            sb
+                    .append(i+1)
+                    .append(".")
+                    .append(tasks.get(i).toString())
+                    .append("\n");
+        }
+        displayMessage(sb.toString());
+    }
 }