Add find functionality

src/main/java/duke/parser/Parser.java

@@ -22,6 +22,10 @@ public String executeCommand(String line, TaskList tasks) throws DukeException {
             String description = line.substring(divider + 1);
 
             return tasks.addTask(new Todo(description));
+        } else if (line.startsWith("find")) {
+            String keyword = line.substring(divider + 1);
+
+            return tasks.getIndexedTasksByKeyword(keyword);
         }
 
         if (line.contains("mark") || line.startsWith("delete")) {

src/main/java/duke/task/Task.java

@@ -23,4 +23,8 @@ public void setStatus(boolean isDone) {
     public String getSerializedString() {
         return isDone + "|" + description;
     }
+
+    public String getDescription() {
+        return description;
+    }
 }

src/main/java/duke/tasklist/TaskList.java

@@ -61,4 +61,16 @@ public String getSerializedTasks() {
 
         return formatted.toString();
     }
+
+    public String getIndexedTasksByKeyword(String keyword) {
+        int idx = 1;
+        StringBuilder formatted = new StringBuilder();
+        for (Task task : tasks) {
+            if (task.getDescription().contains(keyword)) {
+                formatted.append(idx++).append(". ").append(task.getFormattedTask()).append("\n");
+            }
+        }
+
+        return formatted.toString();
+    }
 }