Storage: Add backupAddressBook(ReadOnlyAddressBook)

Storage saves to a single file path.

Users may want to do a backup of the address book in a different
location.

Let's add Storage#backupAddressBook(ReadOnlyAddressBook).

src/main/java/seedu/address/storage/AddressBookStorage.java

@@ -42,4 +42,5 @@ public interface AddressBookStorage {
      */
     void saveAddressBook(ReadOnlyAddressBook addressBook, Path filePath) throws IOException;
 
+    void backupAddressBook(ReadOnlyAddressBook addressBook) throws IOException;
 }

src/main/java/seedu/address/storage/StorageManager.java

@@ -78,6 +78,10 @@ public void saveAddressBook(ReadOnlyAddressBook addressBook, Path filePath) thro
         addressBookStorage.saveAddressBook(addressBook, filePath);
     }
 
+    @Override
+    public void backupAddressBook(ReadOnlyAddressBook addressBook) throws IOException {
+        addressBookStorage.backupAddressBook(addressBook);
+    }
 
     @Override
     @Subscribe

src/main/java/seedu/address/storage/XmlAddressBookStorage.java

@@ -6,6 +6,7 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.Optional;
 import java.util.logging.Logger;
 
@@ -22,10 +23,12 @@ public class XmlAddressBookStorage implements AddressBookStorage {
 
     private static final Logger logger = LogsCenter.getLogger(XmlAddressBookStorage.class);
 
-    private Path filePath;
+    private final Path filePath;
+    private final Path backupFilePath;
 
     public XmlAddressBookStorage(Path filePath) {
         this.filePath = filePath;
+        this.backupFilePath = Paths.get(filePath.toString() + ".backup");
     }
 
     public Path getAddressBookFilePath() {
@@ -60,6 +63,11 @@ public Optional<ReadOnlyAddressBook> readAddressBook(Path filePath) throws DataC
         }
     }
 
+    @Override
+    public void backupAddressBook(ReadOnlyAddressBook addressBook) throws IOException {
+        saveAddressBook(addressBook, backupFilePath);
+    }
+
     @Override
     public void saveAddressBook(ReadOnlyAddressBook addressBook) throws IOException {
         saveAddressBook(addressBook, filePath);