checkstyle: add SingleSpaceSeparator

Most of the code in our the code base only puts a single space between
non-whitespace characters,

    "Like " + "this."

It is highly likely that there will not be a case where we will need to
put more than a single space between non-whitespace characters,

    "Like "  + "this."

To enforce consistency throughout the code base, let's add the
SingleSpaceSeparator module to our checkstyle configuration, and fix all
of the existing violations in our code base.

config/checkstyle/checkstyle.xml

@@ -356,6 +356,25 @@
       <property name="severity" value="warning"/>
     </module>
 
+    <!-- Checks that non-whitespace characters are separated by no more than one whitespace character.
+         a = 1; // Allowed
+         a  = 1; // Not allowed (more than one space before =)
+    -->
+    <module name="SingleSpaceSeparator">
+      <!-- Validate whitespace surrounding comments as well.
+
+           a = 1; // Allowed (single space before start of comment)
+           a = 1; /* Allowed (single space before start of comment) */
+           /* Allowed (single space after end of comment) */ a = 1;
+           a = 1;  // Not allowed (more than one space before start of comment)
+           a = 1;  /* Not allowed (more than one space before start of comment) */
+           /* Not allowed (more than one space after end of comment) */  a = 1;
+
+           This doesn't validate whitespace within comments so a comment /* like  this */ is allowed.
+      -->
+      <property name="validateComments" value="true"/>
+    </module>
+
     <!--
     JAVADOC CHECKS
     -->

src/main/java/seedu/address/commons/util/JsonUtil.java

@@ -60,7 +60,7 @@ public static <T> Optional<T> readJsonFile(
         requireNonNull(filePath);
 
         if (!Files.exists(filePath)) {
-            logger.info("Json file "  + filePath + " not found");
+            logger.info("Json file " + filePath + " not found");
             return Optional.empty();
         }
 

src/main/java/seedu/address/model/person/Email.java

@@ -9,7 +9,7 @@
  */
 public class Email {
 
-    private static  final String SPECIAL_CHARACTERS = "!#$%&'*+/=?`{|}~^.-";
+    private static final String SPECIAL_CHARACTERS = "!#$%&'*+/=?`{|}~^.-";
     public static final String MESSAGE_EMAIL_CONSTRAINTS = "Emails should be of the format local-part@domain "
             + "and adhere to the following constraints:\n"
             + "1. The local-part should only contain alphanumeric characters and these special characters, excluding "

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

@@ -47,7 +47,7 @@ public Optional<ReadOnlyAddressBook> readAddressBook(Path filePath) throws DataC
         requireNonNull(filePath);
 
         if (!Files.exists(filePath)) {
-            logger.info("AddressBook file "  + filePath + " not found");
+            logger.info("AddressBook file " + filePath + " not found");
             return Optional.empty();
         }
 

src/test/java/guitests/GuiRobot.java

@@ -103,7 +103,7 @@ public int getNumberOfWindowsShown(String stageTitle) {
      */
     public Stage getStage(String stageTitle) {
         Optional<Stage> targetStage = listTargetWindows().stream()
-                .filter(Stage.class::isInstance)    // checks that the window is of type Stage
+                .filter(Stage.class::isInstance) // checks that the window is of type Stage
                 .map(Stage.class::cast)
                 .filter(stage -> stage.getTitle().equals(stageTitle))
                 .findFirst();

src/test/java/seedu/address/commons/core/VersionTest.java

@@ -56,56 +56,56 @@ public void versionComparable_validVersion_compareToIsCorrect() {
 
         // Tests equality
         one = new Version(0, 0, 0, true);
-        another = new  Version(0, 0, 0, true);
+        another = new Version(0, 0, 0, true);
         assertTrue(one.compareTo(another) == 0);
 
         one = new Version(11, 12, 13, false);
-        another = new  Version(11, 12, 13, false);
+        another = new Version(11, 12, 13, false);
         assertTrue(one.compareTo(another) == 0);
 
         // Tests different patch
         one = new Version(0, 0, 5, false);
-        another = new  Version(0, 0, 0, false);
+        another = new Version(0, 0, 0, false);
         assertTrue(one.compareTo(another) > 0);
 
         // Tests different minor
         one = new Version(0, 0, 0, false);
-        another = new  Version(0, 5, 0, false);
+        another = new Version(0, 5, 0, false);
         assertTrue(one.compareTo(another) < 0);
 
         // Tests different major
         one = new Version(10, 0, 0, true);
-        another = new  Version(0, 0, 0, true);
+        another = new Version(0, 0, 0, true);
         assertTrue(one.compareTo(another) > 0);
 
         // Tests high major vs low minor
         one = new Version(10, 0, 0, true);
-        another = new  Version(0, 1, 0, true);
+        another = new Version(0, 1, 0, true);
         assertTrue(one.compareTo(another) > 0);
 
         // Tests high patch vs low minor
         one = new Version(0, 0, 10, false);
-        another = new  Version(0, 1, 0, false);
+        another = new Version(0, 1, 0, false);
         assertTrue(one.compareTo(another) < 0);
 
         // Tests same major minor different patch
         one = new Version(2, 15, 0, false);
-        another = new  Version(2, 15, 5, false);
+        another = new Version(2, 15, 5, false);
         assertTrue(one.compareTo(another) < 0);
 
         // Tests early access vs not early access on same version number
         one = new Version(2, 15, 0, true);
-        another = new  Version(2, 15, 0, false);
+        another = new Version(2, 15, 0, false);
         assertTrue(one.compareTo(another) < 0);
 
         // Tests early access lower version vs not early access higher version compare by version number first
         one = new Version(2, 15, 0, true);
-        another = new  Version(2, 15, 5, false);
+        another = new Version(2, 15, 5, false);
         assertTrue(one.compareTo(another) < 0);
 
         // Tests early access higher version vs not early access lower version compare by version number first
         one = new Version(2, 15, 0, false);
-        another = new  Version(2, 15, 5, true);
+        another = new Version(2, 15, 5, true);
         assertTrue(one.compareTo(another) < 0);
     }
 
@@ -124,11 +124,11 @@ public void versionComparable_validVersion_equalIsCorrect() {
         Version another;
 
         one = new Version(0, 0, 0, false);
-        another = new  Version(0, 0, 0, false);
+        another = new Version(0, 0, 0, false);
         assertTrue(one.equals(another));
 
         one = new Version(100, 191, 275, true);
-        another = new  Version(100, 191, 275, true);
+        another = new Version(100, 191, 275, true);
         assertTrue(one.equals(another));
     }
 

src/test/java/seedu/address/commons/util/AppUtilTest.java

@@ -42,6 +42,6 @@ public void checkArgument_falseWithErrorMessage_throwsIllegalArgumentException()
         String errorMessage = "error message";
         thrown.expect(IllegalArgumentException.class);
         thrown.expectMessage(errorMessage);
-        AppUtil.checkArgument(false,  errorMessage);
+        AppUtil.checkArgument(false, errorMessage);
     }
 }

src/test/java/seedu/address/commons/util/StringUtilTest.java

@@ -42,7 +42,7 @@ public void isUnsignedPositiveInteger() {
 
         // EP: numbers with white space
         assertFalse(StringUtil.isNonZeroUnsignedInteger(" 10 ")); // Leading/trailing spaces
-        assertFalse(StringUtil.isNonZeroUnsignedInteger("1 0"));  // Spaces in the middle
+        assertFalse(StringUtil.isNonZeroUnsignedInteger("1 0")); // Spaces in the middle
 
         // EP: number larger than Integer.MAX_VALUE
         assertFalse(StringUtil.isNonZeroUnsignedInteger(Long.toString(Integer.MAX_VALUE + 1)));

src/test/java/seedu/address/logic/parser/EditCommandParserTest.java

@@ -168,7 +168,7 @@ public void parse_oneFieldSpecified_success() {
     @Test
     public void parse_multipleRepeatedFields_acceptsLast() {
         Index targetIndex = INDEX_FIRST_PERSON;
-        String userInput = targetIndex.getOneBased()  + PHONE_DESC_AMY + ADDRESS_DESC_AMY + EMAIL_DESC_AMY
+        String userInput = targetIndex.getOneBased() + PHONE_DESC_AMY + ADDRESS_DESC_AMY + EMAIL_DESC_AMY
                 + TAG_DESC_FRIEND + PHONE_DESC_AMY + ADDRESS_DESC_AMY + EMAIL_DESC_AMY + TAG_DESC_FRIEND
                 + PHONE_DESC_BOB + ADDRESS_DESC_BOB + EMAIL_DESC_BOB + TAG_DESC_HUSBAND;
 

src/test/java/seedu/address/model/person/EmailTest.java

@@ -51,12 +51,12 @@ public void isValidEmail() {
 
         // valid email
         assertTrue(Email.isValidEmail("[email protected]"));
-        assertTrue(Email.isValidEmail("a@bc"));  // minimal
-        assertTrue(Email.isValidEmail("test@localhost"));   // alphabets only
+        assertTrue(Email.isValidEmail("a@bc")); // minimal
+        assertTrue(Email.isValidEmail("test@localhost")); // alphabets only
         assertTrue(Email.isValidEmail("!#$%&'*+/=?`{|}~^[email protected]")); // special characters local part
-        assertTrue(Email.isValidEmail("123@145"));  // numeric local part and domain name
+        assertTrue(Email.isValidEmail("123@145")); // numeric local part and domain name
         assertTrue(Email.isValidEmail("[email protected]")); // mixture of alphanumeric and special characters
-        assertTrue(Email.isValidEmail("[email protected]"));   // long domain name
-        assertTrue(Email.isValidEmail("[email protected]"));    // long local part
+        assertTrue(Email.isValidEmail("[email protected]")); // long domain name
+        assertTrue(Email.isValidEmail("[email protected]")); // long local part
     }
 }

src/test/java/systemtests/ClearCommandSystemTest.java

@@ -27,7 +27,7 @@ public void clear() {
         /* Case: undo clearing address book -> original address book restored */
         String command = UndoCommand.COMMAND_WORD;
         String expectedResultMessage = UndoCommand.MESSAGE_SUCCESS;
-        assertCommandSuccess(command,  expectedResultMessage, defaultModel);
+        assertCommandSuccess(command, expectedResultMessage, defaultModel);
         assertSelectedCardUnchanged();
 
         /* Case: redo clearing address book -> cleared */