From c5a223503ff6a1d23de5e44137c87016f39c7cc7 Mon Sep 17 00:00:00 2001
From: John Lin <johnlinp@gmail.com>
Date: Tue, 8 Jan 2019 11:56:14 +0800
Subject: [PATCH] Add useful isValidHashtag() for java

The implementation references isValidHashtag() in js and valid_hashtag() in ruby
---
 .../com/twitter/twittertext/Validator.java    | 20 +++++++++++++++++++
 .../twitter/twittertext/ValidatorTest.java    | 13 ++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/java/src/main/java/com/twitter/twittertext/Validator.java b/java/src/main/java/com/twitter/twittertext/Validator.java
index f19364416..602429da2 100644
--- a/java/src/main/java/com/twitter/twittertext/Validator.java
+++ b/java/src/main/java/com/twitter/twittertext/Validator.java
@@ -4,6 +4,8 @@
 
 package com.twitter.twittertext;
 
+import java.util.List;
+
 /**
  * A class for validating Tweet texts.
  */
@@ -36,6 +38,24 @@ public boolean isValidTweet(String text) {
     return TwitterTextParser.parseTweet(text).isValid;
   }
 
+  /**
+   * Checks if a given text is a valid hashtag.
+   *
+   * @param text text to validate
+   * @return whether the text is a valid hashtag or not
+   */
+  public boolean isValidHashtag(String text) {
+    if (text == null) {
+      return false;
+    }
+
+    Extractor extractor = new Extractor();
+    List<String> extracted = extractor.extractHashtags(text);
+
+    // Should extract the hashtag minus the # sign, hence the .substring(1)
+    return extracted.size() == 1 && extracted.get(0).equals(text.substring(1));
+  }
+
   public static boolean hasInvalidCharacters(String text) {
     return Regex.INVALID_CHARACTERS_PATTERN.matcher(text).matches();
   }
diff --git a/java/src/test/java/com/twitter/twittertext/ValidatorTest.java b/java/src/test/java/com/twitter/twittertext/ValidatorTest.java
index 9e5cadaae..2b5e7df3f 100644
--- a/java/src/test/java/com/twitter/twittertext/ValidatorTest.java
+++ b/java/src/test/java/com/twitter/twittertext/ValidatorTest.java
@@ -50,4 +50,17 @@ public void testMutiByteCharacters() {
     assertTrue(validator.isValidTweet(builder.append(c).toString()));
     assertFalse(validator.isValidTweet(builder.append(c).toString()));
   }
+
+  public void testValidHashtags() {
+    assertTrue(validator.isValidHashtag("#test"));
+    assertTrue(validator.isValidHashtag("#\u53F0\u7063"));
+  }
+
+  public void testInvalidHashtags() {
+    assertFalse(validator.isValidHashtag("#test #test"));
+    assertFalse(validator.isValidHashtag("#test test"));
+    assertFalse(validator.isValidHashtag("#test,test"));
+    assertFalse(validator.isValidHashtag("test"));
+    assertFalse(validator.isValidHashtag(null));
+  }
 }