diff --git a/java/3-longest-substring-without-repeating-characters.java b/java/3-longest-substring-without-repeating-characters.java
new file mode 100644
index 000000000..31d8efa3d
--- /dev/null
+++ b/java/3-longest-substring-without-repeating-characters.java
@@ -0,0 +1,24 @@
+class Solution {
+    public int lengthOfLongestSubstring(String s) {
+        if (s == null || s.length() == 0)
+            return 0;
+        
+        int start = 0, maxLen = 0;
+        Set<Character> currentWindowSet = new HashSet<>();
+        
+        for (int end = 0 ; end < s.length() ; end++) {
+            char currentChar = s.charAt(end);
+            if (currentWindowSet.contains(currentChar)) {
+                while (s.charAt(start) != currentChar) {
+                    currentWindowSet.remove(s.charAt(start++));
+                }
+                start++;
+            } else {
+                currentWindowSet.add(currentChar);
+                maxLen = Math.max(maxLen, currentWindowSet.size());
+            }
+        }
+        
+        return maxLen;
+    }
+}