diff --git a/Data-Structures/Trees/Trie/PrefixTrie.java b/Data-Structures/Trees/Trie/PrefixTrie.java
index aec9be7..5b5b5f6 100644
--- a/Data-Structures/Trees/Trie/PrefixTrie.java
+++ b/Data-Structures/Trees/Trie/PrefixTrie.java
@@ -1,43 +1,72 @@
-import java.util.*;
-class PrefixNode {
-    PrefixNode[] children = new PrefixNode[26];
-    int count; 
-    PrefixNode() {
-        count = 0;
-    }
-}
-public class PrefixTrie {
-    PrefixNode root;
-    public PrefixTrie() {
-        root = new PrefixNode();
-    }
-    public void insert(String word) {
-        PrefixNode curr = root;
-        for (char ch : word.toCharArray()) {
-            int index = ch - 'a';
-            if (curr.children[index] == null) {
-                curr.children[index] = new PrefixNode();
-            }
-            curr = curr.children[index];
-            curr.count++;
-        }
-    }
-    public int countPrefix(String prefix) {
-        PrefixNode curr = root;
-        for (char ch : prefix.toCharArray()) {
-            int index = ch - 'a';
-            if (curr.children[index] == null) {
-                return 0;
-            }
-            curr = curr.children[index];
-        }
-        return curr.count;
-    }
-    public static void main(String[] args) {
-        PrefixTrie trie = new PrefixTrie();
-        Scanner sc = new Scanner(System.in);
-        System.out.println("Enter a String: ");
-        String prefix = sc.nextLine();
-        trie.insert(prefix);
-    }
-}
\ No newline at end of file
+import java.util.*;
+
+class PrefixNode {
+    PrefixNode[] children = new PrefixNode[26];
+    int count;
+
+    PrefixNode() {
+        count = 0;
+    }
+}
+
+public class PrefixTrie {
+    PrefixNode root;
+
+    public PrefixTrie() {
+        root = new PrefixNode();
+    }
+
+    private int getValidatedIndex(char ch, int position, String input) {
+        char normalized = Character.toLowerCase(ch);
+        int index = normalized - 'a';
+        if (index < 0 || index >= 26) {
+            throw new IllegalArgumentException(
+                "Invalid character '" + ch + "' at position " + position
+                    + " in \"" + input + "\". Please use letters a-z only."
+            );
+        }
+        return index;
+    }
+
+    public void insert(String word) {
+        PrefixNode curr = root;
+        for (int i = 0; i < word.length(); i++) {
+            int index = getValidatedIndex(word.charAt(i), i, word);
+            if (curr.children[index] == null) {
+                curr.children[index] = new PrefixNode();
+            }
+            curr = curr.children[index];
+            curr.count++;
+        }
+    }
+
+    public int countPrefix(String prefix) {
+        PrefixNode curr = root;
+        for (int i = 0; i < prefix.length(); i++) {
+            int index = getValidatedIndex(prefix.charAt(i), i, prefix);
+            if (curr.children[index] == null) {
+                return 0;
+            }
+            curr = curr.children[index];
+        }
+        return curr.count;
+    }
+
+    public static void main(String[] args) {
+        PrefixTrie trie = new PrefixTrie();
+
+        // Valid examples (uppercase is normalized to lowercase)
+        trie.insert("Apple");
+        trie.insert("application");
+        trie.insert("app");
+        System.out.println("countPrefix(app): " + trie.countPrefix("app"));
+        System.out.println("countPrefix(AP): " + trie.countPrefix("AP"));
+
+        // Invalid example
+        try {
+            trie.insert("ab#c");
+        } catch (IllegalArgumentException e) {
+            System.out.println("Invalid example error: " + e.getMessage());
+        }
+    }
+}
diff --git a/Data-Structures/Trees/Trie/Trie.java b/Data-Structures/Trees/Trie/Trie.java
index b216583..14d7d82 100644
--- a/Data-Structures/Trees/Trie/Trie.java
+++ b/Data-Structures/Trees/Trie/Trie.java
@@ -1,59 +1,87 @@
-import java.util.*;
-class TrieNode {
-    TrieNode[] children = new TrieNode[26];
-    boolean isEndOfWord;
-    TrieNode() {
-        isEndOfWord = false;
-        for (int i = 0; i < 26; i++) {
-            children[i] = null;
-        }
-    }
-}
-public class Trie {
-    TrieNode root;
-    public Trie() {
-        root = new TrieNode();
-    }
-    public void insert(String word) {
-        TrieNode curr = root;
-        for (char ch : word.toCharArray()) {
-            int index = ch - 'a';
-            if (curr.children[index] == null) {
-                curr.children[index] = new TrieNode();
-            }
-            curr = curr.children[index];
-        }
-        curr.isEndOfWord = true;
-    }
-    public boolean search(String word) {
-        TrieNode curr = root;
-        for (char ch : word.toCharArray()) {
-            int index = ch - 'a';
-            if (curr.children[index] == null) {
-                return false;
-            }
-            curr = curr.children[index];
-        }
-        return curr.isEndOfWord;
-    }
-    public boolean startsWith(String prefix) {
-        TrieNode curr = root;
-        for (char ch : prefix.toCharArray()) {
-            int index = ch - 'a';
-            if (curr.children[index] == null) {
-                return false;
-            }
-            curr = curr.children[index];
-        }
-        return true;
-    }
-    public static void main(String[] args) {
-        Trie trie = new Trie();
-        trie.insert("apple");
-        trie.insert("app");
-        System.out.println(trie.search("apple"));   
-        System.out.println(trie.search("app"));     
-        System.out.println(trie.search("appl"));    
-        System.out.println(trie.startsWith("app")); 
-    }
-}
\ No newline at end of file
+import java.util.*;
+
+class TrieNode {
+    TrieNode[] children = new TrieNode[26];
+    boolean isEndOfWord;
+
+    TrieNode() {
+        isEndOfWord = false;
+        for (int i = 0; i < 26; i++) {
+            children[i] = null;
+        }
+    }
+}
+
+public class Trie {
+    TrieNode root;
+
+    public Trie() {
+        root = new TrieNode();
+    }
+
+    private int getValidatedIndex(char ch, int position, String input) {
+        char normalized = Character.toLowerCase(ch);
+        int index = normalized - 'a';
+        if (index < 0 || index >= 26) {
+            throw new IllegalArgumentException(
+                "Invalid character '" + ch + "' at position " + position
+                    + " in \"" + input + "\". Please use letters a-z only."
+            );
+        }
+        return index;
+    }
+
+    public void insert(String word) {
+        TrieNode curr = root;
+        for (int i = 0; i < word.length(); i++) {
+            int index = getValidatedIndex(word.charAt(i), i, word);
+            if (curr.children[index] == null) {
+                curr.children[index] = new TrieNode();
+            }
+            curr = curr.children[index];
+        }
+        curr.isEndOfWord = true;
+    }
+
+    public boolean search(String word) {
+        TrieNode curr = root;
+        for (int i = 0; i < word.length(); i++) {
+            int index = getValidatedIndex(word.charAt(i), i, word);
+            if (curr.children[index] == null) {
+                return false;
+            }
+            curr = curr.children[index];
+        }
+        return curr.isEndOfWord;
+    }
+
+    public boolean startsWith(String prefix) {
+        TrieNode curr = root;
+        for (int i = 0; i < prefix.length(); i++) {
+            int index = getValidatedIndex(prefix.charAt(i), i, prefix);
+            if (curr.children[index] == null) {
+                return false;
+            }
+            curr = curr.children[index];
+        }
+        return true;
+    }
+
+    public static void main(String[] args) {
+        Trie trie = new Trie();
+
+        // Valid examples (uppercase is normalized to lowercase)
+        trie.insert("Apple");
+        trie.insert("app");
+        System.out.println("search(apple): " + trie.search("apple"));
+        System.out.println("search(APP): " + trie.search("APP"));
+        System.out.println("startsWith(Ap): " + trie.startsWith("Ap"));
+
+        // Invalid example
+        try {
+            trie.insert("app3");
+        } catch (IllegalArgumentException e) {
+            System.out.println("Invalid example error: " + e.getMessage());
+        }
+    }
+}