diff --git a/Data-Structures/HashTable.java b/Data-Structures/HashTable.java
index f5e7e49..7c14a02 100644
--- a/Data-Structures/HashTable.java
+++ b/Data-Structures/HashTable.java
@@ -1,9 +1,10 @@
 import java.util.Scanner;
 
-public class HashTable {
-    static int size;
-    static Dataitem[] hasharray;
-    static final Dataitem DELETED = new Dataitem(-1, -1);
+public class HashTable {
+    static int size;
+    static Dataitem[] hasharray;
+    static final Dataitem DELETED = new Dataitem(-1, -1);
+    private static final String INVALID_INT_MESSAGE = "Invalid input. Please enter an integer.";
 
     static class Dataitem {
         int data;
@@ -15,12 +16,31 @@ static class Dataitem {
         }
     }
 
-    static void initialise() {
-        Scanner sc = new Scanner(System.in);
-        System.out.print("Enter the size of the Hash Table: ");
-        size = sc.nextInt();
-        hasharray = new Dataitem[size];
-    }
+    static void initialise(Scanner sc) {
+        size = readPositiveInt(sc, "Enter the size of the Hash Table: ");
+        hasharray = new Dataitem[size];
+    }
+
+    static int readInt(Scanner sc, String prompt) {
+        while (true) {
+            System.out.print(prompt);
+            if (sc.hasNextInt()) {
+                return sc.nextInt();
+            }
+            System.out.println(INVALID_INT_MESSAGE);
+            sc.next();
+        }
+    }
+
+    static int readPositiveInt(Scanner sc, String prompt) {
+        while (true) {
+            int value = readInt(sc, prompt);
+            if (value > 0) {
+                return value;
+            }
+            System.out.println("Invalid input. Please enter an integer greater than 0.");
+        }
+    }
 
     static int hashcode(int key) {
         return Math.floorMod(key, size);
@@ -69,22 +89,20 @@ static void display() {
         }
     }
 
-    public static void main(String[] args) {
-        initialise();
-        Scanner sc = new Scanner(System.in);
-
-        System.out.print("Enter the number of Insertions: ");
-        int n = sc.nextInt();
-
-        for (int i = 0; i < n; i++) {
-            System.out.print("Enter the key: ");
-            int key = sc.nextInt();
-            System.out.print("Enter the element: ");
-            int element = sc.nextInt();
-            insert(key, element);
-        }
-
-        System.out.println("Insertion done!");
-        display();
-    }
-}
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        initialise(sc);
+
+        int n = readPositiveInt(sc, "Enter the number of Insertions: ");
+
+        for (int i = 0; i < n; i++) {
+            int key = readInt(sc, "Enter the key: ");
+            int element = readInt(sc, "Enter the element: ");
+            insert(key, element);
+        }
+
+        System.out.println("Insertion done!");
+        display();
+        sc.close();
+    }
+}
diff --git a/Data-Structures/Queue/CircularQueue.java b/Data-Structures/Queue/CircularQueue.java
index f85e347..d098d21 100644
--- a/Data-Structures/Queue/CircularQueue.java
+++ b/Data-Structures/Queue/CircularQueue.java
@@ -77,15 +77,34 @@ public boolean isFull() {
     }
 }
 
-public class CircularQueue {
-    public static void main(String[] args) {
-        Scanner sc = new Scanner(System.in);
-
-        System.out.print("Enter Queue size: ");
-        int capacity = sc.nextInt();
-
-        Queue queue = new Queue(capacity);
-        int choice;
+public class CircularQueue {
+    private static int readInt(Scanner sc, String prompt) {
+        while (true) {
+            System.out.print(prompt);
+            if (sc.hasNextInt()) {
+                return sc.nextInt();
+            }
+            System.out.println("Invalid input. Please enter an integer.");
+            sc.next();
+        }
+    }
+
+    private static int readPositiveInt(Scanner sc, String prompt) {
+        while (true) {
+            int value = readInt(sc, prompt);
+            if (value > 0) {
+                return value;
+            }
+            System.out.println("Invalid input. Please enter an integer greater than 0.");
+        }
+    }
+
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        int capacity = readPositiveInt(sc, "Enter Queue size: ");
+
+        Queue queue = new Queue(capacity);
+        int choice;
 
         do {
             System.out.println("\n--- Circular Queue Operations ---");
@@ -94,18 +113,15 @@ public static void main(String[] args) {
             System.out.println("3. Peek");
             System.out.println("4. Display");
             System.out.println("5. Is Full?");
-            System.out.println("6. Is Empty?");
-            System.out.println("7. Exit");
-            System.out.print("Enter your choice: ");
-
-            choice = sc.nextInt();
-
-            switch (choice) {
-                case 1:
-                    System.out.print("Enter element to Enqueue: ");
-                    int element = sc.nextInt();
-                    queue.enqueue(element);
-                    break;
+            System.out.println("6. Is Empty?");
+            System.out.println("7. Exit");
+            choice = readInt(sc, "Enter your choice: ");
+
+            switch (choice) {
+                case 1:
+                    int element = readInt(sc, "Enter element to Enqueue: ");
+                    queue.enqueue(element);
+                    break;
 
                 case 2:
                     queue.dequeue();
@@ -127,14 +143,14 @@ public static void main(String[] args) {
                     System.out.println(queue.isEmpty() ? "The Queue is Empty." : "The Queue is NOT Empty.");
                     break;
 
-                case 7:
-                    System.out.println("Exiting...");
-                    break;
-
-                default:
-                    System.out.println("Invalid choice!");
-            }
-        } while (choice != 7);
+                case 7:
+                    System.out.println("Exiting...");
+                    break;
+
+                default:
+                    System.out.println("Invalid choice. Please enter a valid menu option.");
+            }
+        } while (choice != 7);
 
         sc.close();
     }
diff --git a/Data-Structures/Queue/Queue.java b/Data-Structures/Queue/Queue.java
index adf2d97..938b6a5 100644
--- a/Data-Structures/Queue/Queue.java
+++ b/Data-Structures/Queue/Queue.java
@@ -1,6 +1,6 @@
 import java.util.Scanner;
 
-class Queue {
+class Queue {
     private int[] arr;
     private int front;
     private int rear;
@@ -74,17 +74,36 @@ public boolean isEmpty() {
         return rear == -1;
     }
 
-    public boolean isFull() {
-        return rear == capacity - 1;
-    }
-
-    public static void main(String[] args) {
-        Scanner sc = new Scanner(System.in);
-
-        System.out.print("Enter Queue size: ");
-        int capacity = sc.nextInt();
-
-        Queue queue = new Queue(capacity);
+    public boolean isFull() {
+        return rear == capacity - 1;
+    }
+
+    private static int readInt(Scanner sc, String prompt) {
+        while (true) {
+            System.out.print(prompt);
+            if (sc.hasNextInt()) {
+                return sc.nextInt();
+            }
+            System.out.println("Invalid input. Please enter an integer.");
+            sc.next();
+        }
+    }
+
+    private static int readPositiveInt(Scanner sc, String prompt) {
+        while (true) {
+            int value = readInt(sc, prompt);
+            if (value > 0) {
+                return value;
+            }
+            System.out.println("Invalid input. Please enter an integer greater than 0.");
+        }
+    }
+
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        int capacity = readPositiveInt(sc, "Enter Queue size: ");
+
+        Queue queue = new Queue(capacity);
 
         while (true) {
             System.out.println("\n--- Queue Operations ---");
@@ -93,18 +112,15 @@ public static void main(String[] args) {
             System.out.println("3. Peek");
             System.out.println("4. Display");
             System.out.println("5. Is Full?");
-            System.out.println("6. Is Empty?");
-            System.out.println("7. Exit");
-            System.out.print("Enter your choice: ");
-
-            int choice = sc.nextInt();
-
-            switch (choice) {
-                case 1:
-                    System.out.print("Enter element to Enqueue: ");
-                    int element = sc.nextInt();
-                    queue.enqueue(element);
-                    break;
+            System.out.println("6. Is Empty?");
+            System.out.println("7. Exit");
+            int choice = readInt(sc, "Enter your choice: ");
+
+            switch (choice) {
+                case 1:
+                    int element = readInt(sc, "Enter element to Enqueue: ");
+                    queue.enqueue(element);
+                    break;
 
                 case 2:
                     queue.dequeue();
@@ -126,14 +142,14 @@ public static void main(String[] args) {
                     System.out.println(queue.isEmpty() ? "The Queue is Empty." : "The Queue is NOT Empty.");
                     break;
 
-                case 7:
-                    System.out.println("Exiting...");
-                    sc.close();
-                    return;
-
-                default:
-                    System.out.println("Invalid choice!");
-            }
-        }
-    }
-}
+                case 7:
+                    System.out.println("Exiting...");
+                    sc.close();
+                    return;
+
+                default:
+                    System.out.println("Invalid choice. Please enter a valid menu option.");
+            }
+        }
+    }
+}
diff --git a/Data-Structures/Stack.java b/Data-Structures/Stack.java
index a70ee29..d39a8e1 100644
--- a/Data-Structures/Stack.java
+++ b/Data-Structures/Stack.java
@@ -1,6 +1,6 @@
 import java.util.Scanner;
 
-class Stack {
+class Stack {
     private int[] arr;
     private int top;
     private int maxSize;
@@ -37,7 +37,7 @@ int pop() {
         return arr[top--];
     }
 
-    void display() {
+    void display() {
         if (isEmpty()) {
             System.out.println("Stack is Empty!");
             return;
@@ -46,34 +46,50 @@ void display() {
         for (int i = 0; i <= top; i++) {
             System.out.print(arr[i] + " ");
         }
-        System.out.println();
-    }
-
-    public static void main(String[] args) {
-        Scanner sc = new Scanner(System.in);
-
-        System.out.print("Enter the Stack size: ");
-        int size = sc.nextInt();
-
-        Stack stack = new Stack(size);
-
-        while (true) {
+        System.out.println();
+    }
+
+    private static int readInt(Scanner sc, String prompt) {
+        while (true) {
+            System.out.print(prompt);
+            if (sc.hasNextInt()) {
+                return sc.nextInt();
+            }
+            System.out.println("Invalid input. Please enter an integer.");
+            sc.next();
+        }
+    }
+
+    private static int readPositiveInt(Scanner sc, String prompt) {
+        while (true) {
+            int value = readInt(sc, prompt);
+            if (value > 0) {
+                return value;
+            }
+            System.out.println("Invalid input. Please enter an integer greater than 0.");
+        }
+    }
+
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        int size = readPositiveInt(sc, "Enter the Stack size: ");
+
+        Stack stack = new Stack(size);
+
+        while (true) {
             System.out.println("\n*** Stack Operations ***");
             System.out.println("1. Push");
             System.out.println("2. Pop");
-            System.out.println("3. Display");
-            System.out.println("4. Exit");
-            System.out.print("Enter your choice: ");
-
-            int choice = sc.nextInt();
-
-            switch (choice) {
-                case 1:
-                    System.out.print("Enter value to Push: ");
-                    int value = sc.nextInt();
-                    stack.push(value);
-                    break;
-                case 2:
+            System.out.println("3. Display");
+            System.out.println("4. Exit");
+            int choice = readInt(sc, "Enter your choice: ");
+
+            switch (choice) {
+                case 1:
+                    int value = readInt(sc, "Enter value to Push: ");
+                    stack.push(value);
+                    break;
+                case 2:
                     stack.pop();
                     break;
                 case 3:
@@ -81,11 +97,11 @@ public static void main(String[] args) {
                     break;
                 case 4:
                     System.out.println("Exiting...");
-                    sc.close();
-                    return;
-                default:
-                    System.out.println("Invalid choice!");
-            }
-        }
-    }
-}
+                    sc.close();
+                    return;
+                default:
+                    System.out.println("Invalid choice. Please enter a valid menu option.");
+            }
+        }
+    }
+}