Fixes: #176 (This issue has been closed earlier but wasn't resolved). Now migrated to null safety, upgraded dart sdk, tested all the changes.

conversions/Decimal_To_Any.dart

@@ -49,7 +49,7 @@ String decimalToAny(int value, int base) {
     int remainder = value % base;
     value = value ~/ base;
     output =
-        (remainder < 10 ? remainder.toString() : ALPHABET_VALUES[remainder]) +
+        (remainder < 10 ? remainder.toString() : ALPHABET_VALUES[remainder])! +
             output;
   }
 

conversions/Decimal_To_Binary.dart

@@ -13,6 +13,7 @@ void bitwiseConversion(var n) {
   print("Bitwise conversion.\n");
   while (n != 0) {
     d = (n & 1);
+
     b += d * (pow(10, c++).toInt());
     n >>= 1;
   }

conversions/Decimal_to_Hexadecimal.dart

@@ -23,15 +23,15 @@ String decimal_to_hexadecimal(int decimal_val) {
   }
   String hex_string = "";
   while (decimal_val > 0) {
-    String hex_val = "";
+    String? hex_val = "";
     int remainder = decimal_val % 16;
     decimal_val = decimal_val ~/ 16;
     if (hex_table.containsKey(remainder.toString())) {
       hex_val = hex_table[remainder.toString()];
     } else {
       hex_val = remainder.toString();
     }
-    hex_string = hex_val + hex_string;
+    hex_string = hex_val! + hex_string;
   }
   return negative ? "-" + hex_string : hex_string;
 }

conversions/Integer_To_Roman.dart

@@ -39,7 +39,7 @@ List<String> RomanNumbers = [
   "I"
 ];
 
-List<String> integer_to_roman(int num) {
+List<String>? integer_to_roman(int num) {
   if (num < 0) {
     return null;
   }

conversions/binary_to_decimal.dart

@@ -4,7 +4,7 @@ import 'package:test/test.dart';
 
 int binaryToDecimal(String binaryString) {
   binaryString = binaryString.trim();
-  if (binaryString == null || binaryString == "") {
+  if (binaryString == "") {
     throw FormatException("An empty value was passed to the function");
   }
   bool isNegative = binaryString[0] == "-";
@@ -14,8 +14,8 @@ int binaryToDecimal(String binaryString) {
     if ("01".contains(binaryString[i]) == false) {
       throw FormatException("Non-binary value was passed to the function");
     } else {
-      decimalValue +=
-          pow(2, binaryString.length - i - 1) * int.parse((binaryString[i]));
+      decimalValue += pow(2, binaryString.length - i - 1) *
+          int.parse((binaryString[i])) as int;
     }
   }
   return isNegative ? -1 * decimalValue : decimalValue;

conversions/binary_to_hexadecimal.dart

@@ -24,7 +24,7 @@ Map<String, String> hexTable = {
 String binaryToHexadecimal(String binaryString) {
   // checking for unexpected values
   binaryString = binaryString.trim();
-  if (binaryString == null || binaryString == "") {
+  if (binaryString == "") {
     throw new FormatException("An empty value was passed to the function");
   }
   try {
@@ -47,7 +47,7 @@ String binaryToHexadecimal(String binaryString) {
   int i = 0;
   while (i != binaryString.length) {
     String bin_curr = binaryString.substring(i, i + 4);
-    hexValue += hexTable[bin_curr];
+    hexValue += hexTable[bin_curr]!;
     i += 4;
   }
 

conversions/binary_to_octal.dart

@@ -21,7 +21,7 @@ void main() {
 
 String binaryToOctal(String binaryString) {
   binaryString = binaryString.trim();
-  if (binaryString == null || binaryString == "") {
+  if (binaryString == "") {
     throw new FormatException("An empty value was passed to the function");
   }
   bool isNegative = binaryString[0] == "-";

conversions/hexadecimal_to_binary.dart

@@ -24,7 +24,7 @@ Map<String, String> bin_table = {
 String hexadecimal_to_binary(String hex_value) {
   // checking for unexpected values
   hex_value = hex_value.trim();
-  if (hex_value == null || hex_value == "") {
+  if (hex_value == "") {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -40,7 +40,7 @@ String hexadecimal_to_binary(String hex_value) {
     if (!bin_table.containsKey(hex_cur)) {
       throw new FormatException("An invalid value was passed to the function");
     }
-    bin_val += bin_table[hex_cur];
+    bin_val += bin_table[hex_cur]!;
     i++;
   }
 

conversions/hexadecimal_to_decimal.dart

@@ -1,3 +1,5 @@
+// ignore_for_file: deprecated_member_use
+
 import "dart:math" show pow;
 
 Map<String, int> hex_table = {
@@ -21,19 +23,22 @@ void main() {
 
 int hexadecimal_to_decimal(String hex_string) {
   hex_string = hex_string.trim().toUpperCase();
-  if (hex_string == null || hex_string == "") {
+  if (hex_string == "") {
     throw Exception("An empty value was passed to the function");
   }
   bool is_negative = hex_string[0] == "-";
   if (is_negative) hex_string = hex_string.substring(1);
   int decimal_val = 0;
   for (int i = 0; i < hex_string.length; i++) {
-    if (int.parse(hex_string[i], onError: (e) => null) == null &&
+    if (int.parse(hex_string[i],
+                onError: ((e) => null) as int Function(String)?) ==
+            null &&
         hex_table.containsKey(hex_string[i]) == false) {
       throw Exception("Non-hex value was passed to the function");
     } else {
       decimal_val += pow(16, hex_string.length - i - 1) *
-          int.parse((hex_string[i]), onError: (e) => hex_table[hex_string[i]]);
+          int.parse((hex_string[i]),
+              onError: (e) => hex_table[hex_string[i]]!) as int;
     }
   }
   return is_negative ? -1 * decimal_val : decimal_val;

conversions/hexadecimal_to_octal.dart

@@ -1,12 +1,13 @@
 import "dart:math" show pow;
+
 import 'package:test/test.dart';
 
 // Hexadecimal number to octal number conversion program
 String hexadecimal_to_octal(String hex_val) {
   int dec = 0;
 
   // checking for null string passed to function
-  if (hex_val == null || hex_val == "") {
+  if (hex_val == "") {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -29,43 +30,42 @@ String hexadecimal_to_octal(String hex_val) {
       case '7':
       case '8':
       case '9':
-        dec = dec + int.parse(ch) * pow(16, c);
+        dec = dec + int.parse(ch) * (pow(16, c) as int);
         c--;
         break;
       case 'a':
       case 'A':
-        dec = dec + 10 * pow(16, c);
+        dec = dec + 10 * (pow(16, c) as int);
         c--;
         break;
       case 'b':
       case 'B':
-        dec = dec + 11 * pow(16, c);
+        dec = dec + 11 * (pow(16, c) as int);
         c--;
         break;
       case 'c':
       case 'C':
-        dec = dec + 12 * pow(16, c);
+        dec = dec + 12 * (pow(16, c) as int);
         c--;
         break;
       case 'd':
       case 'D':
-        dec = dec + 13 * pow(16, c);
+        dec = dec + 13 * (pow(16, c) as int);
         c--;
         break;
       case 'e':
       case 'E':
-        dec = dec + 14 * pow(16, c);
+        dec = dec + 14 * (pow(16, c) as int);
         c--;
         break;
       case 'f':
       case 'F':
-        dec = dec + 15 * pow(16, c);
+        dec = dec + 15 * (pow(16, c) as int);
         c--;
         break;
       default:
         throw new FormatException(
             "An invalid value was passed to the function");
-        break;
     }
   }
 

conversions/octal_to_binary.dart

@@ -1,11 +1,12 @@
 import "dart:math" show pow;
+
 import 'package:test/test.dart';
 
 // octal number to binary number conversion
 String ocatal_to_binary(String oct_val) {
   // checking for unexpected values
   oct_val = oct_val.trim();
-  if (oct_val == null || oct_val == "") {
+  if (oct_val == "") {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -31,7 +32,7 @@ String ocatal_to_binary(String oct_val) {
   // converting octal to decimal
   int dec_val = 0, i = 0, bin_val = 0;
   while (oct != 0) {
-    dec_val = dec_val + ((oct % 10) * pow(8, i));
+    dec_val = dec_val + ((oct % 10) * (pow(8, i) as int));
     i++;
     oct = oct ~/ 10;
   }

conversions/octal_to_decimal.dart

@@ -1,4 +1,5 @@
 import "dart:math" show pow;
+
 import 'package:test/test.dart';
 
 // octal number to decimal number conversion
@@ -7,7 +8,7 @@ import 'package:test/test.dart';
 String ocatal_to_decimal(String oct_val) {
   // checking for unexpected values
   oct_val = oct_val.trim();
-  if (oct_val == null || oct_val == "") {
+  if (oct_val == "") {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -33,7 +34,7 @@ String ocatal_to_decimal(String oct_val) {
   // converting octal to decimal
   int dec_val = 0, i = 0;
   while (oct != 0) {
-    dec_val = dec_val + ((oct % 10) * pow(8, i));
+    dec_val = dec_val + ((oct % 10) * (pow(8, i) as int));
     i++;
     oct = oct ~/ 10;
   }

conversions/octal_to_hexadecimal.dart

@@ -1,4 +1,5 @@
 import "dart:math" show pow;
+
 import 'package:test/test.dart';
 
 // octal number to hex number
@@ -15,7 +16,7 @@ Map<String, String> hex_table = {
 String ocatal_to_hex(String oct_val) {
   // checking for unexpected values
   oct_val = oct_val.trim();
-  if (oct_val == null || oct_val == "") {
+  if (oct_val == "") {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -41,7 +42,7 @@ String ocatal_to_hex(String oct_val) {
   // converting octal to decimal
   int dec_val = 0, i = 0;
   while (oct != 0) {
-    dec_val = dec_val + ((oct % 10) * pow(8, i));
+    dec_val = dec_val + ((oct % 10) * (pow(8, i) as int));
     i++;
     oct = oct ~/ 10;
   }
@@ -52,15 +53,15 @@ String ocatal_to_hex(String oct_val) {
   }
   String hex_string = "";
   while (dec_val > 0) {
-    String hex_val = "";
+    String? hex_val = "";
     int remainder = dec_val % 16;
     dec_val = dec_val ~/ 16;
     if (hex_table.containsKey(remainder.toString())) {
       hex_val = hex_table[remainder.toString()];
     } else {
       hex_val = remainder.toString();
     }
-    hex_string = hex_val + hex_string;
+    hex_string = hex_val! + hex_string;
   }
 
   // returning the value

data_structures/HashMap/Hashing.dart

@@ -2,8 +2,8 @@
 //Email:[email protected]
 
 class Node {
-  int data;
-  Node next;
+  int? data;
+  Node? next;
 
   Node(int data) {
     this.data = data;
@@ -12,8 +12,8 @@ class Node {
 }
 
 class LinkedList {
-  Node head;
-  int size;
+  Node? head;
+  int? size;
 
   LinkedList() {
     head = null;
@@ -23,7 +23,7 @@ class LinkedList {
   void insert(int data) {
     Node newnode = new Node(data);
 
-    size++;
+    if (size != null) size = size! + 1;
 
     if (head == null) {
       head = newnode;
@@ -38,15 +38,16 @@ class LinkedList {
       print("underFlow!");
       return;
     } else {
-      Node curr = head;
+      Node curr = head!;
       if (curr.data == data) {
         head = curr.next;
-        size--;
+
+        if (size != null) size = size! - 1;
         return;
       } else {
-        while (curr.next.next != null) {
-          if (curr.next.data == data) {
-            curr.next = curr.next.next;
+        while (curr.next!.next != null) {
+          if (curr.next!.data == data) {
+            curr.next = curr.next!.next;
             return;
           }
         }
@@ -56,7 +57,7 @@ class LinkedList {
   }
 
   void display() {
-    Node temp = head;
+    Node? temp = head;
     while (temp != null) {
       print(temp.data.toString());
       temp = temp.next;
@@ -66,11 +67,11 @@ class LinkedList {
 }
 
 class HashMap {
-  int hsize;
-  List<LinkedList> buckets;
+  late int hsize;
+  late List<LinkedList?> buckets = [];
 
   HashMap(int hsize) {
-    buckets = new List<LinkedList>(hsize);
+    buckets = new List.filled(hsize, null);
     for (int i = 0; i < hsize; i++) {
       buckets[i] = new LinkedList();
     }
@@ -87,18 +88,18 @@ class HashMap {
 
   void insertHash(int key) {
     int hash = hashing(key);
-    buckets[hash].insert(key);
+    buckets[hash]!.insert(key);
   }
 
   void deleteHash(int key) {
     int hash = hashing(key);
-    buckets[hash].delete(key);
+    buckets[hash]!.delete(key);
   }
 
   void displayHashtable() {
     for (int i = 0; i < hsize; i++) {
       print("Bucket $i:");
-      buckets[i].display();
+      buckets[i]!.display();
     }
   }
 }