Fixed Fibonacci series implementation in Java

fixme.java

@@ -1,11 +1,9 @@
-public class FibonacciChallenge {
+public class fixme {
 
     public static void main(String[] args) {
-        // Fix me: Should we allow negative numbers as input?
         System.out.println("Fibonacci(5) = " + fibonacci(5));
         System.out.println("Fibonacci(10) = " + fibonacci(10));
 
-        // Fix me: What happens with this test case?
         System.out.println("First 8 Fibonacci numbers:");
         printFibonacciSequence(8);
 
@@ -14,34 +12,35 @@ public static void main(String[] args) {
     }
 
     public static int fibonacci(int n) {
-        // Fix me: Remember, Fibonacci sequence starts with 0, 1, ...
+        if (n < 0) {
+            throw new IllegalArgumentException("Input must be a non-negative integer.");
+        }
+        if (n == 0) return 0;
+        if (n == 1) return 1;
 
         return fibonacci(n - 1) + fibonacci(n - 2);
     }
 
     public static void printFibonacciSequence(int n) {
-        // Fix me: Off-by-one error?
-        for (int i = 1; i <= n; i++) {
+        for (int i = 0; i < n; i++) { 
             System.out.print(fibonacci(i) + " ");
         }
         System.out.println();
     }
 
     public static int efficientFibonacci(int n) {
-        // Fix me: How can you avoid recalculating the same Fibonacci numbers repeatedly?
-        int a = 0;
-        int b = 1;
-
-        if (n == 0) {
-            return a;
+        if (n < 0) {
+            throw new IllegalArgumentException("Input must be a non-negative integer.");
         }
+        if (n == 0) return 0;
+        if (n == 1) return 1;
 
-        for (int i = 2; i < n; i++) {
-            int temp = a;
+        int a = 0, b = 1;
+        for (int i = 2; i <= n; i++) { 
+            int temp = a + b;
             a = b;
-            b = temp + a;
+            b = temp;
         }
-
         return b;
     }
-}
+}