I have added changes

AliceAndBobEngine.java

@@ -10,6 +10,9 @@ public class AliceAndBobEngine {
      * @return `true` if `input` is "Alice"
      */
     public Boolean isAlice(String input) {
+        if (input != null){
+            return input.equals("Alice");
+        }
         return null;
     }
 
@@ -19,7 +22,10 @@ public Boolean isAlice(String input) {
      * @return `true` if `input` is "Bob"
      */
     public Boolean isBob(String input) {
-        return null;
+        if (input != null){
+            return input.equals("Bob");
+        }
+        return null;    
     }
 
     /**
@@ -28,6 +34,9 @@ public Boolean isBob(String input) {
      * @return `true` if `input` is "Alice" or "Bob"
      */
     public Boolean isAliceOrBob(String input) {
+        if (input != null){
+            return this.isAlice(input) || this.isBob(input);
+        }
         return null;
     }
 
@@ -42,6 +51,12 @@ public Boolean isAliceOrBob(String input) {
      * @return respective String value
      */
     public String getGreeting(String input) {
+        if(input != null){
+            if(this.isAliceOrBob(input)){
+                return "Hello, " + input + "!";
+            }
+            return "Begone, " + input + "! You're a stranger!";
+        }
         return null;
     }
 }