Handling Null Pointer Exception for Owner ID

src/main/java/org/springframework/samples/petclinic/owner/PetController.java

@@ -56,13 +56,28 @@ public Collection<PetType> populatePetTypes() {
 
 	@ModelAttribute("owner")
 	public Owner findOwner(@PathVariable("ownerId") int ownerId) {
-		return this.owners.findById(ownerId);
+
+		if (ownerId <= 0) {
+			throw new IllegalArgumentException("Owner ID is required");
+		}
+		Owner owner = this.owners.findById(ownerId);
+		if (owner == null) {
+			throw new IllegalArgumentException("Owner ID not found: " + ownerId);
+		}
+		return owner;
 	}
 
 	@ModelAttribute("pet")
 	public Pet findPet(@PathVariable("ownerId") int ownerId,
 			@PathVariable(name = "petId", required = false) Integer petId) {
-		return petId == null ? new Pet() : this.owners.findById(ownerId).getPet(petId);
+		if (ownerId <= 0) {
+			throw new IllegalArgumentException("Owner ID is required");
+		}
+		Owner owner = this.owners.findById(ownerId);
+		if (owner == null) {
+			throw new IllegalArgumentException("Owner ID not found: " + ownerId);
+		}
+		return petId == null ? new Pet() : owner.getPet(petId);
 	}
 
 	@InitBinder("owner")