src/main/java/dev/banking/model/BankAccount.java

@@ -35,6 +35,7 @@ public class BankAccount {
     public void deposit(long amount) {
         lock.writeLock().lock();
         try {
+            if (amount < 0) throw new IllegalArgumentException("Amount cannot be negative");
             balance += amount;
         }
         finally {
@@ -45,6 +46,8 @@ public class BankAccount {
     public void withdraw(long amount) {
         lock.writeLock().lock();
         try {
+            if (amount < 0) throw new IllegalArgumentException("Amount cannot be negative");
+            if  (balance < amount) throw new IllegalArgumentException("insufficient balance");
             balance -= amount;
         }
         finally {
@@ -61,6 +64,8 @@ public class BankAccount {
         try {
             second.lock.writeLock().lock();
             try {
+                if (amount < 0) throw new IllegalArgumentException("Amount cannot be negative");
+                if (amount > balance) throw new IllegalArgumentException("insufficient balance");
                 this.balance -= amount;
                 target.balance += amount;
             } finally {