First solution.

src/main/java/core/basesyntax/Bulldozer.java

@@ -0,0 +1,14 @@
+package core.basesyntax;
+
+public class Bulldozer extends Machine {
+
+    @Override
+    public void doWork() {
+        System.out.println("The Bulldozer is working.");
+    }
+
+    @Override
+    public void stopWork() {
+        System.out.println("The Bulldozer has stopped.");
+    }
+}

src/main/java/core/basesyntax/Excavator.java

@@ -0,0 +1,14 @@
+package core.basesyntax;
+
+public class Excavator extends Machine {
+
+    @Override
+    public void doWork() {
+        System.out.println("The Excavator is working.");
+    }
+
+    @Override
+    public void stopWork() {
+        System.out.println("The Excavator has stopped.");
+    }
+}

src/main/java/core/basesyntax/Machine.java

@@ -0,0 +1,7 @@
+package core.basesyntax;
+
+public abstract class Machine {
+
+    public abstract void doWork();
+    public abstract void stopWork();
+}

src/main/java/core/basesyntax/MainApp.java

@@ -1,5 +1,16 @@
 package core.basesyntax;
 
 public class MainApp {
+    public static void main(String[] args) {
+        Machine[] machines = new Machine[3];
 
+        machines[0] = new Truck();
+        machines[1] = new Bulldozer();
+        machines[2] = new Excavator();
+
+        for (Machine machine : machines) {
+            machine.doWork();
+            machine.stopWork();
+        }
+    }
 }

src/main/java/core/basesyntax/Truck.java

@@ -0,0 +1,14 @@
+package core.basesyntax;
+
+public class Truck extends Machine {
+
+    @Override
+    public void doWork() {
+        System.out.println("The Truck is working.");
+    }
+
+    @Override
+    public void stopWork() {
+        System.out.println("The Truck has stopped.");
+    }
+}