First solution

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

@@ -0,0 +1,13 @@
+package core.basesyntax;
+
+public class Bulldozer extends Machine{
+    @Override
+    public void doWork() {
+        System.out.println("Bulldozer started working");
+    }
+
+    @Override
+    public void stopWork() {
+        System.out.println("Bulldozer stopped working");
+    }
+}

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

@@ -0,0 +1,13 @@
+package core.basesyntax;
+
+public class Excavator extends Machine{
+    @Override
+    public void doWork() {
+        System.out.println("Excavator started working");
+    }
+
+    @Override
+    public void stopWork() {
+        System.out.println("Excavator stopped working");
+    }
+}

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

@@ -0,0 +1,6 @@
+package core.basesyntax;
+
+public abstract class Machine {
+    public abstract void doWork();
+    public abstract void stopWork();
+}

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

@@ -1,5 +1,12 @@
 package core.basesyntax;
 
 public class MainApp {
+    public static void main(String[] args){
+        Machine[] machines = {new Truck(),new Excavator(), new Bulldozer()};
 
+        for (Machine machine : machines){
+            machine.doWork();
+            machine.stopWork();
+        }
+    }
 }

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

@@ -0,0 +1,13 @@
+package core.basesyntax;
+
+public class Truck extends Machine{
+    @Override
+    public void doWork() {
+        System.out.println("Truck started working");
+    }
+
+    @Override
+    public void stopWork() {
+        System.out.println("Truck stopped working");
+    }
+}