created classes, overridden methods and created an array

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

@@ -1,5 +1,50 @@
 package core.basesyntax;
 
+abstract class Machine {
+    public abstract void doWork();
+    public abstract void stopWork();
+}
+class Truck extends Machine {
+    @Override
+    public void doWork() {
+        System.out.println("Truck starts working");
+    }
+    @Override
+    public void stopWork() {
+        System.out.println("Truck stopped working");
+    }
+}
+class Bulldozer extends Machine {
+    @Override
+    public void doWork() {
+        System.out.println("Bulldozer starts working");
+    }
+    @Override
+    public void stopWork() {
+        System.out.println("Bulldozer stopped working");
+    }
+}
+class Excavator extends Machine {
+    @Override
+    public void doWork() {
+        System.out.println("Excavator starts working");
+    }
+    @Override
+    public void stopWork() {
+        System.out.println("Excavator stopped working");
+    }
+}
 public class MainApp {
+    public static void main(String[] args) {
+        Machine truck = new Truck();
+        Machine bulldozer = new Bulldozer();
+        Machine excavator = new Excavator();
 
-}
+        truck.doWork();
+        truck.stopWork();
+        bulldozer.doWork();
+        bulldozer.stopWork();
+        excavator.doWork();
+        excavator.stopWork();
+    }
+}