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Machine.java

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+abstract class Machine {
+    public abstract void doWork();
+
+    public abstract void stopWork();
+}
+

buldozer.java

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+class Buldozer extends Machine {
+    public void doWork() {
+        System.out.println("The Buldozer started working.");
+    }
+
+    public void stopWork() {
+        System.out.println("The Buldozer stopped working.");
+    }
+}
+

excavator.java

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+class Excavator extends Machine {
+    public void doWork() {
+        System.out.println("The Excavator started working.");
+    }
+
+    public void stopWork() {
+        System.out.println("The Excavator stopped working.");
+    }
+}

main.java

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+class Main {
+    public static void main(String[] args) {
+        Machine[] machines = {new Truck(), new Buldozer(), new Excavator()};
+        for (int i = 0; i < machines.length; i++) {
+            machines[i].doWork();
+            machines[i].stopWork();
+        }
+    }
+}

truck.java

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+class Truck extends Machine {
+    public void doWork() {
+        System.out.println("The Truck started working.");
+    }
+
+    public void stopWork() {
+        System.out.println("The Truck stopped working.");
+    }
+}