Phase 11 of 20 · Topic 11.1

Abstract Classes vs Interfaces (When to Use Which)

1Concept

Abstract classes provide a common base with state (instance fields) and partial implementation for closely related classes (IS-A relationship). Interfaces define pure behavioral contracts with no instance state for potentially unrelated classes (CAN-DO relationship). Since Java 8/9, interfaces can have default, static, and private methods.

2Architecture Diagram

Abstract Class (Base):  [ Shape ] (fields: color, x, y) ---> [ Circle ] (IS-A Shape)
Interface (Contract):   [ Serializable, Exportable ]        ---> [ Circle ] (CAN-DO Export)

3Code Example

Core Java
public class AbstractionDemo {
    abstract static class CloudResource {
        protected String resourceId;
        public CloudResource(String resourceId) { this.resourceId = resourceId; }
        public abstract void provision();
    }

    interface Monitorable {
        void collectMetrics();
    }

    static class VirtualMachine extends CloudResource implements Monitorable {
        public VirtualMachine(String id) { super(id); }

        @Override
        public void provision() {
            System.out.println("VM " + resourceId + " allocated on Hypervisor.");
        }

        @Override
        public void collectMetrics() {
            System.out.println("VM " + resourceId + " CPU utilization: 24%");
        }
    }

    public static void main(String[] args) {
        VirtualMachine vm = new VirtualMachine("i-098234fed");
        vm.provision();
        vm.collectMetrics();
    }
}

4Expected Output

VM i-098234fed allocated on Hypervisor.
VM i-098234fed CPU utilization: 24%

5Key Takeaways

  • ✓Use Abstract Classes when sharing state (fields) or protected helper methods.
  • ✓Use Interfaces to define loose contracts across unrelated hierarchies.
  • ✓A class can implement multiple interfaces, but extend only ONE class.