java-virtual-field-pattern.md

Java Virtual Field Pattern

Source

This pattern can be used to add services to an existing class by using multiple inheritance and delegation. This approach is referenced as Virtual Field Pattern.

So lets start with definint our service interface.

interface Switchable {    boolean isActive();

    void setActive(boolean active);
}

We need an interface based on Switchable with an additional abstract method that returns an implementation of Switchable. The inherited methods get a default implementation: they use the previous getter to transmit the call to the Switchable implementation.

public interface SwitchableView extends Switchable {
    Switchable getSwitchable();
 
 
    boolean isActive() default { return getSwitchable().isActive(); }
    void setActive(boolean active) default { getSwitchable().setActive(active); }
}

Then, we create a complete implementation of Switchable.

public class SwitchableImpl implements Switchable {
 
 
    private boolean active;
 
 
    @Override
    public boolean isActive() {
        return active;
    }
 
 
    @Override
    public void setActive(boolean active) {
        this.active = active;
    }
}

Here is an example where we use the virtual field pattern.

public class Device {}
 
 
public class DeviceA extends Device implements SwitchableView {
    private Switchable switchable = new SwitchableImpl();
 
 
    @Override
    public Switchable getSwitchable() {
        return switchable;
    }
}
 
 
public class DeviceB extends Device implements SwitchableView {
    private Switchable switchable = new SwitchableImpl();
 
 
    @Override
    public Switchable getSwitchable() {
        return switchable;
    }
}

DeviceA a = new DeviceA();DeviceB b = new DeviceB();
 
a.setActive(true);
 
assertThat(a.isActive()).isTrue();
assertThat(b.isActive()).isFalse();