Design Patterns - Command

Design Patterns | Command

References
Example

References
- Definition: (source: http://en.wikipedia.org/wiki/Command_pattern)
  
  The Command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time.
- Class diagram: (source: http://en.wikipedia.org/wiki/Command_pattern)

Example

The Receiver:

public interface Payment {
    void apply();
}

The Concrete Receiver:

Implementation 1:

public class Debit implements Payment {
    public void apply() {
        System.out.println("debit ...");
    }
}

Implementation 2:

public class Credit implements Payment {
    public void apply() {
        System.out.println("credit ...");
    }
}

The Command:

public interface Transaction {
    void apply();
}

The Concrete Command:

public class TransactionImpl implements Transaction {
    private Payment payment;

    public TransactionImpl(final Payment payment) {
        this.payment = payment;
    }

    public void apply() {
        payment.apply();
    }
}

The Caller:

public class Operator {
    private Transaction transaction;

    public void setTransaction(final Transaction transaction) {
        this.transaction = transaction;
    }

    public void apply() {
        transaction.apply();
    }
}

A simple class to test the Command design pattern:

public class CommandPatternTest {
    public static void main(String[] args) {
        // create the payment
        Payment payment = new Debit();

        // create the transaction
        Transaction transaction = new TransactionImpl(payment);

        // create the operator
        Operator operator = new Operator();

        operator.setTransaction(transaction);

        // call methods
        operator.apply();
    }
}