The CallBack coordination style differs from the Master Worker style with respect to only one aspect, i.e., the block of business logic written in a CallBack method call is executed only if the result of the callback method call is non erroneous.

Syntax:

	Callback(agent | component.methodName(param_list); variable of 
                                                           method’s return type)
        {
	   // business logic
        }

where,

• agent and component are instances of type Agent and Component

Unlike the Broadcast, Blackboard/Agenda or Parallel Pipe styles of coordination, using Callback does not identify the responder's role and id since invoking a single, and not multiple or arrays of, agent or component instances is possible in Callback.

public Agent CallbackAgent
{
	public void setValue(int x);
	public int getValue();
	public void incrValue();
}

Definition file : CallbackAgent.indus

public class CallbackClass implements CallbackAgent
{
	Context{
		setServantType("USER");
	}

	public int m_value = 0;

	public int getValue()
	{
		return m_value;
	}

	public void setValue(int v)
	{
		m_value = v;
	}

	public void incrValue()
	{
		m_value++;
	}

	public static void main(String [] args)
	{
		try 
		{
			CallbackClass m = newType CallbackClass("cb1");
			int value = 0;
			Callback(m.getValue();value)
			{
				System.out.println("value = " + value);
			}

			Callback(m.setValue(100);) {}
			Callback(m.getValue();value)
			{
				System.out.println("value = " + value);
			}

			Callback(m.incrValue();) {}
			Callback(m.getValue();value)
			{
				System.out.println("value = " + value);
			}

			Callback(m.incrValue();) {}
			Callback(m.getValue();value)
			{
				System.out.println("value = " + value);
			}
		}
		catch (Exception e) 
		{
			e.printStackTrace();
		}	
	}
}

Implementation file : CallbackClass.indus

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