I am designing a UI framework in Java. Every UI component, represented by the class Component, in the framework is identified by a non null unchangeable String key. So I get the key in the constructor itself and provided no setter.

Now I have another class AggregateComponent which represents a component containing array of components. In the AggregateComponent class a Factory is used to create the child components. For some reasons the key of the child components should be parentComponent.key + index, where index is the array index of the child component. So I pass parentComponent.key + index to Factory.create().

The contract is the Factory implementer should construct the child Component using the passed key.

We may enforce this contract in AggregateComponent by checking the key of the created component against the passed key and throw an exception. But I am asking

Is there any solution (even a completely different design solution) without such contracts.