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Message: [QUOTE="rossum, post: 2990928"] When you call any constructor, you need the "new" construction: S = new Stack(); This will not compile in your particular case because you have declared your Stack class to be abstract. Abstract classes cannot be instantiated, and the compiler will tell you so. One way to initialise the stack member variable in Function is to initialise it from a constructor parameter: public Function(Stack ss) { S = ss; } This will compile. Inside your program you could derive a non-abstract class from your abstract Stack and you can pass an instance of that derived class into your Function constructor: class ConcreteStack extends Stack { ... } ConcreteStack myCStack = new ConcreteStack(); Function myFunction = new Function(myCStack); rossum [/QUOTE]