how to declare such a java class?

Question

the following java class declaration is incorrect:

public class BookKeeping<T extends Transaction<K extends Money>> { ... }

here, one cannot use the generic declaration K extends Money .

my question is why this declaration is not allowed? And how should one declare such a class with java generics?

solution

class Money {

}

class Dollar extends Money {

}

class Transaction<T extends Money> {

}

public class BookKeeping<K extends Money,T extends Transaction<K>> {
    public void foo () {
        Dollar d = new Dollar();
        Transaction<Dollar> t = new Transaction<Dollar>();
        BookKeeping<Dollar, Transaction<Dollar>> b = new BookKeeping<Dollar, Transaction<Dollar>>(); 
    }
}

Answer 1

您需要指定通用参数和相应的约束，然后将它们用于任何其他参数。

public class BookKeeping<K extends Money, T extends Transaction<K>> { ... }

Answer 2

public class BookKeeping<K ,T extends Transaction<K extends Money>> { ... }

You need to "declare" the K generic before you can nested or use it. Its the same rule as local variables.