Java Language Strengthen bounded type parameters


Example

Bounded type parameters allow you to set restrictions on generic type arguments:

class SomeClass {

}

class Demo<T extends SomeClass> {

}

But a type parameter can only bind to a single class type.

An interface type can be bound to a type that already had a binding. This is achieved using the & symbol:

interface SomeInterface {

}

class GenericClass<T extends SomeClass & SomeInterface> {

}

This strengthens the bind, potentially requiring type arguments to derive from multiple types.

Multiple interface types can be bound to a type parameter:

class Demo<T extends SomeClass & FirstInterface & SecondInterface> {

}

But should be used with caution. Multiple interface bindings is usually a sign of a code smell, suggesting that a new type should be created which acts as an adapter for the other types:

interface NewInterface extends FirstInterface, SecondInterface {

}

class Demo<T extends SomeClass & NewInterface> {

}