Java generics parameter bounding to any of a range of types

Question

Is there a syntax or workaround to constrain a generic type parameter to any of a range of types?

I am aware that you can constrain a type to be all

Answer 1

Using of instanceof is considered as not very good style of programming, and allowing you to use OR in generics implies you will use one.

Answer 2

No. It wouldn't make any sense unless all the types had a non-empty union type, e.g. an interface they all implemented, or a base class they all extended, in which case you just specify the union type.

Answer 3

public class MyWrapper<T extends A | B> {}

You can't do this for interfaces that you don't have control over, but for your own stuff you could use an empty marker interface:

interface AOrB {
}

interface A extends AOrB {
    someMethodHere();
}

interface B extends AOrB {
    someOtherMethodHere();
}

public class MyClass<T extends AOrB> {}

Regardless of what purists say, using instanceof is perfectly fine when you need it.

Answer 4

The following code would do the same thing as in the provided example, but without runtime type checking and typecasts.

public boolean sameAs(MyClass obj) {
    return this.id.equals(obj.id);
}
public boolean sameAs(String obj) {
    return this.id.equals(obj);
}

NPE checking might be a good idea.

Answer 5

While Java has limited support for "intersection types" like T1 & T2, support for "union types" is scanty.

Generic types with wildcard are actually union types: G<? extends X> is the union of all G<S> where S is a subtype of X.

No support for union of arbitrary two types.

Java 7's multi-catch syntax looks like it supports union of arbitrary exception types

catch (IOException|SQLException ex){ .. }

but not really, the type of ex is a fixed super class, not a union of the two classes.