Comma or ampersand with multiple bounded types?

Question

In Java, are commas and ampersands both valid when declaring a multiply bounded type?

class MyClass <T extends OtherInterface, SomeInterface>

class MyOtherClass <T extends OtherInterface & SomeInterface>

Answer 1

As others have pointed out, this:

class MyOtherClass <T extends OtherInterface & SomeInterface>

defines a multiply bounded type parameter. If you use MyOtherClass, you must give it a type that implements both OtherInterface and SomeInterface.

However, this does not define a multiply bounded type parameter:

class MyClass <T extends OtherInterface, SomeInterface>

It defines a generic with two type parameters. The first one must implement OtherInterface. The second one can be anything. It's just the same as

class MyClass <T extends OtherInterface, U>

except that you named it SomeInterface instead of U. (The convention is that type parameters are normally single upper-case letters, or sometimes an upper-case letter and a digit or a short upper-case identifier. But the compiler doesn't care. It won't look at the form of the identifier to figure out that you really meant it as an interface.)

Answer 2

Multiple Bounds are possible with <T extends B1 & B2 & B3> where B2 and B3 should be interface. B1 can be simple class or interface