Question

Multiple generic interface separator: “,” or “&”

I was always using A<T extends I1, I2> but today, I saw A<T extends I1 & I2>.
What is the difference between these two notation?

Does it have a different meaning? Purpose?

Answer 1

A<T extends I1, I2>

is a type A with two parameters

1. T which must be a subtype of I1
2. I2

You can read this as “A <preposition> type T which extends I1 <preposition> type I2”,
so SortedMap<Key extends comparable, Value> is a “map from comparable type key to type value”

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A<T extends I1 & I2>

is a type A with a single parameter

1. T which must be a subtype of I1 and a subtype of I2

You can read this as “A <preposition> type T which extends I1 and I2” so a SortedSerializableSet<T extends Serializable & Comparable> can be read “a sorted set of elements of type T which is both serializable and comparable”