In my Java 21+ projects, I often have objects with multiple unique keys maps that I keep in multi-key maps like below. How can I build a multi-key map using the stream collector?

For example:

// id and symbol are guaranteed unique
record Foo(int id, String symbol) {};

then I create a multi-key map:

    var fooA = new Foo(1, "A");
    var fooB = new Foo(2, "B");
    var fooList = List.of(fooA, fooB);

    var fooMap = new HashMap();
    for (var foo : fooList) {
        fooMap.put(foo.id(), foo);
        fooMap.put(foo.symbol(), foo);
    }

    assertThat(fooMap.get(1)).isEqualTo(fooA);
    assertThat(fooMap.get("A")).isEqualTo(fooA);
    assertThat(fooMap.get(2)).isEqualTo(fooB);
    assertThat(fooMap.get("B")).isEqualTo(fooB);

What is the streams collector equivalent? Perhaps something like:

var fooMap = fooList.stream().collect(
    Collectors.flatMapping(???...)
    );

or with a custom multiKeyMap varargs helper:

var fooMap = fooList.stream().collect(
    multiKeyMap(Function.identity(), Foo::id, Foo::symbol)
    );