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From: Lew <lewbloch@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Collection to implement linked structure traverse up and down
Date: Mon, 5 Dec 2011 15:08:29 -0800 (PST)
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ruds wrote:
> Roedy Green: Can you elaborate some more on "a HashMap an ArrayList
> and a TreeSet simultaneously all on the same group of objects."

Consider this fragment:

 Map<Foo, Bar> bars = new HashMap<>();
 List<Bar> barVals = new ArrayList<>();
 Set<Foo> foos = new TreeSet<>();
...
 Foo foo = new Foo();
 Bar bar = new Bar();
 bars.put(foo, bar);
 barVals.add(bar);
 foos.add(foo);
...

A pointer to the same 'foo' appears in 'bars' and in 'foos'.  A pointer to the same 'bar' appears in 'bars' and in 'barVals'.

Do that again with a new 'foo' and a new 'bar' a few times and you have "a group of objects" in all three collections simultaneously.

-- 
Lew