Why doesn't java.util.HashSet have a get(Object o) method?

Java Map/Collection Cheat Sheet

Will it contain key/value pair or values only?

1) If it contains pairs, the choice is a map. Is order important?

. 1-1) If yes, follow insertion order or sort by keys?

. . 1-1-1) If ordered, LinkedHashMap

. . 1-1-2) If sorted, TreeMap

. 1-2) If order is not important, HashMap

2) If it stores only values, the choice is a collection. Will it contain duplicates?

. 2-1) If yes, ArrayList

. 2-2) If it will not contain duplicates, is primary task searching for elements (contains/remove)?

. . 2-2-1) If no, ArrayList

. . 2-2-2) If yes, is order important?

. . . 2-2-2-1) If order is not important, HashSet

. . . 2-2-2-2) If yes, follow insertion order or sort by values?

. . . . 2-2-2-2-1) if ordered, LinkedHashSet

. . . . 2-2-2-2-2) if sorted, TreeSet

A Set is a Collection of objects which treats a.equals(b) == true as duplicates, so it doesn't make sense to try to get the same object you already have.

If you are trying to get(Object) from a collection, a Map is likely to be more appropriate.

What you should write is

Map<String, String> map = new LinkedHashMap<>();

map.put("1", "Number 1");
map.put("2", null);
String description = map.get("1");

if an object is not in the set (based on equals), add it, if it is in the set (based on equals) give me the set's instance of that object

In the unlikely event you need this you can use a Map.

Map<Bar, Bar> map = // LinkedHashMap or ConcurrentHashMap

Bar bar1 = new Bar(1);
map.put(bar1, bar1);

Bar bar1a = map.get(new Bar(1));

Your last sentence is the answer.

get(Object o) would run through the HashSet looking for another object being equal to o (using equals(o) method). So it is indeed the same as contains(o), only not returning the same result.