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

Question

I've seen other questions about getting objects from Set's based on index value and I understand why that is not possible. But I haven't been able to find a good explanation for why a get by object is not allowed so thought I would ask.

HashSet is backed by a HashMap so getting an object from it should be pretty straightforward. As it is now, it appears I would have to iterate over each item in the HashSet and test for equality which seems unnecessary.

I could just use a Map but I have no need for a key:value pair, I just need a Set.

For example say I have Foo.java:

package example;

import java.io.Serializable;

public class Foo implements Serializable {

    String _id;
    String _description;

    public Foo(String id){
        this._id = id
    }

    public void setDescription(String description){
        this._description = description;
    }

    public String getDescription(){
        return this._description;
    }

    public boolean equals(Object obj) {
        //equals code, checks if id's are equal
    }

    public int hashCode() {
        //hash code calculation
    }

}

and Example.java:

package example;

import java.util.HashSet;

public class Example {

    public static void main(String[] args){
        HashSet<Foo> set = new HashSet<Foo>();

        Foo foo1 = new Foo("1");
        foo1.setDescription("Number 1");

        set.add(foo1);
        set.add(new Foo("2"));

        //I want to get the object stored in the Set, so I construct a object that is 'equal' to the one I want.
        Foo theFoo = set.get(new Foo("1")); //Is there a reason this is not allowed?
        System.out.println(theFoo.getDescription); //Should print Number 1
    }

}

Is it because the equals method is meant to test for "absolute" equality rather than "logical" equality (in which case contains(Object o) would be sufficient)?

Answer 1

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