Question

I have code that looks like the following:

private void MethodToDo(SpecialObject o) {
    Map<InfoObj, Integer> totalNeeds = new HashMap<InfoObj, Integer>();

    for (ListObject obj : o.getListOfObjects()) {
        InfoObj infoObj = new InfoObj(obj.getTitle(), obj.getId());
        Integer need = totalNeeds.get(infoObj);

        if (need == null) {
           need = new Integer(obj.getNeeded());
        } else {
           need = need + obj.getNeeded();
        }
        totalNeeds.put(infoObj, need); 
    }
}

The object is a private inner class (in the same class as that method) that looks like this:

private class InfoObj {
    private String  title;
    private Integer id;

    public InfoObj(String title, Integer id) {
        this.title = title;
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public Integer getId() {
        return id;
    }

    @Override 
    public boolean equals(Object io2) {
        if (this == io2) { return true; }
        if ( !(io2 instanceof InfoObj) ) { return false; }
        InfoObj temp = (InfoObj) io2;
        return this.id.equals(temp.id) && this.title.equals(temp.title);
    }

    @Override
    public int hashCode() {
        final int prime = 7;
        int result = 1;
        result = prime * result
                + ((this.title == null) ? 0 : this.title.hashCode());
        result = prime * result
                + ((this.id == null) ? 0 : this.id.hashCode());
        return result;
    }

However, despite overriding the equals and hashCode methods, the hashMap will still contain repeat keys (as in title and id are equivalent…but still show up in multiple places). I think I’m doing everything correctly, but realize I could be missing something…

Also, I know there are repeat keys because I loop through the keySet and output the results, which results in objects with the same title and id showing up multiple times.

Answer 1

Per your comment, a HashMap cannot contain the same keys per the implementation the same key would be:

(e.hash == hash && ((k = e.key) == key || key.equals(k)))

And since you are following the contract for equals and hashcode, any object you create here:

InfoObj infoObj = new InfoObj(obj.getTitle(), obj.getId());

With the same id and title, will be considered the same key, and if the map previously contained a mapping for the key, the old value is replaced.