Hashset Containg duplicate values ; Also ArrayList indexOf(string_present_in_list) returning -1

Question

I have made a Hashset of StringBuilder.

The below code returns "true" even if that StringBuilder is present in Hashset:

if(!contains(sb_obj))
{
...
}

Also I have made a ArrayList of StringBuilder.

The below code returns "-1" even if ArrayList contains obj:

if(arr.indexOf(obj)==-1)
{
....
}

Why such behaviour?

Oleg · Accepted Answer · 2017-08-10T16:17:38.617

0

I assume you are comparing different StringBuilder objects that have the same String values.

StringBuilder doesn't override equals and hashCode methods. This causes HashSet and ArrayList to use Object's implementation of them which compares by identity. Since you are comparing different objects they are different.

edited Aug 10 '17 at 16:17

answered Aug 10 '17 at 16:07

Oleg

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I understood that but can you elaborate a little on how to write the override equals method. – ttripdee Aug 10 '17 at 16:10
Yes your assumption is exactly correct. Thats what happening. Please help – ttripdee Aug 10 '17 at 16:11
@ttripdee why do you need to use `StringBuilder`? Can't you use `String`? It's a final class you can't extend it and override equals and hashCode. – Oleg Aug 10 '17 at 16:14
100s of Upvotes Thank you so much!! – ttripdee Aug 10 '17 at 16:28

score 0 · Answer 2 · answered Aug 10 '17 at 16:13

Please use String instead of storing StringBuilder object or if you want to store custom object override hashcode and equals methods whenever you are trying to add and search back that object.

Please read the concept of hascode and equals method, you can find many examples over internet.

Hashset Containg duplicate values ; Also ArrayList indexOf(string_present_in_list) returning -1

2 Answers2