I have created hash map and when I debug it I saw that I have duplicte keys.
I didnt override the hashCode() & equals(Object obj) in the key - Object1
and i wonder how will it affect the performance of the map search?
private HashMap<Object1,Object2> map = new HashMap<Object1,Object2>();
It is not possible to have duplicate keys in a Map, you have different keys that "appear" the same (Maybe based on their toString()? )because you have not overridden equals() and hashCode(), but in reality the keys are different.
This means that in order to get all values from your Map you need to keep every key you created and store it somewhere, which to me defeats the purpose of the Map.
Summary:
Override equals() and hashCode(), then put your key/value pairs into the Map.
MyObject("A", "B") != MyObject("A", "B") if you are only using Object.equals(), but their toString() calls probably look identical. I added a line in response to your comment.HashMap certainly doesn't think they look alike at all ;) The underlying problem here is that the OP doesn't understand that his HashMap uses the same criteria for Object comparison as anything else which your answer covers.It won't really give you bad performances (more the opposite) but it will prevent your objects from being seen as duplicate.
If you want each instance to be seen as a different key, don't override the equals and hashCode methods. But that means you'll need exactly the same instance to retrieve your value in the map.
If you want to retrieve the value with a different instance (for example with the same id), then you need to override the methods.
But the problem isn't really a performance one.
How can you have duplicate keys? You didn't override equals() neither hashcode() so you cannot say "duplicated objects", because that's exactly the purpose of theses methods. You probably debug and saw different values, but for the JVM they are not equals based on the default Object.equals() and Object.hashcode() (well actually the closest superclass)
