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nijan gouda

Dec 13, 2017

How to get the overridden values from map ??

Suppose if i have Map object I am adding. object in map.

As we know map doesn't allow duplicate keys?

Map<Integer, String> map=new HashMap<>();

map.put(1, "Test");

map.put(2, "Testing");

map.put(1, "Duplicate key");

we know the key(1) is overridden with new value "Duplicate",

How do we find all keys which is overridden ???

And please can you Guys how java handles collision in Map??

Thanks in advance.

#java

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stuff ;)

Dec 13, 2017

a map is a key value storage. there is no specific collision handling this is just a "at this key we want this value".

you cannot find the keys in that way, it does not work like that. this is basic memory handling.

a datastructure should be efficient at it's task. so adding a change vector tracking would break the SRP (single responsibility principle). Because mainly it should just store values and have a certain access behaviour.

you need to add a special structure around that tracks the change vectors. Even there you can have different ways of how changes are tracked -> sequencial, distinct.

does this help ?

Bill O'Neil

Feb 1, 2018

That's not entirely true, it's very easy to see if there was a collision in the map.

Map.put has a return value that is often ignored but it is useful in exactly this situation.

@return the previous value associated with <tt>key</tt>, or

<tt>null</tt> if there was no mapping for <tt>key</tt>.

(A <tt>null</tt> return can also indicate that the map

previously associated <tt>null</tt> with <tt>key</tt>,

if the implementation supports <tt>null</tt> values.)

So to detect a collision all you have to do is see if map.put returns a non null value. You can then do something with the previous value if you want. You could also just check to see if the map contains a value before you put to detect it yourself.

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How to get the overridden values from map ??

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