Let's say I have a hashmap. HashMap<String,String> hm = new HashMap<String,String>(); Now I put some values in this map as below. hm.put("A","First"); hm.put("B","First");
After this I again put some values for the already stored key "A".
hm.put("A","Second");
Now, if I try to get the value of "A" , I'll get "Second" as it's value.
sysout(hm.get("A"));
Output Second
Is there any way to get previous value i.e. "First" ?
Help is much appreciated.
No, you can however use a list as value. So it becomes:
Map<String, List<String>> map = ...; And then you can add or remove elements from the list to be able to retrieve previous values.
map.get(key).add(value); This will add to the list in the map and when searching the most recent value, you can use
List<String> list = map.get(key); String value = list.get(list.size()-1); to get the previous value you can assign the return value of put method to string
hm.put("A","First"); String temp = hm.put("A","Second"); System.out.println(temp); // prints first As far as I know you could do it in 2 ways :
Check if the value returned by the old key returns you a value (i.e null or not). Now you can capture that value before putting a new value to the said key.
An illustration :
HashMap<String, String> hmap = new HashMap<>(); hmap.put("A", "First"); if (hmap.get("A") != null) { String oldValue = hmap.get("A"); hmap.put("A", "Second"); } We know that the put() method returns the value of the previous key
Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.
@return the previous value associated with key
So you could do :
String oldValue = hmap.put("A", "Second"); 
hmap.get("A") gives me "First" after alteration (hm.put("A","Second");)First and then it gets replaced with Second. I suggest you to try it out.
println statement as mentioned or maybe store it elsewhere in some other List etc. So +1 from me.Well there are many Opportunities to achieve this, the question is how deep do you want save the values. Do you need also the first value after 10 put operations? Do you need it sorted?
You can achieve this in an unsorted way via a HashTable , see Java HashTable: https://docs.oracle.com/javase/8/docs/api/java/util/Hashtable.html
Hashtable<Key, Element> table; A sorted way could be, using a Stack or something :
HashMap <Key,Stack<Element>> values; https://docs.oracle.com/javase/7/docs/api/java/util/Stack.html
So you can access the first element like this
values.get("A").peek(); If you have to access random Elements in the Stack, use LinkedList for this:
HashMap<Key,LinkedList<Element>> values; values.get("A").getFirst(); values.get("A").getLast(); Be sure that you override always the hashCode() and equals() Method, if you use something like HashMap's.
A normal map does not work that way. In a normal map the old value is overwritten and gone.
There is however a map-like data structure which saves a collection of values that has been put into the map: the multi-map.
Example:
ListMultimap<String, String> m = ArrayListMultimap.create(); m.put("A", "First"); m.put("A", "Second"); for (String s : m.get("A")) { System.out.println(s); } // Prints: // First // Second Multi-maps are very convenient to work with in a lot of situations, and they are well-worth learning about.
The example code here uses a multi-map from the Guava library. It can be downloaded from the Maven Repository.
put. If it'snull, there was nothing mapped to that key. If the result is not null, there you have the previous value associated with the key.putstatements, i.e.hm.put("A","First"); hm.put("B","First");? Is there any relationship between those two mappings and the desire to get"First"as result? Do you want to get it because"A"formerly mapped to it or because"B"has been associated with it beforehm.put("A","Second");? Or is the whole thing a trick question about the actual meaning of “previous”?