What happens when a duplicate key is put into a HashMap?
If I pass the same key multiple times to HashMap 's put method, what happens to the original value? And what if even the value repeats? I didn't find any documentation on this.
Case 1: Overwritten values for a key
Map mymap = new HashMap();
mymap.put("1","one");
mymap.put("1","not one");
mymap.put("1","surely not one");
System.out.println(mymap.get("1"));
We get surely not one .
Case 2: Duplicate value
Map mymap = new HashMap();
mymap.put("1","one");
mymap.put("1","not one");
mymap.put("1","surely not one");
// The following line was added:
mymap.put("1","one");
System.out.println(mymap.get("1"));
We get one .
But what happens to the other values? I was teaching basics to a student and I was asked this. Is the Map like a bucket where the last value is referenced (but in memory)?
By definition, the put command replaces the previous value associated with the given key in the map (conceptually like an array indexing operation for primitive types).
The map simply drops its reference to the value. If nothing else holds a reference to the object, that object becomes eligible for garbage collection. Additionally, Java returns any previous value associated with the given key (or null if none present), so you can determine what was there and maintain a reference if necessary.
More information here: HashMap Doc
You may find your answer in the javadoc of Map#put(K, V) (which actually returns something):
public V put(K key,
V value)
Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for this key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true .)
Parameters:
key - key with which the specified value is to be associated.
value - value to be associated with the specified key.
Returns:
previous value associated with specified key, or null if there was no mapping for key . (A null return can also indicate that the map previously associated null with the specified key , if the implementation supports null values.)
So if you don't assign the returned value when calling mymap.put("1", "a string") , it just becomes unreferenced and thus eligible for garbage collection.
The prior value for the key is dropped and replaced with the new one.
If you'd like to keep all the values a key is given, you might consider implementing something like this:
import org.apache.commons.collections.MultiHashMap;
import java.util.Set;
import java.util.Map;
import java.util.Iterator;
import java.util.List;
public class MultiMapExample {
public static void main(String[] args) {
MultiHashMap mp=new MultiHashMap();
mp.put("a", 10);
mp.put("a", 11);
mp.put("a", 12);
mp.put("b", 13);
mp.put("c", 14);
mp.put("e", 15);
List list = null;
Set set = mp.entrySet();
Iterator i = set.iterator();
while(i.hasNext()) {
Map.Entry me = (Map.Entry)i.next();
list=(List)mp.get(me.getKey());
for(int j=0;j<list.size();j++)
{
System.out.println(me.getKey()+": value :"+list.get(j));
}
}
}
}
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