Java creating a custom Map -

i have 2 questions regarding following code:

1) there way can implement map in case, rather extend 1 direct implementation of interface? not wish write whole map implementation though. nice seeing implementation not care underlying map implementation being used. 2) there bad practices?

extremesmap.java

package ocr.util;  import java.util.arrays; import java.util.hashmap; import java.util.hashset; import java.util.list; import java.util.set;  /**  *  * @author student  */ public class extremesmap extends hashmap<string, integer> {     private set<string> smallest;     private int smallestvalue;      private set<string> biggest;     private int biggestvalue;      public extremesmap() {         super();         smallest = new hashset<>();         smallestvalue = integer.max_value;         biggest = new hashset<>();         biggestvalue = integer.min_value;     }      @override     public void put(string key, integer value) {         if (value == null) {             throw new illegalargumentexception("ocr.util.extremesmap.put: value == null");         }         //todo real performance application implement own type of map directly         integer retrievevalue = super.get(key);         if (retrievevalue != null) {             throw new illegalstateexception("ocr.util.extremesmap.put: not allowed modify existing value: key = " + key);         }         else if (retrievevalue == value) {             return;         }         super.put(key, value);         if (value < smallestvalue) {             smallest = new hashset<>();             smallestvalue = value;         }         else if (value == smallestvalue) {             smallest.add(key);         }         if (value > biggestvalue) {             biggest = new hashset<>();             biggestvalue = value;         }         else if (value == biggestvalue) {             biggest.add(key);         }     }      public string getsmalleststring() {         if (smallest.size() != 1) {             throw new illegalstateexception("ocr.util.extremesmap.getsmallest: smallest.size() != 1: smallest.size() = " + smallest.size());         }         return smallest.iterator().next();     }      public set<string> getsmallestset() {         return smallest;     }      public list<string> getsmallestlist() {         return arrays.aslist(getsmallestarray());     }      public string[] getsmallestarray() {         return smallest.toarray(new string[smallest.size()]);     }      public string getbiggeststring() {         if (biggest.size() != 1) {             throw new illegalstateexception("ocr.util.extremesmap.getbiggest: biggest.size() != 1: biggest.size() = " + biggest.size());         }         return biggest.iterator().next();     }      public set<string> getbiggestset() {         return biggest;     }      public list<string> getbiggestlist() {         return arrays.aslist(getbiggestarray());     }      public string[] getbiggestarray() {         return biggest.toarray(new string[biggest.size()]);     } } 

also there 1 bug unable resolve in put():

method not override or implement method supertype

put(string,integer) in extremesmap cannot implement put(k,v) in map   return type void not compatible integer   k,v type-variables:     k extends object declared in interface map     v extends object declared in interface map 

what goes wrong here?

regards.

as error says, put signature is:

 v put(k key, v value) 

that means method put should return integer , not void:

public integer put(string key, integer value) { ... } 

concerning first 2 questions, not recommended practice extend hashmap, because changing way map working when override put. should better use composition instead (a normal class contains private field map internal use).

for whole code review of code, ask @ code review stackexchange.