Map<String,String> strMap = new HashMap<String,String>(); strMap.put("Key1","Value1"); strMap.put("Key2", "Value2"); String str = strMap.merge("Key4","Value56",(v1,v2)->v1.substring(3).concat(v2)); System.out.println(str); // prints Value56 System.out.println(strMap); // prints {Key2=Value2, Key1=Value1, Key4=Value56}