¿Por qué se actualiza este mapa? (JAVA)

Question

¿Por qué se actualiza este mapa? (JAVA)

I have been looking through java's api of Map for possible reason why a certain Map (map1) in my code gets updated as well when I update another map (map2) or maybe something is wrong about how I wrote it.

void process(Object superObject) {

      Map<Date, Object> map1 = superObject.getValuesForMap();
      Map<Date, Object> map2 = map1;

      updateValueOf(superObject,map2);
}

This is how I updated the value of map2.

updateValueOfMap(Object superObject,Map<Date, Object> map2){

      List<Object> objects = getTheObjectsFromASource;
      for (Object obj : objects) {
         List<Triple<Date, Double, Object>> triples = superObject.getSomeEntriesWithThisAttribute(obj.getCertainAttrib());
         for (Triple<D,D,O> t : triples) {
            Object cache = map2.get(t.first)
            if (cache == null) {
               cache = new Object();
               cache.setThis(t.second);
               cache.setThat(t.third);
            } else {
               Double value = cache.getThis() + t.second; // add the double value from triple to the current cache Object's value
               cache.setThis(value);                      // and update the Object's value in the map
            }
            map2.put(t.first, cache);
         }
  }

}

The problem is certain entries in superObject.getValuesForMap() gets updated too with the same value as the corresponding entries in map2 every iteration in the for (Triple..). Why is that so? Responses will be greatly appreciated. Thanks in advance!

preguntado el 28 de mayo de 14 a las 11:05

eLementary

1 Respuestas

No es la respuesta que estás buscando? Examinar otras preguntas etiquetadas dictionary hashmap or haz tu propia pregunta.

Puneetsri · Accepted Answer

Map map1 = superObject.getValuesForMap(); Map map2 = map1;

All three above , points to the same memory location, so it will indeed be updated.

Prueba de esta manera:

Map map2 = new HashMap();

map2.putAll(map1);

UPDATE : Sample program below(map1 not updated with MAP2 changes.)

public class BaseClass {

Map<String,String> xx = new HashMap<String,String>();

public BaseClass(){
    xx.put("1", "One");
    xx.put("2", "Two");
    xx.put("3", "Three");
}

public Map<String,String> getValuesForMap(){
    return xx;
}

}

public class TestProgram extends BaseClass{

void process() {

      Map<String, String> map1 = getValuesForMap();
      Map<String, String> map2 = new HashMap<String,String>();
          map2.putAll(map1);
      updateValueOf(map1, map2);
}

public void updateValueOf(Map<String, String> map1, Map<String, String> map2){
    String str1 = map2.get("1");
    str1 = str1+"Item";
    map2.put("1", str1);

    String str2 = map2.get("2");
    str2 = str2+"Item";
    map2.put("2", str2);

    String str3 = map2.get("3");
    str3 = str3+"Item";
    map2.put("3", str3);

    System.out.println("Printing Map1 ");
    printit(map1);
    System.out.println("Printing Map2 ");
    printit(map2);
    System.out.println("Printing Map1 Again");
    printit(map1);
    System.out.println("Printing Map2 Again");
    printit(map2);
}


public void printit(Map<String,String> map){
    Iterator iter = map.entrySet().iterator();
        while (iter.hasNext()) {
        Map.Entry pairs = (Map.Entry)iter.next();
        System.out.println(pairs.getKey() + " = " + pairs.getValue());
    }
}

public static void main(String[] args){
    TestProgram ts = new TestProgram();
    ts.process();
}

}