Java Collections HashMap, value changes unexpectedly

Question

public class ListMap {
    HashMap<Integer, List> mp = new HashMap();
    List myList = new ArrayList();
    Integer x = 0;
    Integer y = 5;

    void test() {
    for(Integer i = 0; i < 5; i++) {
        x = y;
        myList.add("check-1a" + i);
        myList.add("check-1a" + i + 1);
        y = null;
        System.out.println(x); // output=5
        mp.put(i, myList);
        myList.clear();
    }
}

1) But after clearing the List with myList.clear() the values that was inside the Map also gets cleared.

I mean to say that the map key remains there but it contains an "empty" List

2) However regarding the Objects x & y, after setting y to null how come x doesn't change?

@David Conrad please elaborate what to use instead of that??? — SOP
– SOP, Commented Jan 22, 2015 at 18:12
You should use parameterized types, such as (in this case) HashMap<Integer, List<String>>, List<String>, and ArrayList<String>. — David Conrad
– David Conrad, Commented Jan 22, 2015 at 18:16

Philipp · Accepted Answer · 2015-01-22 18:01:50Z

When you add an object to a map (or any other collection), you are adding a reference to that object, not a copy. When you then make changes to the object, these changes will also affect the references in the map.

When you want to store a copy of your list, you need to create a new one. This can be done like this:

mp.put(i, new ArrayList(myList));

An alternative (and in my opinion even better) solution would be to reinitialize myList in the beginning of each loop iteration by setting it to a fresh list object:

myList = new ArrayList();
myList.add("check-1a"+i);
myList.add("check-1a"+i+1);
mp.put(i,myList);

Note that the list doesn't get destroyed when you reinitialize the variable myList. You can think of the object living on inside the map*.

_{*although a more technically accurate description would be "the object lives on in memory and isn't garbage-collected as long as there is still a reference to it stored in the map"}

David Conrad · Accepted Answer · 2015-01-22 18:31:54Z

1

You're adding the same list to the Map multiple times and clearing it each time, so no surprise that its empty. Solution: Don't clear the list, create a new one within the for loop. This way, the Map will hold a unique List for each Integer.

public class ListMap {
     HashMap<Integer,List<String>> mp=new HashMap<>();
     // List<String> myList=new ArrayList<String>(); // **** get rid of
    Integer x=0;
    Integer y=5;
    void test(){
        for(Integer i=0;i<5;i++){
            List<String> myList=new ArrayList<String>();  // ****** here 
            x=y;
            myList.add("check-1a"+i);
            myList.add("check-1a"+i+1);
            y=null;
            System.out.println(x);//output=5
            mp.put(i,myList);
            // myList.clear();  // **** get rid of
        }
   }

Also as per Tom, don't use raw types if possible, and so declare your lists as List<String> and ArrayList<String>.

edited Jan 22, 2015 at 18:31

David Conrad

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answered Jan 22, 2015 at 17:51

Hovercraft Full Of Eels

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8 Comments

SOP Over a year ago

I handled it but I am unable to understand why the values of List that I put inside the map prior to clearing the list ,clears the values inside the map as well

Hovercraft Full Of Eels Over a year ago

@ShashiRanjan: you handled what?? You're only creating one List and re-adding this single same cleared List multiple times. Don't do this. The key concept being that there is only one ArrayList object in all your original code.

SOP Over a year ago

and in similar way I used two objects X & Y and putting NULL inside the Y object then why X contains the previous values of Y ???

Tom Over a year ago

@HovercraftFullOfEels Don't do this. ... since you've mentioned that ... how about your usage of raw types (HashMap and List/ArrayList)? :P

Joffrey Over a year ago

@ShashiRanjan x and y in your code are just variables holding object references, not objects. When you write x=y, you're making x point to the same Integer object as y. Then, when you set y to null, you're setting that variable so that it points to no object. x still contains the reference to the object previously referenced by y.

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