Java: Object as Value in HashMap being overwritten?

Question

I am adding/editing objects that are values in a hashmap-- with different keys.

However, editing one object in the hashmap seems to edit them all(?)

What am I doing wrong here?

First, my (poorly named) hashmap class:

import java.util.HashMap;
public class hashmap {

static HashMap<Integer, exObj> hm;

hashmap(){
    hm = new HashMap<Integer, exObj>();
}
public void createVal(){
    for (int i = 0; i<10; i++){
        hm.put(i, new exObj(i));
    }
    hm.get(2).setValue();
}
public void printVal(){
    for (int i = 0; i<10; i++){
        System.out.println(hm.get(i).getValue());
    }
}
public static void main(String args[]){
    hashmap hmap = new hashmap();
    hmap.createVal();
    hmap.printVal();    
}

}

Second, my simple exObj Class:

public class exObj {

private static int value;

exObj(int i){
    value = i;
}

public void setValue(){
    value = value + 1;
}

public int getValue(){
    return value; 
}

}

returns output:

Jeff Foster · Accepted Answer · 2013-04-02 07:43:37Z

5

You have static data which is shared amongst all instances of a class.

Try changing your class to have instance data (i.e. simply remove the static on private int value).

answered Apr 2, 2013 at 7:43

Jeff Foster

44.9k11 gold badges90 silver badges106 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Jesper Over a year ago

Since he is using exObj only as the value, not as the key, in the HashMap, it's not really necessary to override hashCode and equals.

Jesper · Accepted Answer · 2013-04-02 07:45:40Z

2

This is because value in your class exObj is static:

public class exObj {

private static int value;

Because it is static, there is only one copy of the variable, which is shared by all exObj objects.

See Understanding Instance and Class Members.

answered Apr 2, 2013 at 7:45

Jesper

208k47 gold badges325 silver badges361 bronze badges

2 Comments

quannabe Over a year ago

Got it--I didn't realize all of the objects shared static variables!

Tom Hawtin - tackline Over a year ago

@quannabe That's the point of static fields. It's best not to use them with mutable data.

yannicuLar · Accepted Answer · 2013-04-02 08:48:47Z

0

As @Jeff Foster and @Jesper said, it's all about using a static variable. Just to add some more info on your example, by running the code

for (int i = 0; i<10; i++){
    hm.put(i, new exObj(i));
}

the last exObj that gets initialized get the value '9', which is shared for all the instances. Then by calling

hm.get(2).setValue();

the value is set as '10'

You can simply declare the 'value' as

private int value;

so every instance of exObj will have it's own value. Also remember to have your Class names starting with a capital letter, as a good practice

edited Apr 2, 2013 at 8:48

answered Apr 2, 2013 at 8:36

yannicuLar

3,1333 gold badges35 silver badges52 bronze badges

Collectives™ on Stack Overflow

Java: Object as Value in HashMap being overwritten?

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related