Java Hashmap two identical objects stored seperate

Question

I overrided the hashCode and equals method of a class. I also wrote unit tests for it and it's all green. When I tested it out with a hashmap, I noticed something weird. I created 2 identical objects:

obj1 = new PacmanVeld(field2);
obj2 = new PacmanVeld(field2);

I tested it out with this piece of code:

Assert.assertTrue(obj1.hashCode() == obj2.hashCode()); //works
Assert.assertTrue(obj1.equals(obj2)); //works

HashMap<PacmanVeld, Integer> testMap = new HashMap<>();
testMap.put(obj1, 5);

Assert.assertTrue(testMap.put(obj2, 7) == 5); //fails throws nullpointerexception
Assert.assertTrue(testMap.get(obj1) == 7); //fails

I don't understand why this won't work as I understand that in the algorithms of a HashMap, obj1 and obj2 are the same objects.

The PacmanVeld class:

public class PacmanVeld
{
    private Node[][] nodes;

    public PacmanVeld(char[][] veld)
    {
        this.nodes = new Node[veld.length][veld[0].length];

        for (int i = 0; i < veld.length; i++)
        {
            for (int j = 0; j < veld[i].length; j++)
            {
                switch (veld[i][j])
                {
                    case '%':
                        nodes[i][j] = new Node(i, j, NodeType.WALL);
                        break;
                    case ' ':
                        nodes[i][j] = new Node(i, j, NodeType.EMPTY);
                        break;
                    case '.':
                        nodes[i][j] = new Node(i, j, NodeType.CRUMB);
                        break;
                    case 'P':
                        nodes[i][j] = new Node(i, j, NodeType.PACMAN);
                        break;
                }
            }
        }
        initFinish();
        initPacman();
    }

    //getters, setters & methods

    public boolean equals(PacmanVeld p)
    {
        if (p.nodes.length != nodes.length) { return false; }
        for (int i = 0; i < nodes.length; i++)
        {
            if (!Arrays.deepEquals(nodes[i], p.nodes[i])) { return false; }
        }
        return true;
    }

    @Override
    public int hashCode()
    {
        List<Node> nodeList = getNodeList();
        return Arrays.deepHashCode(nodeList.toArray());
    }

    private void initPacman()
    {
        for (Node[] rij : this.nodes)
        {
            for (Node n : rij)
            {
                if (n.isPacman())
                {
                    pacman = n;
                }
            }
        }
    }
}

And include the return from your two testMap.put assertions calls. — eebbesen
– eebbesen, Commented Nov 30, 2013 at 16:50
How are you getting a NPE on testMap.put(obj2, 7) == 5? put can't throw an exception according to the docs. What value is null exactly? — Jeroen Vannevel
– Jeroen Vannevel, Commented Nov 30, 2013 at 17:00

Alexis C. · Accepted Answer · 2013-11-30 17:45:40Z

3

The problem is that you didn't override the equals() method herited from the object class.

@Override
public boolean equals(Object o)
    {
        PacmanVeld p = (PacmanVeld)o;
        if (p.nodes.length != nodes.length) { return false; }
        for (int i = 0; i < nodes.length; i++)
        {
            if (!Arrays.deepEquals(nodes[i], p.nodes[i])) { return false; }
        }
        return true;
    }

answered Nov 30, 2013 at 17:45

Alexis C.

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1 Comment

Jeroen Vannevel Over a year ago

As we thought: wrong HashCode or Equals implementation :)

Jeroen Vannevel · Accepted Answer · 2013-11-30 17:30:32Z

0

I wasn't able to reproduce your error. Since it is too long to post as a comment, I will add it as an answer and edit my post to explain your mistake once you have showed your HashCode and Equals implementation.

public class Test {
    public static void main(String[] args) {
        Map<MyPersonalClass, Integer> map = new HashMap<>();
        MyPersonalClass obj1 = new MyPersonalClass();
        obj1.someInt = 5;
        obj1.someString = "test";

        MyPersonalClass obj2 = new MyPersonalClass();
        obj2.someInt = 5;
        obj2.someString = "test";

        System.out.println(obj1.equals(obj2));
        System.out.println(obj1.hashCode() == obj2.hashCode());

        map.put(obj1, 5);
        System.out.println(Arrays.toString(map.values().toArray()));

        System.out.println(map.put(obj2, 10) == 5);
        System.out.println(Arrays.toString(map.values().toArray()));
        System.out.println(map.get(obj1));
    }
}

class MyPersonalClass {
    public String someString;
    public int someInt;

    @Override
    public boolean equals(Object arg0) {
        if(arg0 == this) { return true; }
        if(!(arg0 instanceof MyPersonalClass)) { return false; }

        MyPersonalClass obj = (MyPersonalClass) arg0;

        return obj.someString.equals(this.someString) && obj.someInt == this.someInt;
    }

    @Override
    public int hashCode() {
        return this.someString.hashCode() * 37 + this.someInt;
    }
}

Output:

true
true
[5]
true
[10]
10

edited Nov 30, 2013 at 17:30

answered Nov 30, 2013 at 17:22

Jeroen Vannevel

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1 Comment

Jeroen Vannevel Over a year ago

@Masud: that's the entire idea behind the question. Quote from the question "I created 2 identical objects" and Assert.assertTrue(obj1.equals(obj2)); //works

A J A Y · Accepted Answer · 2020-02-14 14:19:36Z

0

HashMap return null for new value. Your Assert.assertTrue(testMap.put(obj2, 7) == 5); test case throws NPE exception because, obj2 is absent in HashMap. You can test with obj1 which already put on HashMap

  Assert.assertTrue(testMap.put(obj2, 7) == 5); 
                  //throws NPE because obj2 is not inserted

Try, with obj1

  Assert.assertTrue(testMap.put(obj1, 7) == 5); 
          //will retrun true because, 5 is last inserted value with obj1
  Assert.assertTrue(testMap.put(obj1, 7) == 8); 
         // will retrun false because

edited Feb 14, 2020 at 14:19

A J A Y

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answered Nov 30, 2013 at 16:47

Masudul

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9 Comments

wardva Over a year ago

Yes of course, that's true. But The first assertion throws a nullpointer exception so that's not the only problem I think...

Jeroen Vannevel Over a year ago

I don't believe this to be the issue. This works perfectly fine: Map<String, Integer> map = new HashMap<>(); map.put("test", 7); System.out.println(map.put("test", 9) == 7);

Jeroen Vannevel Over a year ago

@AmirAfghani: Integer caches until 127. It still works with 700 & 999.

Sage Over a year ago

@AmirAfghani, The comparison with two Integer reference might be an issue if any one of them has value higher than 127. But comparison with direct constant expression(plain integer) will not be, because it will automatically unbox the Integerreference to int before the comparison. Masud's answer is wrong

Jeroen Vannevel Over a year ago

I strongly doubt your edit is the source of the issue as well. The entire idea behind the question is that obj1 and obj2 are the same and thus it shouldn't matter which one is put in. I suspect a wrongly implemented Equals/Hashcode instead.

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