Why does the contains() method in Java not work as expected?

Question

I am writing a method that creates an ArrayList S_Comp that contains all elements which are included in the array allNodes, but which are not included in the ArrayList solution (whose elements are all included in allNodes). When I print solution and allNodes, I can easily see that S_Comp should be containing 7 elements. However, all elements from allNodes are added to S_Comp, also the elements which are included in nodesS. What could be the problem here?

Below you can find my code for the method. Before calling this method, S_Comp has only been initialized as new ArrayList<MyNodesData>, so its size is 0.

public void generateSComp(DataFile testDataFile, ArrayList<Route> solution, ArrayList<MyNodesData> S_Comp)
    {
        System.out.println("solution: " + solution.toString());
        ArrayList<MyNodesData> nodesS = getNodesS(solution);
        System.out.println("NodesS: " + nodesS.toString());
        System.out.println("Size nodesS: " + nodesS.size());
        MyNodesData[] allNodes = testDataFile.getNodes();
        System.out.println("allNodes: " + Arrays.toString(allNodes));
        for(MyNodesData node : allNodes)
        {
            if(!nodesS.contains(node))
            {
                System.out.println(node.toShortString() + " is not in nodesS");
                S_Comp.add(node);
            }
        }

@Michal it does not, but .contains() used to work properly on MyNodesData earlier — Mike
– Mike, Commented Jun 5, 2018 at 13:44
@Mike Oh god, with our 3 names this is going to be so confusing... Without overriding equals, contains is using reference equality. If the objects are distinct instances with the same fields then contains will return false. — Michael
– Michael, Commented Jun 5, 2018 at 13:46
without overriding equals, equals method() compares references (take a look into Object.class). So as long as you work with the same objects it works. If you create new objects with same attribute values it doesn't work anymore — Michal
– Michal, Commented Jun 5, 2018 at 13:46

Shubham Kadlag · Accepted Answer · 2018-06-05 14:13:03Z

2

Refer here for the contains method of ArrayList javadoc here: https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html#contains-java.lang.Object-

It says

public boolean contains(Object o)

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

So it will work only if you override equals method of Object Class as you want in MyNodesData class. Moreover as you may already be knowing the contract between equals() and hashcode() method, it is highly recommended to override hashcode() along with equals().

For more information : Why do I need to override the equals and hashCode methods in Java?

edited Jun 5, 2018 at 14:13

answered Jun 5, 2018 at 14:05

Shubham Kadlag

2,3281 gold badge16 silver badges33 bronze badges

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

Comments

vishal ms · Accepted Answer · 2018-06-05 13:51:02Z

0

override hashcode and equals method in MyNodesData class and then check.

answered Jun 5, 2018 at 13:51

vishal ms

443 bronze badges

Comments

A. Llorente · Accepted Answer · 2018-06-05 13:52:51Z

0

contains works just as expected, it will check if the object you pass equals the object in the list. So if you are not getting the expected behavior is because those objects do not equal.

You will have to debug that code to check why the equals is not returning what you expect.

Edit: I read that you are not overriding equals and hashcode in MyNodesData class, so you should do it as other people suggest

edited Jun 5, 2018 at 13:52

answered Jun 5, 2018 at 13:45

A. Llorente

1,1826 silver badges18 bronze badges

Collectives™ on Stack Overflow

Why does the contains() method in Java not work as expected?

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related