How do I remove repeated elements (objects of myClass) from ArrayList

Question

I have an Arraylist

ArrayList<WorkPosition> info = new ArrayList<>();

where in some point, I add some objects. At the end this arrayList has the same object multiple times.. The object has some int values and 2 arraylists

97 [9, 10, 15] [1202, 1204, 2082]
97 [9, 10, 15] [1202, 1204, 2082]
97 [9, 10, 15] [1202, 1204, 2082]
42 [14, 16, 29] [404, 1081, 1201D]
42 [14, 16, 29] [404, 1081, 1201D]
42 [14, 16, 29] [404, 1081, 1201D]
57 [18, 30, 32] [2081, 435, 1032]
57 [18, 30, 32] [2081, 435, 1032]
57 [18, 30, 32] [2081, 435, 1032]
34 [19, 27, 28] [4501, 831, 201]
34 [19, 27, 28] [4501, 831, 201]
34 [19, 27, 28] [4501, 831, 201]

Is there any way to delete the extra appearances? I read this How do I remove repeated elements from ArrayList? but in my case I don't have Strings but objects type of WorkPosition..

public class WorkPosition {

    private int id;
    ArrayList<Integer> machines = new ArrayList<>();
    ArrayList<String> bottles= new ArrayList<>();


    public WorkPosition(int id, ArrayList<Integer> machines, ArrayList<String> bottles) {

        this.id = id;
        this.machines = machines;
        this.kaloupia = kaloupia;
    }

    //getters and setters...
}

you should give the WorkPosition source code, the key is overide equals & hashcode of that class — yelliver, Oct 13 '15 at 10:07

score 4 · Accepted Answer · answered Oct 13 '15 at 10:08

I read this How do I remove repeated elements from ArrayList? but in my case I don't have Strings but objects type of WorkPosition

Since the solution from the answer does not care if you use Strings or some other well-behaved objects, all you need to do is making your WorkPosition objects well-behaved from the point of view of the solution.

To that end, you have to override two specific methods - hashCode() and equals(Object), and make sure that they behave according to specification. Once this is done, the method of eliminating duplicates is going to work for your objects as well as it does for Strings.

class WorkPosition {
    @Override
    public boolean equals(Object obj) {
        // The implementation has to check that obj is WorkPosition,
        // and then compare the content of its attributes and arrays
        // to the corresponding elements of this object
        ...
    }
    @Override
    public int hashCode() {
        // The implementation needs to produce an int based on
        // the values set in object's fields and arrays.
        // The actual number does not matter too much, as long as
        // the same number is produced for objects that are equal.
        ...
    }
}

Laurentiu L. · Answer 2 · 2015-10-13T10:16:20.103

1

Just cast the ArrayList to a Set implementation. The Set collection contains no duplicate elements. Example:

 Set<WorkPosition> set = new HashSet<WorkPosition>(list);

All objects placed in a List, Set, or Map (as either a key or value) should have an appropriate definition of equals. See Collection.contains , Map.containsKey, and Map.containsValue.

Read more on the practice of implementing equals.

edited Oct 13 '15 at 10:16

answered Oct 13 '15 at 10:05

Laurentiu L.

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score 1 · Answer 3 · answered Oct 13 '15 at 10:07

You can convert your List to a Set.

ArrayList<WorkPosition> info = new ArrayList<>();
Set<WorkPosition> distincts = new HashSet<>(info);

This requires that the WorkPosition object overrides the equals(obj) method and hashCode(), like this:

public class WorkPosition {

   // your code

    @Override
    public boolean equals(Object obj) {
        // logic here
    }

    @Override
    public int hashCode() {
        // logic here
    }
}

How do I remove repeated elements (objects of myClass) from ArrayList

3 Answers3