Check if two objects are Equal in Array List

Question

I have a main class that creates an arraylist of type Element:

public static void main(String[] args) throws IOException {
String input = "";
String id = ""; //combination of letters and numbers
String name = "";

ArrayList<Element> element = new ArrayList<> ();

BufferedReader in = new BufferedReader( new InputStreamReader(System.in));

while(!(input.equalsIgnoreCase("quit"))) {
    System.out.println("Please enter 'e' to enter an element, or 'quit' to quit");
    input = in.readLine();

if(input.equalsIgnoreCase("e")) {

    System.out.println("Please enter a name for the element");
    name = in.readLine();

    System.out.println("Please enter an id for the element");
    id = in.readLine(); 

    element.add(new Element(name,id));
    //only add if id and name don't exist already
}

}

}

Then I have a element clas:

public class Element {

private String name;
private String id;

public Element(String name, String id) {

this.name = name;
this.id = id;

}


}

I want to check before adding an element to a list (it's id and name), to check if another element already in the list already has those exact values (id and name). I know I can use the toString method to do this, but I'm not sure how I can override it to pass on an id and name, before adding the elements to the list. Is their a way to do this? Ideally I'd only want to add an element, if it doesn't already exist.

hagrawal7777 · Accepted Answer · 2015-10-10 21:55:57Z

2

Below overridden equals method will work.

public class Element {
    private String name;
    private String id;

    public Element(String name, String id) {
        this.name = name;
        this.id = id;
    }

    @Override
    public boolean equals(Object obj) {
        if(obj instanceof Element){
            Element element = (Element) obj;
            if(element != null && this.name.equals(element.name) && this.id.equals(element.id)){
                return true;
            }
        }
        return false;
    }
  }

And then try doing check before adding to your list, as below:

        ArrayList<Element> element = new ArrayList<Element> ();
        Element element1 = new Element("a", "1");
        Element element2 = new Element("b", "2");
        Element element3 = new Element("b", "2");
        element.add(element1);
        element.add(element2);
        if(element.contains(element3)){
            System.out.println("Yes");
        } else{
            System.out.println("No");
        }

Time for some concept:

Below is the contains implementation from Java:

Part which should concern you is this - o.equals(elementData[i]. Internally equals method of the object will be used to do equals check, if you have not overridden the equals method then the memory location of 2 objects will be compared, and since it would be different ArrayList.contains will return false, so by overriding the equals method we are establishing a logical condition on when to treat 2 Element object as same.

public boolean contains(Object o) {
    return indexOf(o) >= 0;
}

  public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

edited Oct 10, 2015 at 21:55

answered Oct 10, 2015 at 21:22

hagrawal7777

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13 Comments

A.Panzer Over a year ago

Your equals method does not follow the contract of equals method. It throws NPE for null parameter: For any non-null reference value x, x.equals(null) should return false

user3739406 Over a year ago

Just one question, how would I call the equals method in the main class? Something like this: element.equals(element)?

Mateusz Korwel Over a year ago

If you don't needed duplication in your collection, you should use Set instead of List.

hagrawal7777 Over a year ago

@A.Panzer I didn't get you, clearly, if you mean if(element.contains(null)){ will throw NPE then it will not because of this if (o == null) { condition in indexOf and there is a NULL in the list then contains will return true.

A.Panzer Over a year ago

@hagrawal Take a look at Javadoc of Object.equals method. There are rules for equals() method. This is no matter whether your code pass null or not. Element class might be reused. docs.oracle.com/javase/7/docs/api/java/lang/…

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Mateusz Korwel · Accepted Answer · 2015-10-10 22:06:19Z

2

You should use a Set. It is a data structure that does exactly what you want it to - it can't contain duplicates. In order for the set to work, you should have a correct implementation of the equals() and hashCode() methods of your class.

edited Oct 10, 2015 at 22:06

Mateusz Korwel

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answered Oct 10, 2015 at 21:17

Danail Alexiev

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4 Comments

user3739406 Over a year ago

I'm not allowed to use anything other than an arraylist.

Danail Alexiev Over a year ago

You should still override the equals() method and rely on the contains method of the ArrayList... But it will be grossly inefficient

hagrawal7777 Over a year ago

@DanailAlexiev Please specify which implementation of Set you are recommending. Why contains will be inefficient?

Mateusz Korwel Over a year ago

@hagrawal The Set implementation depend on what you needed. You can use a simple HashCode or if you what keep the order you can use LinkedHashSet.

A.Panzer · Accepted Answer · 2015-10-10 21:16:49Z

0

Override equals method of Element class. Use contains method of List to check whether element is already in the list.

answered Oct 10, 2015 at 21:16

A.Panzer

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