Question

this is am learning java now. I just want to know whether should we write any comparator for comparing the list of objects with a particular object? And i also want to know why the particular line is not comparing the list

myList.contains(obj1)

import java.io.File;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

public class createItem {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String [] itemid = null;

        try{
            File itemFile =  new File("C:\\"+"Sano"+".xml");
            itemFile.createNewFile();

            List<ObjectIdName> myList = new ArrayList<ObjectIdName>();
            ObjectIdName obj1 = new ObjectIdName("ABC","ABC");

            myList.add(new ObjectIdName("DEF","DEF"));
            myList.add(new ObjectIdName("ABC","ABC"));
            if(myList.contains(obj1)){
                System.out.println("Has");

            }
            System.out.println("MyList:" + myList.size());

        }           
        catch(Exception e){
            e.printStackTrace();
        }
    }


}
class ObjectIdName implements Serializable{

    private static final long serialVersionUID = 1L;

    private String id;

    private String name ;

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

    }
}

Answer 1

By default, equals() compares the references:

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Since obj1 and the second object you insert into the list are two distinct objects, myList.contains(obj1) returns false.

To compare the fields, ObjectIdName has to implements its own equals() method.

Note that if you implement equals(), it is also a good practice to implement hashCode() (even if it’s not used by your code).