Overriding equals() & hashcode() in collections

[ramya narayanan]

Dear all,
What is the need for overriding equals() & hashcode() in Collections .
Normally every instance by default inherits Object class which has equals() & hashcode() . Then why we should override it?
Regards.

[Campbell Ritchie]

public class Name
{
  private final String name;
 
  public Name(String name)
  {
    this.name = name;
  }
 
  public String getName()
  {
    return name;
  }
 
/**
 * This method calls "super" and performs exactly the same as the Object class
 * method of the same name.
 */
  @Override public boolean equals(Object other)
  {
    return super.equals(other);
  }
}
 
public class NameDemo
{
  /**
   * Two command-line arguments required.
   */
  public static void main(String[] args)
  {
     Name name1 = new Name(args[0]);
     Name name2 = new Name(args[1]);
     System.out.printf("Name %s == Name %s: %b%n%n", name1.getName(), name2.getName(), name1.equals(name2));
  }
}

java NameDemo ramya ramya

See what comes out. You need to have two identical objects recoognised as identical to each other for the Collections to work.