equals() and contains()

To support Collection's contains() method, we should override Object's equals() method.

Object

+ equals(Object) : boolean
+ hashCode() : int
+ toString() : String
...more, of course

Implementations of the equals() method should have these properties:

x.equals(x) should return true (reflexive).
x.equals(y) should return true if and only if
y.equals(x) returns true (symmetric).
If x.equals(y) returns true
and y.equals(z) returns true,
then x.equals(z) should return true (transitive).
x.equals(null) should return false.
The default implementation of equals returns true if and only if x and y refer to the same object (x == y).

public class Book {
    String title;
    int    isbn;

    Book(String title, int isbn) {
        this.title = title;
        this.isbn  = isbn;
    }

    public boolean equals(Object object) {
        if (object == null) {
            return false;
        }
        if (object == this) {
            return true;
        }
        if (object.getClass() == this.getClass()) {
            Book other = (Book) object;
            return this.title.equals(other.title) 
               && (this.isbn == other.isbn);
        }
        return false;
    }
}

import java.util.*;

public class TestCollections {
    public static void main(String[] args) {
        List list = new LinkedList();
        list.add(new Book("Patterns", 12345));
        list.add(new Book("Examples", 23456));

        Book patterns = new Book("Patterns", 12345);

        System.out.println("We have it: " 
                           + list.contains(patterns));
    }

}