How to write equals and hashCode methods in java for attribute that is a list of other attributes

Question

I have a HashMap where the key is a class and value is an integer. I need to check if an object of the class already exists in the map. I use containsKey() , but for some reason it does not work when I include attribute sideDish in the equals() and hashCode() . Here is my code for the classes:

OrderItem class:

@ToString
@Entity
@Table(name="OrderItem")
public class OrderItem implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Getter @Setter 
    private Long id;

    @ManyToOne
    @Getter @Setter 
    private Food food;

    @ManyToMany
    @Getter @Setter 
    private List<SideDish> sideDishes;

    public OrderItem() {}

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((food == null) ? 0 : food.hashCode());
        result = prime * result + ((sideDishes == null) ? 0 : sideDishes.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        OrderItem other = (OrderItem) obj;
        if (food == null) {
            if (other.food != null)
                return false;
        } else if (!food.equals(other.food))
            return false;
        if (sideDishes == null) {
            if (other.sideDishes != null)
                return false;
        } else if (!sideDishes.equals(other.sideDishes))
            return false;
        return true;
    }
}

Food class:

@ToString
@Entity
@Table(name="Food")
public class Food implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Getter @Setter 
    private Long id;

    @Column(nullable = false, unique = true)
    @NotNull(message = "Name cannot be null.")
    @Getter @Setter 
    private String name;

    @ManyToMany
    @Getter @Setter
    private List<SideDish> sidedishes;

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((foodtype == null) ? 0 : foodtype.hashCode());
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Food other = (Food) obj;
        if (foodtype == null) {
            if (other.foodtype != null)
                return false;
        } else if (!foodtype.equals(other.foodtype))
            return false;
        if (id == null) {
            if (other.id != null)
                return false;
        } else if (!id.equals(other.id))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
}

SideDish class:

@Entity
@ToString(exclude= {"id","dishtype"})
@Table(name="SideDish")
public class SideDish implements Serializable, Comparable<SideDish>{

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Getter @Setter
    private Long id;

    @Getter @Setter
    @Column(nullable = false, unique = true)
    private String name;

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        SideDish other = (SideDish) obj;
        if (id == null) {
            if (other.id != null)
                return false;
        } else if (!id.equals(other.id))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
}

For some reason, if I removethe sideDish attribute from equals() and hashCode() in the OrderItem class, it works perfectly.

But I also need sideDish to be checked as part of the object identity.

Here is how I use it:

HashMap<OrderItem, Integer> orderItemsToSend = new HashMap<OrderItem, Integer>();

for (Order order : orders) {
    for (OrderItem orderItem : order.getOrderItems()) {
        int numSimilarOrders = getNumOfSimilarOrders(orderItem, orders);
        if(!orderItemsToSend.containsKey(orderItem)) {
            orderItemsToSend.put(orderItem, numSimilarOrders);
        }else {
            System.out.println("Vec je dodat item koji isti kao: " + orderItem.getFood().getName());
        }
    }
}

Answer 1

In your OrderItem class, both your hashCode() and equals() depend on the property List<SideDish> sideDishes .

Thus, if sideDishes changes, so does the hashCode() (and so does equality).

---

A HashMap uses both hashCode() and equals() to store and find the object which is the key . It uses a concept called " hash buckets ". If you put a key into a HashMap , and then the hashCode() changes, that object will be in the wrong hash bucket, and you won't be able to find it again.

A key is something which is used for lookup purposes - that's what the word "key" means. An important quality of a key, whether in a database, or a hashmap, is immutability. So in Java, that means an object which changes its hashCode() makes for a bad key.

It's a bit like if a file system did lookups by the hash of the filename, but then you changed the filename, but it didn't update the hash. You'd only find the file by doing a lookup with the old name.

---

This simple test program will illustrate the point.

We store 2 objects in a HashMap , and then change the hashCode() . The map still contains both objects, but now one of them cannot be found or used for lookup.

The solution is use some simple immutable object as the key , such as a Long of its database ID.

Sample output is below the code.

public class HashTest {

   static class Hashable {
     String name;

    @Override
    public int hashCode() {
      return ((name == null) ? 0 : name.hashCode());
    }

    @Override
    public boolean equals(Object object) {
      return (object instanceof Hashable) && equals((Hashable) object); 
    }

    private boolean equals(Hashable that) {
      return Objects.equals(this.name, that.name);
    }

    @Override
    public String toString() {
      // Use identityHashCode() so we can really see which object is which
      return "[" + name + ":" + System.identityHashCode(this) + "]";
    }
   }

   public static void main(String[] args) {
     Hashable one = new Hashable();
     one.name = "one";
     Hashable two = new Hashable();
     two.name = "one";

     print(one, two);
     two.name = "two";
     print(one, two);

     HashMap<Hashable, Integer> map = new HashMap<>();

     map.put(one, 1);
     map.put(two, 2);
     find(map, one, two);

     one.name = "two"; // Let's confuse things
     print(one, two);
     find(map, one, two);
   }

   private static void print(Hashable one, Hashable two) {
     System.out.print("Names:" + one.name + ":" + two.name);
     System.out.print("\tHashcodes:" + one.hashCode() + ":" + two.hashCode());
     System.out.println("\tEquals:" + one.equals(two));
   }

   private static void find(HashMap<Hashable, Integer> map, Hashable one, Hashable two) {
     System.out.print(map);
     System.out.print("\tFound: " + map.get(one));
     System.out.println("\tFound: " + map.get(two));
   }
}

---

Sample output:

Names:one:one   Hashcodes:110182:110182 Equals:true
Names:one:two   Hashcodes:110182:115276 Equals:false
{[one:366712642]=1, [two:1829164700]=2} Found: 1    Found: 2
Names:two:two   Hashcodes:115276:115276 Equals:true
{[two:366712642]=1, [two:1829164700]=2} Found: 2    Found: 2