Similarity of objects

Let's revise the equals method used to compare objects, and become familiar with the hashCode method used in making approximate comparisons.

Method to Test For Equality - "equals"

This can be illustrated with the following example. The Book class does not have its own implementation of the equals method, so it falls back on the default implementation provided by Java.

Book bookObject = new Book("Book object", 2000, "...");
Book anotherBookObject = bookObject;

if (bookObject.equals(anotherBookObject)) {
    System.out.println("The books are the same");
} else {
    System.out.println("The books aren't the same");
}

// we now create an object with the same content that's nonetheless its own object
anotherBookObject = new Book("Book object", 2000, "...");

if (bookObject.equals(anotherBookObject)) {
    System.out.println("The books are the same");
} else {
    System.out.println("The books aren't the same");
}

The internal structure of the book objects (i.e., the values of their instance variables ) in the previous example is the same, but only the first comparison prints "The books are the same". This is because the references are the same in the first case, i.e., the object is compared to itself. The second comparison is about two different entities, even though the variables have the same values.

For strings, equals works as expected in that it declares two strings identical in content to be 'equal' even if they are two separate objects. The String class has replaced the default equals with its own implementation.

If we want to compare our own classes using the equals method, then it must be defined inside the class. The method created accepts an Object-type reference as a parameter, which can be any object. The comparison first looks at the references. This is followed by checking the parameter object's type with the instanceof operation - if the object type does not match the type of our class, the object cannot be the same. We then create a version of the object that is of the same type as our class, after which the object variables are compared against each other.

public boolean equals(Object comparedObject) {
    // if the variables are located in the same place, they're the same
    if (this == comparedObject) {
        return true;
    }

   // if comparedObject is not of type Book, the objects aren't the same
   if (!(comparedObject instanceof Book)) {
        return false;
    }

    // let's convert the object to a Book-olioksi
    Book comparedBook = (Book) comparedObject;

    // if the instance variables of the objects are the same, so are the objects
    if (this.name.equals(comparedBook.name) &&
        this.published == comparedBook.published &&
        this.content.equals(comparedBook.content)) {
        return true;
    }

    // otherwise, the objects aren't the same
    return false;
}

The Book class in its entirety.

public class Book {
    private String name;
    private String content;
    private int published;

    public Book(String name, int published, String content) {
        this.name = name;
        this.published = published;
        this.content = content;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getPublished() {
        return this.published;
    }

    public void setPublished(int published) {
        this.published = published;
    }

    public String getContent() {
        return this.content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public String toString() {
        return "Name: " + this.name + " (" + this.published +   ")\n"
            + "Content: " + this.content;
    }

    @Override
    public boolean equals(Object comparedObject) {
        // if the variables are located in the same place, they're the same
        if (this == comparedObject) {
            return true;
        }

        // if comparedObject is not of type Book, the objects aren't the same
        if (!(comparedObject instanceof Book)) {
            return false;
        }

        // let's convert the object to a Book-object
        Book comparedBook = (Book) comparedObject;

        // if the instance variables of the objects are the same, so are the objects
        if (this.name.equals(comparedBook.name) &&
            this.published == comparedBook.published &&
            this.content.equals(comparedBook.content)) {
            return true;
        }

        // otherwise, the objects aren't the same
        return false;
    }
}

Now the book comparison returns true if the instance variables of the books are the same.

Book bookObject = new Book("Book Object", 2000, "...");
Book anotherBookObject = new Book("Book Object", 2000, "...");

if (bookObject.equals(anotherBookObject)) {
    System.out.println("The books are the same");
} else {
    System.out.println("The books aren't the same");
}

The ArrayList also uses the equals method in its internal implementation. If we don't define the equals method in our objects, the contains method of the ArrayList does not work properly. If you try out the code below with two Book classes, one with the equals method defined and another without it, you'll see the difference.

ArrayList<Book> books = new ArrayList<>();
Book bookObject = new Book("Book Object", 2000, "...");
books.add(bookObject);

if (books.contains(bookObject)) {
    System.out.println("Book Object was found.");
}

bookObject = new Book("Book Object", 2000, "...");

if (!books.contains(bookObject)) {
    System.out.println("Book Object was not found.");
}

This reliance on default methods such as equals is actually the reason why Java demands that variables added to ArrayList and HashMap are of reference type. Each reference type variable comes with default methods, such as equals, which means that you don't need to change the internal implementation of the ArrayList class when adding variables of different types. Primitive variables do not have such default methods.

Approximate Comparison With HashMap

In addition to equals, the hashCode method can also be used for approximate comparison of objects. The method creates from the object a "hash code", i.e, a number, that tells a bit about the object's content. If two objects have the same hash value, they may be equal. On the other hand, if two objects have different hash values, they are certainly unequal.

Hash codes are used in HashMaps, for instance. HashMap's internal behavior is based on the fact that key-value pairs are stored in an array of lists based on the key's hash value. Each array index points to a list. The hash value identifies the array index, whereby the list located at the array index is traversed. The value associated with the key will be returned if, and only if, the exact same value is found in the list (equality comparison is done using the equals method). This way, the search only needs to consider a fraction of the keys stored in the hash map.

So far, we've only used String and Integer-type objects as HashMap keys, which have conveniently had ready-made hashCode methods implemented. Let's create an example where this is not the case: we'll continue with the books and keep track of the books that are on loan. We'll implement the book keeping with a HashMap. The key is the book and the value attached to the book is a string that tells the borrower's name:

HashMap<Book, String> borrowers = new HashMap<>();

Book bookObject = new Book("Book Object", 2000, "...");
borrowers.put(bookObject, "Pekka");
borrowers.put(new Book("Test Driven Development", 1999, "..."), "Arto");

System.out.println(borrowers.get(bookObject));
System.out.println(borrowers.get(new Book("Book Object", 2000, "...")));
System.out.println(borrowers.get(new Book("Test Driven Development", 1999, "...")));

We find the borrower when searching for the same object that was given as a key to the hash map's put method. However, when searching by the exact same book but with a different object, a borrower isn't found, and we get the null reference instead. The reason lies in the default implementation of the hashCode method in the Object class. The default implementation creates a hashCode value based on the object's reference, which means that books having the same content that are nonetheless different objects get different results from the hashCode method. As such, the object is not being searched for in the right place.

For the HashMap to work in the way we want it to, that is, to return the borrower when given an object with the correct content (not necessarily the same object as the original key), the class that the key belongs to must overwrite the hashCode method in addition to the equals method. The method must be overwritten so that it gives the same numerical result for all objects with the same content. Also, some objects with different contents may get the same result from the hashCode method. However, with the HashMap's performance in mind, it is essential that objects with different contents get the same hash value as rarely as possible.

We've previously used String objects as HashMap keys, so we can deduce that the String class has a well-functioning hashCode implementation of its own. We'll delegate, i.e., transfer the computational responsibility to the String object.

public int hashCode() {
    return this.name.hashCode();
}

The above solution is quite good. However, if name is null, we see a NullPointerException error. Let's fix this by defining a condition: if the value of the name variable is null, we'll return the year of publication as the hash value.

public int hashCode() {
    if (this.name == null) {
        return this.published;
    }

    return this.name.hashCode();
}

Now, all of the books that share a name are bundled into one group. Let's improve it further so that the year of publication is also taken into account in the hash value calculation that's based on the book title.

public int hashCode() {
    if (this.name == null) {
        return this.published;
    }

    return this.published + this.name.hashCode();
}

It's now possible to use the book as the hash map's key. This way the problem we faced earlier gets solved and the book borrowers are found:

HashMap<Book, String> borrowers = new HashMap<>();

Book bookObject = new Book("Book Object", 2000, "...");
borrowers.put(bookObject, "Pekka");
borrowers.put(new Book("Test Driven Development",1999, "..."), "Arto");

System.out.println(borrowers.get(bookObject));
System.out.println(borrowers.get(new Book("Book Object", 2000, "...")));
System.out.println(borrowers.get(new Book("Test Driven Development", 1999)));

Output:

Let's review the ideas once more: for a class to be used as a HashMap's key, we need to define for it:

• the equals method, so that all equal or approximately equal objects cause the comparison to return true and all false for all the rest
• the hashCode method, so that as few objects as possible end up with the same hash value

Remember to check your points from the ball on the bottom-right corner of the material!