The document provides an overview of core Java concepts related to the Object class, including methods like toString(), equals(), and hashCode(). It details how these methods aid in object representation and comparison, alongside the importance of understanding object equality and hashing for data structures. Additionally, it sets the agenda for future sessions on arrays and the Collections API.

1. Core Java Training
Object Class

2. Page 1Classification: Restricted
Quick recap: Topics covered till now
• Data Types – Primitive (byte, short, int, long, double, float, char, boolean)
• Data Types – Wrapper (Byte, Short, Integer …)
• Data Types – Classes for String (String - immutable, StringBuilder – mutable,
StringBuffer – mutable+threadsafe); Concept of String pooling / interning
• Arrays
• Conditional statements, Loops
• OOP with Java –
• Polymorphism – Method overloading (Compile-time), Method Overriding
(Runtime)
• Inheritance – extending classes, implementing interfaces; multiple inheritance
• Abstraction – abstract classes vs interfaces
• Encapsulation – POJO classes / Beans
• Packaging, Class Modifiers (public, protected, default, private)
• Static members vs Instance members
• Exception handling

3. Page 2Classification: Restricted
Agenda
• Object Class
• toString()
• equals()
• Hashing
• hashCode()

4. Object Class
as
“The basic unit of any collection”

5. Page 4Classification: Restricted
Object class
• Superclass for all Java classes
• Any class without explicit extends clause is a direct subclass of Object
• Methods of Object include:
• String toString()
• boolean equals (Object other)
• int hashCode()

6. Page 5Classification: Restricted
5
Method toString()
• Returns String representation of object; describes state of object
• Automatically called when:
• Object is concatenated with a String
• Object is printed using print() or println()
• Object reference is passed to assert statement of the form:
assert condition : object

7. Page 6Classification: Restricted
6
Example
Rectangle r = new Rectangle (0,0,20,40);
System.out.println(r);
Prints out:
java.awt.Rectangle[x=0,y=0,width=20,height=40]

8. Page 7Classification: Restricted
7
More on toString()
• Default toString() method just prints (full) class name & hash code of object
• Not all API classes override toString()
• Good idea to implement for debugging purposes:
• Should return String containing values of important fields along with
their names
• Should also return result of getClass().getName() rather than hard-
coded class name

9. Page 8Classification: Restricted
Overriding toString(): example
public class Employee
{
public String toString()
{
return getClass().getName()
+ "[name=" + name
+ ",salary=" + salary
+ "]";
}
...
}
Typical String produced: Employee[name=John Doe,salary=40000]

10. Page 9Classification: Restricted
9
Overriding toString in a subclass
• Format superclass first
• Add unique subclass details
• Example:
public class Manager extends Employee
{
public String toString()
{
return super.toString()
+ "[department=" + department + "]";
}
...
}

11. Page 10Classification: Restricted
10
Example continued
• Typical String produced:
Manager[name=Mary Lamb,salary=75000][department=Admin]
• Note that superclass reports actual class name

12. Page 11Classification: Restricted
11
Equality testing
• Method equals() tests whether two objects have same contents
• By contrast, == operator test 2 references to see if they point to the same
object (or test primitive values for equality)
• Need to define for each class what “equality” means:
Compare all fields
Compare 1 or 2 key fields

13. Page 12Classification: Restricted
12
Equality testing
• Object.equals tests for identity:
public class Object {
public boolean equals(Object obj) {
return this == obj;
}
...
}
• Override equals if you don't want to inherit that behavior

14. Page 13Classification: Restricted
13
Overriding equals()
• Good practice to override, since many API methods assume objects have
well-defined notion of equality
• When overriding equals() in a subclass, can call superclass version by using
super.equals()

15. Page 14Classification: Restricted
14
Requirements for equals() method
• If x is not null, then x.equals(null) must be false

16. Page 15Classification: Restricted
The perfect equals() method
public boolean equals(Object otherObject)
{
if (this == otherObject) return true;
if (otherObject == null) return false;
if (getClass() != otherObject.getClass()) return false;
...
}

17. Page 16Classification: Restricted
16
Hashing
• Technique used to find elements in a data structure quickly, without doing
full linear search
• Important concepts:
• Hash code: integer value used to find array index for data
storage/retrieval
• Hash table: array of elements arranged according to hash code
• Hash function: computes hash code for element; uses algorithm likely to
produce different hash codes for different objects to minimize collisions

18. Page 17Classification: Restricted
17
Hashing in Java
• Java library contains HashSet and HashMap classes
• Use hash tables for data storage
• Since Object has a hashCode method (hash function), any type of object
can be stored in a hash table

19. Page 18Classification: Restricted
18
Default hashCode()
• Hashes memory address of object; consistent with default equals() method
• If you override equals(), you should also redefine hashCode()
• For class you are defining, use product of hash of each field and a prime
number, then add these together – result is hash code

20. Page 19Classification: Restricted
19
Example
public class Employee
{
public int hashCode()
{
return 11 * name.hashCode()
+ 13 * new Double(salary).hashCode();
}
...
}

21. Page 20Classification: Restricted
20
Topics to be covered in next session
• Recap of Arrays
• Introduction to Collections API
• Lists – ArrayList, Vector, LinkedList

22. Page 21Classification: Restricted
21
Thank you!