This document provides an overview of the Object class in Java. It discusses the toString(), equals(), and hashCode() methods. The toString() method returns a string representation of an object. The equals() method tests for equality between two objects. The hashCode() method generates a hash value for an object used in hash tables. It is important to override these methods in subclasses to define object equality and hashing behavior. The document also covers concepts like hashing, hash tables, and hash functions.

1. Java & JEE Training
Day 14 – Object Class

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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

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Agenda
• Object Class
• toString()
• equals()
• Hashing
• hashCode()

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

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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()

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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

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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]

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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

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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]

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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 + "]";
}
...
}

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

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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

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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

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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()

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Requirements for equals() method
• If x is not null, then x.equals(null) must be false

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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;
...
}

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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

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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

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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

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Example
public class Employee
{
public int hashCode()
{
return 11 * name.hashCode()
+ 13 * new Double(salary).hashCode();
}
...
}

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Topics to be covered in next session
• Recap of Arrays
• Introduction to Collections API
• Lists – ArrayList, Vector, LinkedList

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Thank you!