In this session you will learn: 1. Object Class

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!