CHAPTER 3 part2.pdf

Class: TY.BSc Computer Science
Subject Code :- CS-335
Subject Name :- Programming in Java – I
Chapter 3 – Part 2
-By Anupama Alagannawar
MIT ACSC, Department of ComputerScience 2020-21

3.6 Predefined Class – Object Class Methods
What is an Object class? Java.lang.Object
● Object class is present in java.lang package.
● The Object class is the parent class of all the classes in java by default. In
other words, it is the topmost class of java.
● Every class in Java is directly or indirectly derived from the Object class.
● If a Class does not extend any other class then it is direct child class
of Object and if extends other class then it is an indirectly derived.
Therefore the Object class methods are available to all Java classes.
● Hence Object class acts as a root of inheritance hierarchy in any Java
Program.
MIT ACSC, Department of Computer Science 2020-21

Java.lang.Object class
● Object class is the parent class of the classes in java.
● Class A
● {
● }
● When we compile the above code , the compiler verifies java syntax and checks whether class A
has any parent.
● Java compiler adds java.lang.Object as the parent of A.
● Every class in java is child of Object class either directly or indirectly.
● Class A extends java.lang.Object // directly
● {
● }
● Class B extends A // Java.lang.object //(indirectly)java.lang.Object is indirectly the parent of B
● {
● } A
B MIT ACSC, Department of Computer Science 2020-21

Methods of Object Class
● int hashCode() - Used to get a hash code value for the object.
● String toString() - Used to get a string representation of the object.
● boolean equals(Object obj) - Used to indicate whether some other object is
"equal to" this one.
● protected void finalize() - garbage collector calls this method on an object
when it determines that there are no more references to the object.
● Class<?> getClass() - Used to get the runtime class of this Object.
● protected Object clone() - Used to create and return a copy of this object.

int hashcode()
● To identify object uniquely.
● JVM assigns one unique value.
● It is not an address.
Syntax
public int hashCode()
Parameters
This is a default method and this will not accept any parameters.
Return Value
This method returns a hash code value for this object.

int hashcode()
● Example
● import java.io.*;
● public class Test
● {
● public static void main(String args[])
● {
● String Str = new String("Welcome to JAVA Programming”);
● System.out.println("Hashcode for Str :" + Str.hashCode() );
}
● }
● Output
● Hashcode for Str :1186874997

String toString()
● toString() : toString() provides String representation of an Object and used
to convert an object to String.
● If you want to represent any object as a string, toString() method is used.
● Object class contains toString() method. We can use toString() method to get
string representation of an object.
● Whenever we try to print the Object reference then internally toString()
method is invoked.
● If we did not define toString() method in your class then Object class
toString() method is invoked otherwise our implemented/Overridden
toString() method will be called.

toString()
● Syntax of Object class toString() method:
● public String toString() {
return getClass().getName()+"@"+Integer.toHexString(hashCode()); }
Default behavior of toString() is to print class name, then @, then
unsigned hexadecimal representation of the hash code of the object
● It is always recommended to override toString() method to get our
own String representation of Object.
● Note : Whenever we try to print any Object reference, then
internally toString() method is called.
● Student s = new Student(); // Below two statements are equivalent
System.out.println(s); System.out.println(s.toString());

Understanding problem without toString() method
(Let's see the simple code that prints reference.
● class Student{
● int rollno;
● String name;
● String city;
●
● Student(int rollno, String name, String city){
● this.rollno=rollno;
● this.name=name;
● this.city=city;
● }
● public static void main(String args[]){
Student s1=new Student(101,"Raj","lucknow");
Student s2=new Student(102,"Vijay","ghaziabad"
);
● System.out.println(s1);
● //compiler writes here s1.toString()
● System.out.println(s2);
● //compiler writes here s2.toString()
● }
● }
● Output:
Student@1fee6fc
Student@1eed786

Example of Java toString() method (Now let's see
the real example of toString() method.)
● class Student{
● int rollno;
● String name;
● String city;
●
● Student(int rollno, String name, String city)
● { this.rollno=rollno;
● this.name=name;
● this.city=city;
● }
● public String toString(){
● //overriding the toString() method
● return rollno+" "+name+" "+city;
● }
●
● Student s1=new Student(101,"Raj","luckn
ow");
● Student s2=new Student(102,"Vijay","gha
ziabad");
●
● System.out.println(s1);//compiler writes h
ere s1.toString()
● System.out.println(s2);//compiler writes h
ere s2.toString()
● }
● }
● Output:101 Raj lucknow
102 Vijay ghaziabad

boolean equals(Object obj)
● Java Object equals(Object obj) Method
● equals(Object obj) is the method of Object class. This method is
used to compare the given objects. It is suggested to override
equals(Object obj) method to get our own equality condition on
Objects.
● Syntax
public boolean equals(Object obj)
● Parameter example: s1.equals(s2)
obj - it is the reference object.
● Returns
It returns true if this object is same as the obj argument else it
returns false otherwise.

Example of equals() Object Method
● public class JavaObjectequalsExample1
● {
● static int a = 10, b=20;
● int c;
● // Constructor
● JavaObjectequalsExample1()
● {
● System.out.println("Addition of 10 a
nd 20 : ");
● c=a+b;
● System.out.println("Answer : "+c);
● }
●
● {
● System.out.println("1st object created...");
● JavaObjectequalsExample1 obj1 = new Jav
aObjectequalsExample1();
● System.out.println("2nd object created...");
● JavaObjectequalsExample1 obj2 = new Java
ObjectequalsExample1();
System.out.println("Objects are equal:" + o
bj1.equals(obj2));
● }
● } //for any non-null reference values obj1 and obj2, this
method returns true if and only if obj1 and obj2 refer to
the same object (obj1 == obj2 has the value true).

Difference between == and equals() Method
● == is used for reference
comparison(address comparison). It means
the == operator checks if both objects
refer to the same memory location or not.
● String s1=new String(“KARAN”);
● SOP(s1==s2); //false // s1.equals(s2)
● String s3=“Naman”;
● String s4=“Naman”;
● SOP(s3==s4); //true
Heap Area
○ S1
○ S2
S3
S4
○ SCP – String Constant Pool
SCP
KARAN
KARAN
KARAN
Naman

Equals() method in Object class and String class
Object Class
● equals() method in Object class is used to compare
the reference or address of two Objects, that is if two
objects point to the same memory location.
● Class Object
● {
● public boolean equals(Object obj)
● {
● return(this==obj) //s1.equals(s2)
● }
● }
● Class Demo {
● p.s.v.m{
● SOP(s1==s2); //false
● Equals() method and == are same if you are talking
about Object class
String class
● equals() method in String class is used for content
comparison. It means the equals method is used to
check object values.
● Class String extends Object
● {
● --------
● //override equals
● public boolean equals(Object obj)
● {
● //statements (Content check)
● }
● }
● SOP(s1.equals(s2)) ; //true

finalize() method
● The java.lang.Object.finalize() is called by the garbage collector on an object when
garbage collection determines that there are no more references to the object. A
subclass overrides the finalize method to dispose of system resources or to perform
other cleanup.
● Declaration
protected void finalize()
● Parameters : NA
● Return Value
This method does not return a value.
● The finalize() method of Object class is a method that the Garbage
Collector always calls just before the deletion/destroying the object which is eligible
for Garbage Collection, so as to perform clean-up activity. Clean-up activity means
closing the resources associated with that object like Database Connection, Network
Connection or we can say resource de-allocation. Remember it is not a reserved
keyword. Once the finalize method completes immediately Garbage Collector destroy
that object.

getclass() method
● The java.lang.Object.getClass() method returns the runtime class of an object.
● The Java Object getClass() method returns the class name of the object.
● The syntax of the getClass() method is: object.getClass()
● getClass() Parameters
● The getClass() method does not take any parameters.
● getClass() Return Values
● the class of the object that calls the method
● class student
● {
● {
● student s1=new student();
● System.out.println(s1.getClass());
● String ss=new String();
● System.out.println(ss.getClass());
● }
● }

clone()
● Java provides an assignment operator to copy the values but no operator
to copy the object.
● Object class has a clone method which can be used to copy the values of
an object without any side-effect.
● Assignment operator has a side-effect that when a reference is assigned to
another reference then a new object is not created and both the reference
point to the same object. This means if we change the value in one object
then same will reflect in another object as well.
● clone() method handles this problem.
● //cloning method copies the object
● B b2 = b1.clone(); //b2 and b1 are now pointing to different object
● b2.a = 3; //modify b2 and changes will not reflect in b1

Methods of Object Class continued.....
● void notify() - Used to wake up a single thread that is waiting on this object's
monitor.
● void notifyAll() - Used to wake up all threads that are waiting on this object's
monitor.
● void wait() - marks the current thread to wait until another thread invokes
the notify() method or the notifyAll() method for this object.
● void wait(long timeout) - marks the current thread to wait until either
another thread invokes the notify() method or the notifyAll() method for this
object, or a specified amount of time has elapsed.
● void wait(long timeout, int nanos) - marks the current thread to wait until
another thread invokes the notify() method or the notifyAll() method for this
object, or some other thread interrupts the current thread, or a certain
amount of real time has elapsed.

Inner Classes
● Nested Classes
● In Java, just like methods, variables of a class too can have another
class as its member. Writing a class within another is allowed in
Java. The class written within is called the nested class, and the
class that holds the inner class is called the outer class.
● Syntax
● The class Outer_Demo is the outer class and the
class Inner_Demo is the nested class.
● class Outer_Demo
● {
● class Inner_Demo
● {
● }
● }

Nested classes
● Nested classes are divided into two types −
● Non-static nested classes − These are the non-static members
of a class.
● Static nested classes − These are the static members of a class.

Inner Classes (Non-static Nested Classes)
● Any non-static nested class is known as inner class in java. Java inner class is
associated with the object of the class and they can access all the variables and
methods of the outer class.
● Since inner class are associated with the instance , we can’t have any static variables in
them.
● Java inner class or nested class is a class that is declared inside the class or interface.
We use inner classes to logically group classes and interface in one place so that it can
be more readable and maintainable.
● It can access all the members of the outer class including private data members and
methods.
● Inner classes are a security mechanism in Java. We know a class cannot be associated
with the access modifier private, but if we have the class as a member of other class,
then the inner class can be made private. And this is also used to access the private
members of a class.
● Inner classes are of three types depending on how and where you define them. They are
● Inner Class, Method –local inner class, Anonymous Inner class

Advantages of Java Inner class
● There are basically three advantages of inner classes in java. They
are as follows:
1) Nested classes represent a special type of relationship that is it
can access all the members (data members and methods)
of outer class including private.
2) Nested classes are used to develop more readable and
maintainable code because it logically group classes and
interfaces in one place only.
3) Code Optimization: It requires less code to write

Anonymous inner class
● A class that have no name is known as anonymous inner class in java. It
should be used if you have to override method of class or interface.
● abstract class Person{
● abstract void eat();
● }
● class TestAnonymousInner{
● Person p=new Person(){
● void eat(){System.out.println("nice fruits");}
● };
● p.eat();
● }
● }

Wrapper Classes in Java
● A Wrapper class is a class whose object wraps or contains primitive data
types. When we create an object to a wrapper class, it contains a field and
in this field, we can store primitive data types. In other words, we can wrap
a primitive value into a wrapper class object.
Need of Wrapper Classes
● They convert primitive data types into objects. Objects are needed if we
wish to modify the arguments passed into a method (because primitive
types are passed by value).
● The classes in java.util package handles only objects and hence wrapper
classes help in this case also.
● Data structures in the Collection framework, such as ArrayList and Vector,
store only objects (reference types) and not primitive types.
● An object is needed to support synchronization in multithreading.

Wrapper Class in Java .Continued.......
● Wrapper classes are used to convert primitive data types into Object and
Vice Versa
● Eight classes of java.lang package are known as wrapper class in java
● Primitive Data types and their Corresponding Wrapper class

Autoboxing and Unboxing in wrapper class
● Autoboxing: Converting primitive data
types into Object.
● Eg: Primitive to Wrapper
class Wrapper1
{
public static void main(String args[])
{
int i=100;
Integer j = Integer.valueOf(i);
Integer ab==i; //Autoboxing (JVM
will write Integer.valueOf automatically)
System.out.println( i, j, ab);
}
}
● Unboxing: Converting Object to Primitive
data types.
● Eg. Wrapper to Primitive
Class Wrapper2
{
public static void main(String args[])
{
Integer i = new Integer(10);
int ab = i.intValue();
int j = i; //unboxing
System.out.println( i, ab, j);
}
}

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