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![• I/O Package – for I/O manipulation
• Networking Package – for communicating with other computers via the internet
• AWT Package – implements platform-independent GUI
• Applet Package – allows us to create java applets
iii) JRE (Java Runtime Environment)
facilitates the execution of programs developed in Java
• JVM – it is a program that interprets the intermediate Java byte code and generates the desired o/p.
• Runtime class libraries – a set of core class libraries that are required for the execution of java programs.
• User interface toolkit – support varied i/p methods for the users to interact with the application program.
• Deployment technologies – comprises
a) Java plug-in – enbles the execution of a java applet on the browser
b) Java web start – enables remote-deployment of an application.
OVERVIEW OF JAVA LANGUAGE
To create a simple java program, you need to create a class that contains main method. Let's understand the
requirement first.
REQUIREMENT FOR HELLO JAVA EXAMPLE:
For executing any java program, you need to
• install the JDK if you don't have installed it, download the JDK and install it.
• set path of the jdk/bin directory.
• create the java program
• compile and run the java program
CREATING HELLO JAVA EXAMPLE :
class Simple{
public static void main(String args[]){
System.out.println("Hello Java");](https://image.slidesharecdn.com/javafull1recovered-231001024003-a4536b77/85/java-full-1-Recovered-docx-13-320.jpg)
![}
}
save this file as Simple.java
To compile : javac Simple.java
To execute :java Simple
JAVA PROGRAM STRUCTURE
• Let’s use example of HelloWorld Java program to understand structure and features of class.
• This program is written on few lines, and its only task is to print “Hello World from Java” on the screen. Refer th
following picture.
1.“PACKAGES”:
2. “PUBLIC CLASS HELLOWORLD”:
3. COMMENTS SECTION:
4. “PUBLIC STATIC VOID MAIN (STRING [ ] ARGS)”:
5. SYSTEM.OUT.PRINTLN("HELLO WORLD FROM JAVA")
JAVA TOKENS
• A Java program is basically a collection of classes.
• A class is defined by a set of declaration statements and methods containing executable statements.
• Most statements contain expressions, which describe the actions carried out on data.
• Smallest individual unit in a program are known as tokens.
• The compiler recognizes them for building up expressions and statements.
5 Types of Tokens](https://image.slidesharecdn.com/javafull1recovered-231001024003-a4536b77/85/java-full-1-Recovered-docx-14-320.jpg)
![• I/O Package – for I/O manipulation
• Networking Package – for communicating with other computers via the internet
• AWT Package – implements platform-independent GUI
• Applet Package – allows us to create java applets
iii) JRE (Java Runtime Environment)
facilitates the execution of programs developed in Java
• JVM – it is a program that interprets the intermediate Java byte code and generates the desired o/p.
• Runtime class libraries – a set of core class libraries that are required for the execution of java programs.
• User interface toolkit – support varied i/p methods for the users to interact with the application program.
• Deployment technologies – comprises
a) Java plug-in – enbles the execution of a java applet on the browser
b) Java web start – enables remote-deployment of an application.
OVERVIEW OF JAVA LANGUAGE
To create a simple java program, you need to create a class that contains main method. Let's understand the
requirement first.
REQUIREMENT FOR HELLO JAVA EXAMPLE:
For executing any java program, you need to
• install the JDK if you don't have installed it, download the JDK and install it.
• set path of the jdk/bin directory.
• create the java program
• compile and run the java program
CREATING HELLO JAVA EXAMPLE :
class Simple{
public static void main(String args[]){
System.out.println("Hello Java");](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-13-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![}
}
save this file as Simple.java
To compile : javac Simple.java
To execute :java Simple
JAVA PROGRAM STRUCTURE
• Let’s use example of HelloWorld Java program to understand structure and features of class.
• This program is written on few lines, and its only task is to print “Hello World from Java” on the screen. Refer th
following picture.
1.“PACKAGES”:
2. “PUBLIC CLASS HELLOWORLD”:
3. COMMENTS SECTION:
4. “PUBLIC STATIC VOID MAIN (STRING [ ] ARGS)”:
5. SYSTEM.OUT.PRINTLN("HELLO WORLD FROM JAVA")
JAVA TOKENS
• A Java program is basically a collection of classes.
• A class is defined by a set of declaration statements and methods containing executable statements.
• Most statements contain expressions, which describe the actions carried out on data.
• Smallest individual unit in a program are known as tokens.
• The compiler recognizes them for building up expressions and statements.
5 Types of Tokens](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-14-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![long float, double
float double
VARIABLE ARGUMENT (VARARGS):
The varrags allows the method to accept zero or muliple arguments. Before varargs either we use overloaded
method or take an array as the method parameter but it was not considered good because it leads to the maintenance
problem. If we don't know how many argument we will have to pass in the method, varargs is the better approach.
ADVANTAGE OF VARARGS:
WE DON'T HAVE TO PROVIDE OVERLOADED METHODS SO LESS CODE.
SYNTAX OF VARARGS:
The varargs uses ellipsis i.e. three dots after the data type. Syntax is as follows:
1. return_type method_name(data_type... variableName){}
SIMPLE EXAMPLE OF VARARGS IN JAVA:
1.
2. class VarargsExample1{
3.
4. static void display(String... values){
5. System.out.println("display method invoked ");
6. }
7.
8. public static void main(String args[]){
9.
10. display();//zero argument
11. display("my","name","is","varargs");//four arguments
12. }
13. }
14.
Test it Now
Output:display method invoked
display method invoked
ANOTHER PROGRAM OF VARARGS IN JAVA:](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-24-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![1.
2. class VarargsExample2{
3.
4. static void display(String... values){
5. System.out.println("display method invoked ");
6. for(String s:values){
7. System.out.println(s);
8. }
9. }
10.
11. public static void main(String args[]){
12.
13. display();//zero argument
14. display("hello");//one argument
15. display("my","name","is","varargs");//four arguments
16. }
17. }
18.
Test it Now
Output:display method invoked
display method invoked
hello
display method invoked
my
name
is
varargs
download this example
Rules for varargs:
While using the varargs, you must follow some rules otherwise program code won't compile. The rules are as
follows:
o There can be only one variable argument in the method.
o Variable argument (varargs) must be the last argument.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-25-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![EXAMPLES OF VARARGS THAT FAILS TO COMPILE:
1.
2. void method(String... a, int... b){}//Compile time error
3.
4. void method(int... a, String b){}//Compile time error
5.
EXAMPLE OF VARARGS THAT IS THE LAST ARGUMENT IN THE METHOD:
1.
2. class VarargsExample3{
3.
4. static void display(int num, String... values){
5. System.out.println("number is "+num);
6. for(String s:values){
7. System.out.println(s);
8. }
9. }
10.
11. public static void main(String args[]){
12.
13. display(500,"hello");//one argument
14. display(1000,"my","name","is","varargs");//four arguments
15. }
16. }
17.
Test it Now
Output:number is 500
hello
number is 1000
my
name
is
varargs
Varargs is a short name for variable arguments. In Java, an argument of a method can accept arbitrary number of
values. This argument that can accept variable number of values is called varargs.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-26-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![The syntax for implementing varargs is as follows:
accessModifier methodName(datatype… arg) {
// method body
}
In order to define vararg, ... (three dots) is used in the formal parameter of a method.
A method that takes variable number of arguments is called a variable-arity method, or simply a varargs method.
First, let’s look at the example without using varargs:
class NoVararg {
public int sumNumber(int a, int b){
return a+b;
}
public int sumNumber(int a, int b, int c){
return a+b+c;
}
public static void main( String[] args ) {
NoVararg obj = new NoVararg();
System.out.println(obj.sumNumber(1, 2));
System.out.println(obj.sumNumber(1, 2, 3));
}
}
When you run this program, the output will be:
3
6
As you can clearly see, you had to overload sumNumber() method to make it work for 3 arguments.
THINGS TO REMEMBER WHILE USING VARARGS
Here are a couple of things you should remember while working with Java vargars:](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-27-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![• Classes provide a convenient method for packing together a group of logically related data items and
functions
• In Java The data items are called fields and the functions are called methods.
DEFINING A CLASS :
• A class is a user-defined data type with a template that serves to define its properties
• These variables are called instance variables because they are created whenever an object of the class is
instantiated.
Ex : class A
{
int length;
Int width;
}
3. FIELDS DECLARATION :
• Data is encapsulated in a class by placing data fields inside the body of the class definition
• Once the class type has been defined, we can create variables of that type using declarations that are similar to
the basic-type declarations.
• These termed as instances of classes, which are the actual objects.
Syntax
class classname [extends superclasname]
{
[ fields declaration; ]
[methods declaration;] }
Ex : class A
{ }
4. METHODS DECLARATION :
• A class with only data fields & without methods that operate on that data has no life.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-37-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![class Operation
{ static float mul(float x, float y)
{ return x*y; }
static float div(float x, float y)
{ return x/y; }}
Class MathOp
{ public void static main(String args[])
{ float a = Operation.mul(5.0,2.0);
float b = Operation.div(6.0,2.0);
System.out.ptintln(“a=“+a);
System.out.ptintln(“b=“+b) }}
10. NESTING OF METHODS :
Def : A method can be called by using only its name by another method of the same class is known as nesting
of methods.
class Nest
{ int large()
{ …. }
class display( )
{ int l = large( ); }
Class Ntest
{ public void static main(String args[])
{ ….. }
INHERITANCE IN JAVA :
1. Inheritance
2. Types of Inheritance
3. Why multiple inheritance is not possible in Java in case of class?
Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a parent
object. It is an important part of OOPs (Object Oriented programming system).](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-42-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![Java Inheritance Example
As displayed in the above figure, Programmer is the subclass and Employee is the superclass. The relationship
between the two classes is Programmer IS-A Employee. It means that Programmer is a type of Employee.
class Employee{ float salary=40000; }
class Programmer extends Employee{
int bonus=10000;
public static void main(String args[]){
Programmer p=new Programmer();
System.out.println("Programmer salary is:"+p.salary);
System.out.println("Bonus of Programmer is:"+p.bonus);
} }
Test it Now
Programmer salary is:40000.0
Bonus of programmer is:10000
In the above example, Programmer object can access the field of own class as well as of Employee class i.e. code
reusability.
Types of inheritance in java
On the basis of class, there can be three types of inheritance in java: single, multilevel and hierarchical.
In java programming, multiple and hybrid inheritance is supported through interface only. We will learn about
interfaces later.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-44-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![Note: Multiple inheritance is not supported in Java through class.
When one class inherits multiple classes, it is known as multiple inheritance. For Example:
Single Inheritance Example
When a class inherits another class, it is known as a single inheritance. In the example given below, Dog class
inherits the Animal class, so there is the single inheritance.
TestInheritance.java
class Animal{
void eat(){System.out.println("eating...");}
}
class Dog extends Animal{ void bark(){System.out.println("barking..."); } }
class TestInheritance{ public static void main(String args[]) {
Dog d=new Dog();
d.bark();
d.eat(); }}
Output:
barking...
eating...](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-45-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![Multilevel Inheritance Example
When there is a chain of inheritance, it is known as multilevel inheritance. As you can see in the example given
below, BabyDog class inherits the Dog class which again inherits the Animal class, so there is a multilevel
inheritance.
class Animal{
void eat(){System.out.println("eating...");}
}
class Dog extends Animal{
void bark(){System.out.println("barking...");}
}
class BabyDog extends Dog{
void weep(){System.out.println("weeping...");} }
class TestInheritance2{
public static void main(String args[]){
BabyDog d=new BabyDog();
d.weep();
d.bark();
d.eat(); }}
Output:
weeping...
barking...
eating...
Hierarchical Inheritance Example
When two or more classes inherits a single class, it is known as hierarchical inheritance. In the example given
below, Dog and Cat classes inherits the Animal class, so there is hierarchical inheritance.
TestInheritance3.java
class Animal{
void eat(){System.out.println("eating...");}
}
class Dog extends Animal{
void bark(){System.out.println("barking...");}
}
class Cat extends Animal{](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-46-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![void meow(){System.out.println("meowing...");}
}
class TestInheritance3{
public static void main(String args[]){
Cat c=new Cat();
c.meow();
c.eat();
//c.bark();//C.T.Error
}}
Output:
meowing...
eating...
Q) Why multiple inheritance is not supported in java?
To reduce the complexity and simplify the language, multiple inheritance is not supported in java.
Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes
have the same method and you call it from child class object, there will be ambiguity to call the method of A or B
class.
Since compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So
whether you have same method or different, there will be compile time error.
class A{
void msg(){System.out.println("Hello");}
}
class B{
void msg(){System.out.println("Welcome");}
}
class C extends A,B{//suppose if it were
public static void main(String args[]){
C obj=new C();
obj.msg();//Now which msg() method would be invoked? } }
Test it Now
Compile Time Error](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-47-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![Method Overriding
Method Overloading and Method Overriding are the topics that should be on your to-do list before appearing for
the interview. In this post, I will share what is method overriding, the rules of method overriding, and examples. So
let's dive in and understand method overriding in java.
What is Method Overriding
When the child class(subclass) has the method which has the same name, same parameters and same return type (or
covariant return type) as a method in its parent class(or super-class), then the child method has overridden the
parent class method.
Runtime polymorphism can be achieved through method overriding. Remember that at runtime, Object type
(not reference variable's type) determines which overridden method is used.
If the parent class object is used to invoke the method, then the parent class method will be executed. Otherwise, if,
a child class object is used to invoke the method, then the child class method will be executed.
Example of Method Overriding
// Method Overriding Example
//Base class
class Parent { public void display() {
System.out.println("parent method is executed"); }}
//Derived or Inherited class
class Child extends Parent { //Below method overrides the Parent display() method
@Override
public void display() { System.out.println("child method is executed"); }}
//Driver class
public class MethodOverriding1 {
public static void main(String args[])](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-48-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![// Access Modifiers and Overriding Example
//Base class
class Parent { //Access modifier of parent's display() method is protected
protected void display() { System.out.println("parent method is executed"); }}
//Derived class
class Child extends Parent { //Below method overrides the Parent display() method
//Access modifier public is less restrictive than protected
@Override
public void display() { System.out.println("child method is executed"); } }
//Driver class
PublicclassMethodOverriding2{
public static void main(String args[])
{ Parent parentObject = new Parent();
parentObject.display();
Parent childObject = new Child();
childObject.display(); }}
Output :
parent method is executed
child method is executed
2. Private methods can not be overridden
Another rule regarding access modifiers is you can not override private methods in the parent class. If subclass
tries to override the private method of a parent class then the program will throw a compile-time error as shown
below.
// Access Modifiers and Overriding Example
//Base class or Superclass
class Parent { //Access modifier of parent method is private
// This method can not override the child class method
private void display() { System.out.println("parent method is executed"); }}
//Derived class or Subclass
class Child extends Parent {
//Below method can not overrides the Parent display() method
//This method is unique to the child class
private void display() { System.out.println("child method is executed"); }}
//Driver class
public class MethodOverriding3 {
public static void main(String args[])
{ Parent parentObject = new Parent();
parentObject.display(); // this line when execute will throw compiler error](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-50-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![//This method hides the Parent display() method
static void display() { System.out.println("child static method is executed"); }
// This method overrides the Parent show() method
void show() { System.out.println("child non-static (instance) method is executed"); }}
//Driver Class
public class MethodOverriding4 { public static void main(String args[])
{ Parent childObject = new Child();
// static method can not be overridden,
// so below line calls parent display() method
childObject.display();
// Expected child method will run
childObject.show(); }}
Output:
parent static method is executed
child non-static (instance) method is executed
5. Invoking a superclass (Parent class) version of overridden method
It's easy to call the superclass(Parent class) version of overridden method by using the super keyword.
// Invoking SuperClass version of Overridden Method Example
//Base class or SuperClass
class Parent {
void display() {
System.out.println("parent method is executed");
}
}
//Derived class or SubClass
class Child extends Parent {
//This method overrides the Parent display() method
@Override
void display() {
super.display();](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-52-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![System.out.println("child method is executed");
}
}
public class MethodOverriding5 {
public static void main(String args[])
{
Parent childObject = new Child();
childObject.display();
}
}
Output :
parent method is executed
child method is executed
6. Overriding method must have the same return type (or Covariant return type) :
The method return type must be the same, or a subtype of, the return type declared in the original
overridden..method..in..the..superclass. Since java 5.0 it is made possible to have a different return type for an
overriding method in a child class. But child's return type should be a subtype of the parent's
return..type…This..phenomenon..is..called..covariant..return..type.
Example of Covariant Return Type
class Alpha { Alpha doStuff(char c) {
return new Alpha(); }}
class Beta extends Alpha {
Beta doStuff(char c) { // legal override in Java 1.5
return new Beta(); }}
public class MethodOverriding6 {
public static void main(String args[])
{ Alpha childObject = new Beta();
childObject.doStuff('a'); }}
The above code will compile and run.
7. Constructor and Overriding
You can not override the constructor as the constructor name of the base class and child class can never be the same.
(Constructor name is always the same as the class name).](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-53-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![8. Exception Handling and Overriding
Rule 1. The overriding method can throw any unchecked(runtime) exception regardless of whether the overridden method
declares the exception.
// Overriding when superclass does not throw an exception
//Base class or SuperClass
class Parent { //overridden method
void display() {
System.out.println("parent method is executed"); }}
//Derived class or SubClass
class Child extends Parent {
//This method overrides the Parent display() method
//It throws unchecked exception i.e Arithmetic Exception
@Override
void display() throws ArithmeticException{
System.out.println("child method is executed"); }}
public class MethodOverriding7 {
public static void main(String args[])
{ Parent childObject = new Child();
childObject.display(); }}
Output: child method is executed
Rule 2. The overriding method must not throw checked exceptions that are new or broader than those declared by
the overridden method. For example, a method that declares a FileNotFoundException can not be overridden by a
method that declares a SQLException, Exception, or any other non-runtime exception unless it's a subclass of
FileNotFoundException.
Note : The overriding method can throw narrower or fewer exceptions. Just because an overridden method "takes
risks" doesn't mean that the overriding subclass' exception takes the same risks.
Bottom line: an overriding method doesn't have to declare any exceptions that it will never throw, regardless of
what the overridden method declares.
9. Abstract method and Overriding
Abstract methods can only be overridden by the first concrete class (classes that have no abstract methods)
otherwise a compiler error will be thrown.
//Abstract class
abstract class Parent
{ public abstract void display( int x, String j); }](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-54-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![//Subclass or Derived Class
class Child extends Parent
{ @Override
public void display( int x, String j )
{ System.out.println("child method is executed"); }}
//Driver class
public class MethodOverriding8 {
public static void main(String args[])
{ Parent childObject = new Child();
childObject.display(1, "Alive is Awesome"); }}
Output : child method is executed
10. Synchronized/strictfp method and Overriding
Synchronized or strictfp methods do not have any impact on the overriding method. It's possible that a non-
synchronized or non-strictfp method can override a synchronized/strictfp method or vice versa.
MultiLevel Method Overriding in Java
In the multilevel method overriding we create a grandchild class and override the display() method of the
parent class. Let's find out what will happen if we perform runtime polymorphism in multilevel..overriding.
// MultiLevel Overriding
//Base class or SuperClass
class Parent { //overridden method
void display() {
System.out.println("parent method is executed"); }}
//Derived class or SubClass
class Child extends Parent {
//This method overrides the Parent display() method
@Override
void display() {
System.out.println("child method is executed"); }}
//Inherited Class
class GrandChild extends Child {
// This method overrides the Parent display() method
@Override
void display() { System.out.println("grand child method is executed"); }}
//Driver Class
public class MethodOverriding9 {
public static void main(String args[])](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-55-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![1) Java final variable
If you make any variable as final, you cannot change the value of final variable(It will be constant).
Example of final variable
There is a final variable speedlimit, we are going to change the value of this variable, but It can't be changed
because final variable once assigned a value can never be changed.
1. class Bike9{
2. final int speedlimit=90;//final variable
3. void run(){
4. speedlimit=400;
5. }
6. public static void main(String args[]){
7. Bike9 obj=new Bike9();
8. obj.run();
9. }](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-65-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![10. }//end of class
Test it Now
Output:Compile Time Error
2) Java final method
If you make any method as final, you cannot override it.
Example of final method
1. class Bike{
2. final void run(){System.out.println("running");}
3. }
4.
5. class Honda extends Bike{
6. void run(){System.out.println("running safely with 100kmph");}
7.
8. public static void main(String args[]){
9. Honda honda= new Honda();
10. honda.run();
11. }
12. }
Test it Now
Output:Compile Time Error
3) Java final class
If you make any class as final, you cannot extend it.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-66-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![Example of final class
1. final class Bike{}
2.
3. class Honda1 extends Bike{
4. void run(){System.out.println("running safely with 100kmph");}
5.
6. public static void main(String args[]){
7. Honda1 honda= new Honda1();
8. honda.run();
9. }
10. }
Test it Now
Output:Compile Time Error
Q) Is final method inherited?
Ans) Yes, final method is inherited but you cannot override it. For Example:
1. class Bike{
2. final void run(){System.out.println("running...");}
3. }
4. class Honda2 extends Bike{
5. public static void main(String args[]){
6. new Honda2().run();
7. }
8. }](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-67-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![6. System.out.println(speedlimit);
7. }
8.
9. public static void main(String args[]){
10. new Bike10();
11. }
12. }
Test it Now
Output: 70
static blank final variable
A static final variable that is not initialized at the time of declaration is known as static blank final variable. It can
be initialized only in static block.
Example of static blank final variable
1. class A{
2. static final int data;//static blank final variable
3. static{ data=50;}
4. public static void main(String args[]){
5. System.out.println(A.data);
6. }
7. }
Q) What is final parameter?](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-69-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![If you declare any parameter as final, you cannot change the value of it.
1. class Bike11{
2. int cube(final int n){
3. n=n+2;//can't be changed as n is final
4. n*n*n;
5. }
6. public static void main(String args[]){
7. Bike11 b=new Bike11();
8. b.cube(5);
9. }
10. }
Test it Now
Output: Compile Time Error
Q) Can we declare a constructor final?
No, because constructor is never inherited.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-70-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![Java Object finalize() Method
Finalize() is the method of Object class. This method is called just before an object is garbage collected. finalize()
method overrides to dispose system resources, perform clean-up activities and minimize memory leaks.
Syntax
1. protected void finalize() throws Throwable
Throw
Throwable - the Exception is raised by this method
Example 1
1. public class JavafinalizeExample1 {
2. public static void main(String[] args)
3. {
4. JavafinalizeExample1 obj = new JavafinalizeExample1();
5. System.out.println(obj.hashCode());
6. obj = null;
7. // calling garbage collector
8. System.gc();
9. System.out.println("end of garbage collection");
10.
11. }
12. @Override](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-71-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![In this example, Bike is an abstract class that contains only one abstract method run. Its implementation is provided
by the Honda class.
1. abstract class Bike{
2. abstract void run();
3. }
4. class Honda4 extends Bike{
5. void run(){System.out.println("running safely");}
6. public static void main(String args[]){
7. Bike obj = new Honda4();
8. obj.run();
9. }
10. }
Test it Now
running safely
Understanding the real scenario of Abstract class
In this example, Shape is the abstract class, and its implementation is provided by the Rectangle and Circle classes.
Mostly, we don't know about the implementation class (which is hidden to the end user), and an object of the
implementation class is provided by the factory method.
A factory method is a method that returns the instance of the class. We will learn about the factory method later.
In this example, if you create the instance of Rectangle class, draw() method of Rectangle class will be invoked.
File: TestAbstraction1.java
1. abstract class Shape{ abstract void draw(); }
2. //In real scenario, implementation is provided by others i.e. unknown by end user
3. class Rectangle extends Shape{
4. void draw(){System.out.println("drawing rectangle");} }
5. class Circle1 extends Shape{
6. void draw(){System.out.println("drawing circle");} }
7. //In real scenario, method is called by programmer or user
8. class TestAbstraction1{ public static void main(String args[]){
9. Shape s=new Circle1();//In a real scenario, object is provided through method, e.g., getShape() method
10. s.draw(); } }
Test it Now](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-75-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![drawing circle
Another example of Abstract class in java
File: TestBank.java
1. abstract class Bank{
2. abstract int getRateOfInterest(); }
3. class SBI extends Bank{
4. int getRateOfInterest(){return 7;} }
5. class PNB extends Bank{
6. int getRateOfInterest(){return 8;} }
7. class TestBank{ public static void main(String args[]){ Bank b;
8. b=new SBI();
9. System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %");
10. b=new PNB();
11. System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %"); }}
Test it Now
Rate of Interest is: 7 %
Rate of Interest is: 8 %
VARIABLE ARGUMENT (VARARGS):
The varrags allows the method to accept zero or muliple arguments. Before varargs either we use overloaded
method or take an array as the method parameter but it was not considered good because it leads to the maintenance
problem. If we don't know how many argument we will have to pass in the method, varargs is the better approach.
ADVANTAGE OF VARARGS:
We don't have to provide overloaded methods so less code.
Syntax of varargs:
The varargs uses ellipsis i.e. three dots after the data type. Syntax is as follows:
return_type method_name(data_type... variableName){}
SIMPLE EXAMPLE OF VARARGS IN JAVA:
15.
16. class VarargsExample1{
17. static void display(String... values){](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-76-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![18. System.out.println("display method invoked "); }
19. public static void main(String args[]){
20. display();//zero argument
21. display("my","name","is","varargs");//four arguments } }
Test it Now
Output:display method invoked
display method invoked
ANOTHER PROGRAM OF VARARGS IN JAVA:
19.
20. class VarargsExample2{
21. static void display(String... values){
22. System.out.println("display method invoked ");
23. for(String s:values){
24. System.out.println(s); } }
25. public static void main(String args[]){
26. display();//zero argument
27. display("hello");//one argument
28. display("my","name","is","varargs");//four arguments } }
Test it Now
Output:display method invoked
display method invoked
hello
display method invoked
my
name
is
varargs
download this example
RULES FOR VARARGS:
While using the varargs, you must follow some rules otherwise program code won't compile. The rules are as
follows:
o There can be only one variable argument in the method.
o Variable argument (varargs) must be the last argument.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-77-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![EXAMPLES OF VARARGS THAT FAILS TO COMPILE:
6. void method(String... a, int... b){}//Compile time error
7. void method(int... a, String b){}//Compile time error
EXAMPLE OF VARARGS THAT IS THE LAST ARGUMENT IN THE METHOD:
18. class VarargsExample3{ static void display(int num, String... values){
19. System.out.println("number is "+num);
20. for(String s:values){
21. System.out.println(s);
22. } }
23. public static void main(String args[]){
24. display(500,"hello");//one argument
25. display(1000,"my","name","is","varargs");//four arguments } }
26.
Test it Now
Output:number is 500
hello
number is 1000
my
name
is
varargs
Varargs is a short name for variable arguments. In Java, an argument of a method can accept arbitrary number of
values. This argument that can accept variable number of values is called varargs.
The syntax for implementing varargs is as follows:
accessModifier methodName(datatype… arg) { // method body }
In order to define vararg, ... (three dots) is used in the formal parameter of a method.
A method that takes variable number of arguments is called a variable-arity method, or simply a varargs method.
First, let’s look at the example without using varargs:
class NoVararg {](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-78-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![public int sumNumber(int a, int b){
return a+b; }
public int sumNumber(int a, int b, int c){
return a+b+c; }
public static void main( String[] args ) {
NoVararg obj = new NoVararg();
System.out.println(obj.sumNumber(1, 2));
System.out.println(obj.sumNumber(1, 2, 3)); }}
When you run this program, the output will be:
3
6
As you can clearly see, you had to overload sumNumber() method to make it work for 3 arguments.
THINGS TO REMEMBER WHILE USING VARARGS
Here are a couple of things you should remember while working with Java vargars:
1. While defining method signature, always keep varargs at last.
The variable argument must be the last argument passed to the method. Let's consider, you
invoked doSomething() method like this:
doSomething(1, 2, 3, 4);
And, your doSomething() method is defined as:
// incorrect method declaration
public void doSomething(int ... nums, int p){ // method body }
In this case, compiler cannot figure out the number of arguments passed to nums.
However, if you define your method as:
public void doSomething(int p, int ... nums) { // method body }
The Java compiler assigns the first argument to p, and the remaining int arguments are assigned to nums.
2. A method can have only one varargs parameter.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-79-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![1. class A{
2. private int data=40;
3. private void msg(){System.out.println("Hello java");} }
4. public class Simple{
5. public static void main(String args[]){
6. A obj=new A();
7. System.out.println(obj.data);//Compile Time Error
8. obj.msg();//Compile Time Error } }
Role of Private Constructor
If you make any class constructor private, you cannot create the instance of that class from outside the class. For
example:
1. class A{
2. private A(){}//private constructor
3. void msg(){System.out.println("Hello java");} }
4. public class Simple{
5. public static void main(String args[]){
6. A obj=new A();//Compile Time Error } }
Note: A class cannot be private or protected except nested class.
2) Default
If you don't use any modifier, it is treated as default by default. The default modifier is accessible only within
package. It cannot be accessed from outside the package. It provides more accessibility than private. But, it is more
restrictive than protected, and public.
Example of default access modifier
In this example, we have created two packages pack and mypack. We are accessing the A class from outside its
package, since A class is not public, so it cannot be accessed from outside the package.
1. //save by A.java
2. package pack;
3. class A{
4. void msg(){System.out.println("Hello");} }
1. //save by B.java
2. package mypack;
3. import pack.*;
4. class B{ public static void main(String args[]){](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-84-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![5. A obj = new A();//Compile Time Error
6. obj.msg();//Compile Time Error } }
In the above example, the scope of class A and its method msg() is default so it cannot be accessed from outside the
package.
3) Protected
The protected access modifier is accessible within package and outside the package but through inheritance only.
The protected access modifier can be applied on the data member, method and constructor. It can't be applied on the
class.
It provides more accessibility than the default modifer.
Example of protected access modifier
In this example, we have created the two packages pack and mypack. The A class of pack package is public, so can
be accessed from outside the package. But msg method of this package is declared as protected, so it can be
accessed from outside the class only through inheritance.
1. //save by A.java
2. package pack;
3. public class A{
4. protected void msg(){System.out.println("Hello");} }
1. //save by B.java
2. package mypack;
3. import pack.*;
4. class B extends A{
5. public static void main(String args[]){
6. B obj = new B();
7. obj.msg(); } }
Output:Hello
4) Public
The public access modifier is accessible everywhere. It has the widest scope among all other modifiers.
Example of public access modifier
1. //save by A.java](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-85-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
![2.
3. package pack;
4. public class A{
5. public void msg(){System.out.println("Hello");}
6. }
1. //save by B.java
2. package mypack;
3. import pack.*;
4. class B{
5. public static void main(String args[]){
6. A obj = new A();
7. obj.msg(); } }
Output:Hello
Java Access Modifiers with Method Overriding
If you are overriding any method, overridden method (i.e. declared in subclass) must not be more restrictive.
1. class A{
2. protected void msg(){System.out.println("Hello java");} }
3. public class Simple extends A{
4. void msg(){System.out.println("Hello java");}//C.T.Error
5. public static void main(String args[]){
6. Simple obj=new Simple();
7. obj.msg(); } }
The default modifier is more restrictive than protected. That is why, there is a compile-time error.
JAVA ACCESS MODIFIERS:
In Java, We have seen that the variables and methods of a class are visible everywhere in the program. However, it
may be necessary for some situations to restrict access to certain variables and methods from outside the class. We
can achieve this in Java by applying visibility modifiers to the instance variables and methods. These visibility
modifiers are also known as Java Access Modifiers. There are five types of Java Access Modifiers are exists:](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fjavafull1recovered-231001024003-a4536b77%2F85%2Fjava-full-1-Recovered-docx-86-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fjava-full-1-recovereddocx%2F261640390&__type=image)
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This document summarizes the evolution of Java, including its origins in 1991, key features like portability and object-orientation, differences from C/C++, and enhancements in later versions. It discusses how Java was designed to be simple, reliable, and portable by compiling to bytecode and avoiding pointers. It also covers Java's support for distributed applications, multithreading, and dynamic features. Later sections summarize additional features introduced in J2SE 5.0 like generics and annotations, as well as enhancements in Java SE 6 like scripting language integration and XML processing.
