Package in Java
Package is a group of similar types of classes, interfaces and sub-packages. Packages can be built-in or user defined.
Built-in packages - java, util, io etc.
import java.util.Scanner;
import java.io.IOException;
Access Modifiers in Java
Private: The access level of a private modifier is only within the class. It cannot be accessed from outside the class.
Default: The access level of a default modifier is only within the package. It cannot be accessed from outside the package. If you do not specify any access level, it will be the default.
Protected: The access level of a protected modifier is within the package and outside the package through child class. If you do not make the child class, it cannot be accessed from outside the package.
Public: The access level of a public modifier is everywhere. It can be accessed from within the class, outside the class, within the package and outside the package.
package newpackage;
class Account {
public String name;
protected String email;
private String password;
public void setPassword(String password) {
this.password = password;
}
}
public class Sample {
public static void main(String args[]) {
Account a1 = new Account();
a1.name = "Apna College";
a1.setPassword("abcd");
a1.email = "hello@apnacollege.com";
}
}
Encapsulation
Encapsulation is the process of combining data and functions into a single unit called class. In Encapsulation, the data is not accessed directly; it is accessed through the functions present inside the class. In simpler words, attributes of the class are kept private and public getter and setter methods are provided to manipulate these attributes. Thus, encapsulation makes the concept of data hiding possible.(Data hiding: a language feature to restrict access to members of an object, reducing the negative effect due to dependencies. e.g. "protected", "private" feature in Java).
Abstraction
We try to obtain an abstract view, model or structure of a real life problem, and reduce its unnecessary details. With definition of properties of problems, including the data which are affected and the operations which are identified, the model abstracted from problems can be a standard solution to this type of problems. It is an efficient way since there are nebulous real-life problems that have similar properties.
In simple terms, it is hiding the unnecessary details & showing only the essential parts/functionalities to the user.
Data binding : Data binding is a process of binding the application UI and business logic. Any change made in the business logic will reflect directly to the application UI.
Abstraction is achieved in 2 ways :
Abstract class
Interfaces (Pure Abstraction)
Abstract Class
An abstract class must be declared with an abstract keyword.
It can have abstract and non-abstract methods.
It cannot be instantiated.
It can have constructors and static methods also.
It can have final methods which will force the subclass not to change the body of the method.
abstract class Animal {
abstract void walk();
void breathe() {
System.out.println("This animal breathes air");
}
Animal() {
System.out.println("You are about to create an Animal.");
}
}
class Horse extends Animal {
Horse() {
System.out.println("Wow, you have created a Horse!");
}
void walk() {
System.out.println("Horse walks on 4 legs");
}
}
class Chicken extends Animal {
Chicken() {
System.out.println("Wow, you have created a Chicken!");
}
void walk() {
System.out.println("Chicken walks on 2 legs");
}
}
public class OOPS {
public static void main(String args[]) {
Horse horse = new Horse();
horse.walk();
horse.breathe();
}
}
2. Interfaces
All the fields in interfaces are public, static and final by default.
All methods are public & abstract by default.
A class that implements an interface must implement all the methods declared in the interface.
Interfaces support the functionality of multiple inheritance.
interface Animal {
void walk();
}
class Horse implements Animal {
public void walk() {
System.out.println("Horse walks on 4 legs");
}
}
class Chicken implements Animal {
public void walk() {
System.out.println("Chicken walks on 2 legs");
}
}
public class OOPS {
public static void main(String args[]) {
Horse horse = new Horse();
horse.walk();
}
}
Static Keyword
Static can be :
Variable (also known as a class variable)
Method (also known as a class method)
Block
Nested class
class Student {
static String school;
String name;
}
public class OOPS {
public static void main(String args[]) {
Student.school = "JMV";
Student s1 = new Student();
Student s2 = new Student();
s1.name = "Meena";
s2.name = "Beena";
System.out.println(s1.school);
System.out.println(s2.school);
}
}
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