'Back-End/Java_1'에 해당되는 글 48건

 Package

- Package is a group of similar types of classes, interface and sub-packages

- built-in package(java, lang, awt, javax...) and user-defined package

1. Advantage of Java Package

- Java package is used to categorize the classes and interface so that they can be easily maintained

- It provides access protection

- It removes naming collision

2. How to compile and run java package

1) compile

//javac -d directory javafilename javac -d . Simple.java

2) run

java mypack.Simple  

Interface

- interface is a blueprint of a class

- It has static constants and abstract methods only

- It is used to achieve fully abstraction and multiple inheritance in java

1. Why use it

- It is used to achieve fully abstraction

- By interface, we can support the functionality of multiple inheritance

- It can be used to achieve loose coupling

- The java compiler adds public and abstract keywords before the interface method and public, static and final keywords before data members

2. Example

interface printable{ void print(); } class A implements printable{ public void print(){ System.out.println("Hello"); } } public class Test{ public static void main(String[]args){ A obj = new A(); obj.print(); } }

3. Multiple inheritance 

interface printable{ void print(); } interface showable{ void show(); } class A implements printable, showable{ public void print(){ System.out.println("Hello"); } public void show(){ System.out.println("Show"); } } public class Test{ public static void main(String[]args){ A obj = new A(); obj.print(); obj.show(); } }  

1) Is it possible by interface, why?

- There is no ambiguity as implementation is provided by the implementation class

4. What is marker or tagged interface?

- An interface that have no member is known as marker or tagged interface

- Serializable, Cloneable, Remote....

- They are used to provide some essential information to the JVM so that JVM may perform some useful operation

public interface Serializable{ }  

Abstract class

- A class that is declared with abstract keyword, is known as abstract class

- It can have abstract and non-abstract methods

1. Abstraction in java

- Abstraction is a process of hiding the implementation details 

- It showing only functionality to the user

- Abstraction lets you focus on what the object does instead of how it does it

1) Ways to achieve Abstraction

- Abstract class

- Interface

2. Example

abstract class Shape{ abstract void draw(); } class Rectangle extends Shape{ void draw(){ System.out.println("Drawing rec"); } } class Circle extends Shape{ void draw(){ System.out.println("draw circle"); } } public class Test{ public static void main(String[]args){ Shape s = new Circle(); s.draw(); } }

3. Abstract class having constructor, data member, methods

abstract class Bike{ Bike(){ System.out.println("Bike is created"); } abstract void run(); void changeGear(){ System.out.println("gear changed"); } } class Honda extends Bike{ void run(){ System.out.println("running"); } } public class Test{ public static void main(String[]args){ Bike obj = new Honda(); obj.run(); obj.changeGear(); } }

- If there is any abstract method in a class, that class must be abstract

- If you are extending any abstract class that have abstract method, you must either provide the implementation of the method or make this class abstraction

 Instanceof

- instanceof operator is used to test whether the object is an instance of the specified type

- It is also known as type comparison operator

- It returns either true or false

1. Example

public class Test{ public static void main(String[]args){ Test a = new Test(); System.out.println(a instanceof Test); } }

- true

2. instanceof in java with a variable that have null value

public class Test{ public static void main(String[]args){ Test a = null; System.out.println(a instanceof Test); } }  

- false

3. Downcasting with java instanceof operator

1) Downcasting

- When subclass type refers to the object of Parent class, it is known as downcasting

Dog d = new AnImal(); //Compilation error  

Dog d = (Dog) new Animal() // ClassCastException  

2) Possibility of downcasting with instanceof

package statics; class Animal{ } class Dog extends Animal{ static void method(Animal a){ if(a instanceof Dog){ Dog d = (Dog)a;//down } } } public class Test{ public static void main(String[]args){ Animal a = new Dog(); Dog.method(a); } }  

3) Downcasting without the use of instanceof

class Animal{ } class Dog extends Animal{ static void method(Animal a){ Dog d = (Dog)a;//down } } public class Test{ public static void main(String[]args){ Animal a = new Dog(); Dog.method(a); } }