What is Composition in JAVA !!!

Composition is restricted form of agreegation. For example, a class car can't exist without engine.


class Car
{
  private Engine engine;
  Car(Engine en)
  {
    engine en;
  }
}

Why String[] args in main() method in JAVA?

The main objective is designed very intelligently by developers. Actual thinking is very deep. Which is basically developed under consideration of C & C++ based on Command line argument but nowadays nobody uses it more.

Thing 1- User can enter any type of data from the command line can be Number or String & necessary to accept it by the compiler which datatype we should have to use?

Thing 2- String is the datatype which supports all of the primitive datatypes like int, long, float, double, byte, shot, char in Java. You can easily parse it in any primitive datatype.

let's see the following example, 

If input is -> 1 1

// one class needs to have a main() method

public class HelloSilan

{

  // arguments are passed using the text field below this editor

  public static void main(String[] x)

  {   

System.out.println(x[0] + x[1]); // Output is 11

//Comment out below code in case of String

    System.out.println(Integer.parseInt(parameter[0]) + Integer.parseInt(parameter[1])); //Output is 2

    System.out.println(Float.parseFloat(parameter[0]) + Float.parseFloat(parameter[1])); //Output is 2.0   

    System.out.println(Long.parseLong(parameter[0]) + Long.parseLong(parameter[1])); //Output is 2   

    System.out.println(Double.parseDouble(parameter[0]) + Double.parseDouble(parameter[1])); //Output is 2.0   

  }

}

What is checked exception?

The exception which is checked by the compiler at the time of compilation for smooth execution of the program is known as checked exception.
Example:
import java.io.*;
class Test{
public static void main(String[] args){
PrintWriter pw=new PrintWriter("abc.txt");
pw.println("SILAN TECHNOLOGY");
}
}
sometimes the compiler check some specific program for smooth execution at run time, so when we compile this program we will get following compile time error :

unreported exception java.lang.FileNotFoundException must be caught or declared to be thrown