Answers to exercises and questions in Java Course Material

// Perfect Number

import java.util.Scanner;
public class PerfectNumber {
    public static void main(String[] args) {

        // take numbers from user
        Scanner s = new Scanner(System.in);
        int num = s.nextInt();
        int sum = 0;
        for ( int i = 1;  i <= num/2 ; i ++) {
          if ( num % i == 0 ) {
               sum += i;
          }
        }
        if ( sum == num)
              System.out.println("Perfect Number");
        else
              System.out.println("Not Perfect Number");
    }
}

// Accept ten numbers from user and display the largest of the given numbers.

import java.util.Scanner;

public class LargestNumber {
    public static void main(String[] args) {
        // take number from user
        Scanner s = new Scanner(System.in);
        int num, largest = 0;
        
        for ( int i = 1; i <= 10 ;i ++) {
         System.out.print("Enter a number : ");
         num = s.nextInt();
         if ( num > largest)
              largest = num;
        }
        System.out.println("Largest Number : " + largest);
    }
}

// Accept two numbers and display the GCD of the numbers

import java.util.Scanner;
public class GCD {
    public static void main(String[] args) {
        // take numbers from user
        Scanner s = new Scanner(System.in);
        int first = s.nextInt();
        int second = s.nextInt();
        
        int i = first < second ? first : second;
        
        for ( ;  i > 0 ; i --) {
          if ( first % i == 0 &&  second % i == 0) {
               System.out.println("GCD = " + i);
               break;
          }
        }
    }
}

// display Fibonacii series 
public class Fibonacii {
    public static void main(String[] args) {
        int first = 1,second = 1, result;
        
        System.out.println(first);
        System.out.println(second);
        
        for ( int i = 3; i <= 10 ; i ++) {
             result = first + second;
             System.out.println( result );
             first = second;
             second = result;
        } // for
    } // end of main
}

// Accept two numbers on command line and raise base (first number) to power (second number).

public class Power {
    public static void main(String[] args) {
        int base = Integer.parseInt( args[0]);
        int power  = Integer.parseInt( args[1]);
        int result =1;
        for ( int i = 1; i <= power ; i ++) {
              result *= base;
        }
        System.out.printf("%d raised to %d = %d", base, power, result);  
    }
    
}

import java.util.Scanner;

public class HighestFactor {

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        System.out.print("Enter a number : ");
        int num = s.nextInt();

        for ( int i = num/2; i > 0 ; i --) {
            if ( num % i == 0 ) {
                System.out.printf("Highest Factor : %d\n",i);
                break;
            }
        }

    }
}

import java.util.Scanner;

public class ReverseDigits {


    public static void main(String[] args) {

        Scanner s = new Scanner(System.in);
        System.out.print("Enter a number : ");
        int num = s.nextInt();

        while( num != 0 )
        {
            System.out.print( num % 10);
            num /= 10;

        }

    }

}

public class AverageOfNumbers {


    public static void main(String[] args) {
       System.out.println( average(10,20,45,33));
       System.out.println( average(12,22,88));
    }

    public static int average(int ... numbers) {
        int total = 0;
        for(int n : numbers)
            total += n;

        return total / numbers.length;
    }
}

import java.util.Scanner;

public class InterchangeArray {


    public static void main(String[] args) {

            int a[] = new int[10];

            Scanner s = new Scanner(System.in);

            // read values from keyboard into array
            for ( int i = 0 ; i <  a.length ; i ++) {
                System.out.printf("Enter a number for [%d] element :",i);
                a[i] = s.nextInt();
            }

            System.out.println("Orginal Array");
            printArray(a);

            // interchange array
            for ( int i = 0, j = a.length -1 ; i < a.length / 2; i ++, j --) {
                int t = a[i];
                a[i] = a[j];
                a[j] = t;
            }

            System.out.println("Interchanged Array");
            printArray(a);
        

    }

    public static void printArray(int [] a) {
            for(int n : a) {
                System.out.printf("%5d",n);
            }
    }

}

public class PalindromeNumbers {

    public static void main(String[] args) {

        for ( int i = 100; i <= 1000; i ++)  {

              int n = i, rn  = 0;
              while (n > 0 ) {
                  int d = n % 10;  // get right most digit
                  rn = rn * 10 + d;
                  n /= 10;   // remove right most digit
              }
              if ( i == rn )
                  System.out.println(i);
        }

    }

}

import java.io.FileReader;
import java.util.Scanner;

public class CountLines {
    
    public static void main(String[] args) throws Exception {
        
        System.out.println("Enter a filename : ");
        Scanner s = new Scanner(System.in);
        String filename =  s.nextLine();
        
        FileReader fr = new FileReader(filename);
        int ch = fr.read();
        int nolines = 0;
        while ( ch != -1) {
             if ( ch == '\n')
                    nolines ++;
             ch = fr.read();
        }
        fr.close();
        System.out.printf("No. of lines =  %d\n",nolines);
        
    }
    
}

import java.io.BufferedReader;
import java.io.FileReader;
import java.util.Scanner;

public class NonBlankLines {
    
    public static void main(String[] args) throws Exception {
        Scanner s = new Scanner(System.in);
        String filename =  s.nextLine();
        FileReader fr = new FileReader(filename);
        BufferedReader br  = new BufferedReader(fr);
        String line = br.readLine();     
        while ( line != null) {
             if ( line.trim().length() > 0 )
                   System.out.println(line);
             line = br.readLine();
        }
        br.close();
        fr.close();
    }
    
}

import  java.io.*;
import  java.util.Scanner;

public class FileList
{
 public static void main(String args[])
 {
  Scanner s = new Scanner(System.in);
  System.out.println("Enter directory name :");
  String dir = s.nextLine();
  File path = new File(dir);
  if (! path.isDirectory())
  {
     System.out.println( dir + " is not a directory");
     return;
  }
  File list [] = path.listFiles();
  for ( File f : list)
  {
     if ( f.isDirectory())
      System.out.println(f.getName() + "\t" +  "");
     else
     System.out.println( f.getName() + "\t" +  f.length());
  } // end of for loop 
 } // end of main
} // end of class 
    

import java.util.*;
import java.io.*;

public class  ReverseFile
{
   public static void main(String args[])  throws Exception  {

     System.out.println("Enter a filename : ");
     Scanner keyboard = new Scanner(System.in);
     String filename =  keyboard.nextLine();
        
     FileReader fr = new FileReader(filename);
     BufferedReader br = new BufferedReader(fr);
     
     Stack s = new Stack();
     String line = br.readLine();
     while ( line != null ) {
       s.push(line);
       line = br.readLine();
     }
     br.close();
     
     // display stack until it is empty
     while ( ! s.empty())
          System.out.println( s.pop());

  }
}