Two's Complement

/*
 * WAP to enter a negative number in base 10 and 
 * find its 2's complement.
 */
import java.util.*;
class TwosComplement
{
public static void main (String args[])throws InputMismatchException
{
Scanner scan = new Scanner(System.in);
int n,i,p,b,x,carry;

System.out.println("Enter a number (in base 10): ");
    n = scan.nextInt();
    if(n<0){
int a[] = new int[100];

p = -1*n;
x=0;
//Convert the number to binary
    while(p>0){

       b = p%2;
       a[x++] = b;
       p=p/2;

    }
    //Convert the array to store the 1s complement
    for(i=0; i< x; i++){
        if(a[i] == 0){
            a[i] = 1;
        }else{
            a[i] = 0;
        }
    }
    System.out.print("1s complement: ");
    for(i=x-1; i>=0; i--){
        System.out.print(a[i]);
    }
    //Add 1 to the last digit of 1s complement to get 2s complement
    a[0] += 1;
    carry = 0;
    for(i=0; i < x; i++){
        a[x] = a[x] + carry;
        if(a[x] == 2){
            a[x] = 0;
            carry = 1;
        }
    }
    System.out.print("\n2s complement: ");
    for(i=x-1; i>=0; i--){
        System.out.print(a[i]);
    }
}else{
    System.out.println("Invalid number. Please enter a negative number.");
} 
}//end of main
}//end of class