gistfile1.java

import java.util.Scanner;
import java.util.Arrays;

public class BinaryConverter {

    public static String userOption;
    
    public static void main(String[] args) {
        
        Scanner keyboard = new Scanner(System.in);

        userOption = getInput(keyboard);
        
        while (!userOption.equalsIgnoreCase("QUIT")) {
        
            if (userOption.equalsIgnoreCase("BD")) {
                System.out.print("Please enter a binary string: ");
                String binaryNumber = keyboard.next();
                
                System.out.println(binaryToDecimal(binaryNumber));
            } else if (userOption.equalsIgnoreCase("DB")) {
                System.out.print("Please enter a number: ");
                int number = keyboard.nextInt();     
                
                System.out.println(Arrays.toString(decimalToBinary(number)));
            } else {
                System.out.println("Invalid Command.");
            }
            
            userOption = getInput(keyboard);
        }
    }


    // ############################################
    // ############## Get User Input ##############
    // ############################################  
    public static String getInput(Scanner keyboard) {
        System.out.print("Please enter an option: ");
        return keyboard.next();
    }
    
    // ############################################
    // ############# Binary to Decimal ############
    // ############################################   
    public static int binaryToDecimal(String binaryNumber) {
        int decimal = 0;

        for (int i = 0; i < binaryNumber.length(); i++) {
            decimal *= 2;
            decimal += binaryNumber.charAt(i) - '0';
        }

        return decimal;
    }

    // ############################################
    // ############# Decimal to Binary ############
    // ############################################
    
    // Let's get the length of an array
    public static int getLength(int number) {
        int length = 0;
        
        while (number >= 1) {
            // Divide number by two until the number 
            //   is less than 1
            number >>>= 1;
            length++;
        }
        
        return length;
    } 
    
    // Fill in the array
    public static int[] decimalToBinary(int number) {
        // Make an array with the appropriate number of (empty) values
        //    For 4:
        //    [0, 0, 0]
        int arrayLength = getLength(number);
        int[] numArray = new int[arrayLength];
        
        int i = numArray.length - 1;

        while (number >= 1) {
            // Retains number 1 & 1
            //   0011
            //   0110
            //   ----
            //   0010 <-- result
            //   http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op3.html
            numArray[i--] = number & 1;
           
            number >>>= 1;
        }
        
        return numArray;
    }
}