hi I was wondering if I could get help with this program. I have the program jus

Question

hi I was wondering if I could get help with this program. I have the program just about fully written. just at the last method where It converts from a binary number into a decimal it does not always add up. it works fine when i try to do the positive numbers. But some of the negative numbers that i try inputting comes up with the wrong answer. For example 10000000 comes up as 128, not -128 and 11111111 comes up as -127. if you could help with it at all. most of the program should be good up to that point but maybe not? I think it is something to do with the binary to decimal method. thanks in advance

/*


* Description: Simple program to convert decimal to binary
* and binary to decimal
*/
import java.util.*;
import java.lang.Math;
public class lab1 {

public static void main (String [] args){
  Scanner keyIn = new Scanner(System.in);
  int response;
  // menu options to choose how to run the program and continue running the
  // program until user decides to quit.
  do{
   System.out.println("Enter 1 to convert decimal to binary");
   System.out.println("Enter 2 to convert binary to decimal");
   System.out.println("Enter 3 to quit");
   System.out.print("Please enter your choice: ");
   response = keyIn.nextInt();
   //choosing 1 uses case 1 for decimal conversion and 2 for binary conversion
   switch(response)
   {case 1:
    System.out.println("");
    System.out.print("Enter a whole number between -128 and +127: ");
    int decimal = keyIn.nextInt();
    while(decimal < -128 || decimal > 127)
    {
     System.out.print("Enter a whole number between -128 and +127: ");
     decimal = keyIn.nextInt();
    }
    System.out.println("");
    System.out.print(decimal + " converted into a binary is: ");
    decConvert(decimal);
    System.out.println("");
    break;
   case 2:
    System.out.println("");
    System.out.println("Enter a 8-bit binary number:");
    String a = keyIn.next();
    System.out.println("");
    System.out.println(a + " converted into a decimal is: ");
    int i = binconvert(a);
    System.out.println(i);
    break;
   default:
    break;
   }
   System.out.println("");
  }while(response!=3);
  System.out.println("The program has ended");
}
//method to convert from decimal to binary
public static void decConvert(int num){
  String out = "";
  if(num >= 0)
  {
   while(num > 0)
   {
    out += num % 2;
    num = num/2;
   }
   int length = out.length();
   while(length < 8)
   {
    out += 0;
    length++;
   }
   String temp = "";
   int y = 7;
   //Start from right of string moving to left
   while(y > -1){
    temp += out.charAt(y);
    y--;
   }
   System.out.print(temp);
  }
  else
  { num += 1;
  num *= -1;
  // flip the numbers
  while(num > 0){
   out += num % 2;
   num = num/2;
  }
  int length = out.length();
  while(length < 8){
   out += 0;
   length++;
  }
  String temp = "";
  int y = 7;
  while(y > -1){
   temp += out.charAt(y);
   y--;
  }
  out = temp;
  int bin [] = new int [8];
  //do{
  for(int i = 0;i < 8; i++){
   bin [i]= out.charAt(i)-48;
   if(bin[i] == 0)
    bin[i]= 1;
   else
    bin [i] = 0;
   System.out.print(bin[i]);}

  //Start from right of string moving to left
  }
}
//method to convert from binary to decimal
private static int binconvert(String a)
{
  double j = 0;
  for(int i = 0; i<a.length(); i++)
  {
   if(a.charAt(i)== '1')
    j=j+ Math.pow(2, a.length()-1-i);
  }
  if(j > 128)
   j = 128 - j;
  else if(j < 0)
   j =-128;
  return (int) j;
}
}

Explanation / Answer

You can modify the method as below:

//method to convert from binary to decimal
private static int binconvert(String a)
{
   int binaryNum = Integer.parseInt(a);
   int decimal = 0;
    int i = 0;
    while(true){
      if(binaryNum == 0){
        break;
      } else {
          int temp = binaryNum%10;
          decimal += temp*Math.pow(2, i);
          binaryNum = binaryNum/10;
          i++;
       }
    }

    decimal = (decimal + 128)%256 - 128; //this is the key line which change 128 to -128
    return decimal;  
}

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