二进制到十进制-Java

Question

I want to write a program which receive a string value and print the decimal number.

In addition, if the string value is not 1 or 0, I need to print a message.

I wrote this code but it is always getting inside the if command.

I Would appreciate your support!

Thank you

import java.util.Random; 
public class Decimal {

    public static void main(String[] args) {
        String input = (args[0]);
        int sum = 0;
        for (int i = 0; i <= input.length(); i++) {
            if (!(input.charAt(i) == '0') || (input.charAt(i) == '1')) {
                System.out.println("wrong string");
                break;
            }
            char a = input.charAt(i);
            if (a == '1') {
                sum |= 0x01;
            }
            sum <<= 1;
            sum >>= 1;
            System.out.println(sum);
        }
    }
}

Answer 1

The ! (not) operator of the if statement only applies to the first part:

if ( ! (input.charAt(i) == '0')
     ||
     (input.charAt(i) == '1')
   ) {

So that is the same as:

if ((input.charAt(i) != '0') || (input.charAt(i) == '1')) {

When you actually meant to do:

if (input.charAt(i) != '0' && input.charAt(i) != '1') {

It's a good thing though, because once that works, you're going to get an IndexOutOfBoundsException when i == input.length() . Change the loop to:

for (int i = 0; i < input.length(); i++) {

And for performance, move variable a up and use it in that first if statement. Rename to c or ch is more descriptive/common.

Doing both sum <<= 1 and sum >>= 1 leaves you where you started. Is that what you wanted? You should also do the left-shift before setting the right-most bit.

Applying all that, I believe you meant to do this:

String input = args[0];
int sum = 0;
for (int i = 0; i < input.length(); i++) {
    char c = input.charAt(i);
    if (c != '0' && c != '1') {
        System.out.println("wrong string");
        break;
    }
    sum <<= 1;
    if (c == '1')
        sum |= 1;
}
System.out.println(sum);