Java实现进制转换

import java.math.BigInteger;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;

public class Main {
    public static void main(String [] args){
      Scanner sc = new Scanner(System.in);
      String str = sc.next();///输入16进制
      String str1="";
      StringBuilder stringBuilder = new StringBuilder();
      for(int j=0;j< str.length();j++){
         str1 = Integer.toBinaryString(Integer.valueOf(str.charAt(j)+"",16));
         ///16进制的每一位转化为二进制
         while(str1.length()<4&&j!=0) str1="0"+str1;///加0
         stringBuilder.append(str1);///加入字符串容器中
      }
      int numtmp = 0;
       BigInteger value = new BigInteger("0");///BigInteger包装超大数据
       long valuetmp;
       for(int k = stringBuilder.length()-1;k>=0;k--){
          valuetmp = Integer.parseInt(stringBuilder.charAt(k)+"")*(long)Math.pow(2,numtmp);
          ///二进制中一位位转化为十进制

          value = value.add(BigInteger.valueOf(valuetmp));///a.add(b)为a+b
          numtmp++;
       }
       System.out.println(value);
    }
}

import java.util.Scanner;

public class Main {
    public static void main(String [] args){
      Scanner sc = new Scanner(System.in);
      String str = sc.next();
      Long ans = Long.parseLong(str,16);
      System.out.println(ans);
    }
}