1. 朴素法

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNext()) { // 注意 while 处理多个 case
            String s = in.next().toUpperCase().substring(2);
            int res = 0;
            for(int i = 0; i < s.length(); i ++){
                char ch = s.charAt(i);
                int num;
                if(ch >= 'A') num = ch - 55;
                else num = ch - '0';
                res = res * 16 + num;
            }
            System.out.println(res);
        }
    }
}

2. API

import java.io.*;
import java.util.*;

public class Main{
    public static void main(String[] args) throws Exception{
        Scanner sc = new Scanner(System.in);
        while(sc.hasNextLine()){
            String s = sc.nextLine();
            System.out.println(Integer.parseInt(s.substring(2,s.length()),16));
        }
    }
}

import java.util.*;

public class Main {
    public static void main(String[] args) {
        String s = "0xAa";
        //或者
        String s = "0xAA";
        System.out.println(Integer.parseInt(s.substring(2), 16));
    }
}