try {
    String s = "b7c5b4f3"; //思路就是把String转成对应的byte[]数组
    byte[] b = new byte[s.length()/2];
    for (int i=0, j=0; i<s.length(); i+=2) {
        b[j++] = (byte)Integer.parseInt(s.substring(i,i+2), 16);
    }
    String data = new String(b, "UTF-16BE"); //自己调整编码试试看，如UTF-16LE什么的？
    System.out.println(data);
} catch (Exception e) {
    e.printStackTrace();
}

------解决方案--------------------

Java code
package com;
import java.io.UnsupportedEncodingException;
import java.util.Scanner;

public class Decoder {
public static void main(String[]args){
 System.out.println("请输入转换的16进制代码：");
 Scanner scan=new Scanner(System.in);
 String getb=scan.nextLine();
   String result=decodeString(getb, "GBK");
   System.out.println(result);
} 
private static String decodeString(String string, String encoding) {
try {
byte[]data=string2Bytes(string);
return new String(data, encoding);
} catch (UnsupportedEncodingException ex) {
ex.printStackTrace();

}
return encoding;
} 
private static byte[] string2Bytes(String string) {
int blen=string.length()/2;
byte[]data=new byte[blen];
for(int i=0;i<blen;i++){
String Str=string.substring(2*i,2*(i+1));
data[i]=(byte)Integer.parseInt(Str,16);
}
return data;
}
}

------解决方案--------------------
必须先知道 你这个字符串是什么编码 utf-8